Eds battery formula. Battery electromotive force

The batteries are filled with sulfuric acid and, during the normal charge-discharge cycle, they release explosive gases (hydrogen and oxygen). To avoid personal injury or damage to your vehicle, always follow these safety guidelines:

Before working on any electrical components of the vehicle, disconnect the power cable from the negative terminal of the battery. With the negative power cable disconnected, all electrical circuits in the vehicle will be open to ensure that any electrical component is not accidentally shorted to ground. An electrical spark creates a potential injury and fire hazard.
Any work involving the battery must be carried out with protective goggles.
Wear protective clothing to protect against skin contact with sulfuric acid in the battery.
Do not violate the safety instructions in the maintenance procedures when handling equipment used for battery maintenance and testing.
It is strictly forbidden to smoke or use open flames in the immediate vicinity of the battery.

Routine maintenance of the battery

Routine maintenance of the battery consists in checking the cleanliness of the battery case and, if necessary, adding clean water to it. All battery manufacturers recommend the use of distilled water for this purpose, but if not available, you can use clean drinking water with a low salt content. Since water is the only consumable component in the battery, do not add acid to the battery. Part of the water from the electrolyte escapes during the charging and discharging of the battery, but the acid contained in the electrolyte remains in the battery. Do not overfill the battery with electrolyte, because in this case, normal bubbling (gassing) that occurs in the electrolyte during battery operation will lead to electrolyte leakage, causing corrosion of the battery terminals, its mounting brackets and the pan. Batteries should be filled with electrolyte approximately one and a half inches (3.8 cm) below the top of the filler neck.

The terminals of the power cables connected to the battery and the terminals of the battery itself must be inspected and cleaned to avoid voltage drop across them. One of the common reasons an engine won't start is loose or corroded power cable pins connected to the battery terminals.

Figure: Severely corroded battery terminal

Figure: This power cable, connected to the battery, was found to be highly corroded under the insulation. Although the corrosion eaten through the insulation, it went unnoticed until the cable was thoroughly inspected. This cable must be replaced

Figure: Carefully check all battery terminals for signs of corrosion. In this car, two power cables are connected to the positive terminal of the battery with a long bolt. This is a common cause of corrosion that causes electrical engine start failure.

Battery EMF measurement

Electromotive force (EMF) is the potential difference between the positive and negative electrodes of the battery when the external circuit is open.

The EMF value depends mainly on the electrode potentials, i.e. on the physical and chemical properties of the substances from which the plates and electrolyte are made, but does not depend on the size of the battery plates. The EMF of an acid battery also depends on the density of the electrolyte.

Measurement of electromotive force (EMF) of a battery using a voltmeter is a simple way to determine its state of charge. EMF of a storage battery is not an indicator that guarantees the performance of a storage battery, but this parameter more fully characterizes the condition of a storage battery than just its inspection. A rechargeable battery that looks good, may not actually be as good as it sounds.

This test is called an open-circuit voltage measurement (EMF test) of the battery because the measurement is carried out at the battery terminals without a load connected to it, at zero current consumption.

If the check is carried out immediately after the end of charging the battery or in the car at the end of the trip, before the measurement, it is necessary to release the battery from the polarization EMF. Polarization EMF is an increased, compared to normal, voltage, which occurs only on the surface of the battery plates. The polarization EMF quickly disappears when the battery is operating under load, so it does not give an accurate estimate of the state of charge of the battery.
To release the battery from the EMF polarization, turn on the headlights in high beam mode for one minute, and then turn them off and wait a couple of minutes.
With the engine off and all other electrical equipment, with the doors closed (so that the interior lights are turned off), connect a voltmeter to the battery terminals. Connect the red, positive, voltmeter wire to the positive terminal of the storage battery, and the black, negative, wire to its negative terminal.
Record the voltmeter reading and compare it with the table of the state of charge of the battery. The table below is suitable for assessing the state of charge of a battery by the EMF value at room temperature - from 70 ° F to 80 ° F (from 21 ° C to 27 ° C).

Table

EMF of the storage battery (V)	State of charge
12.6V and above	100% charged
12,4	75% charged
12,2	50% charged
12	Charged by 25%
11.9 and below	Discharged

Figure: The voltmeter shows the battery voltage one minute after turning on the headlights (a). After turning off the headlights, the voltage measured on the battery quickly recovered to 12.6 V (b)

NOTE

If the voltmeter gives a negative reading, then either the battery is charged in reverse polarity (and then must be replaced), or the voltmeter is connected to the battery in reverse polarity.

Measuring battery voltage under load

One of the most accurate ways to determine the health of a battery is to measure the voltage of the battery under load. Most car battery cranking and charging testers use a carbon rheostat as the battery load. The load parameters are determined by the nominal capacity of the tested battery. The rated capacity of a battery is characterized by the starting current that the battery can provide at 0 ° F (-18 ° C) for 30 seconds. Previously, the characteristic of the nominal capacity of batteries in ampere-hours was used. Measurement of the battery voltage under load is carried out at a value of the discharge current equal to half of the rated CCA current of the storage battery or three times the rated capacity of the storage battery in ampere-hours, but not less than 250 amperes. Measurement of the battery voltage under load is performed after checking the degree of its charge using the built-in hydrometer or by measuring the EMF of the battery. The battery must be charged at least 75%. An appropriate load is connected to the battery and after 15 seconds of battery operation under load, the voltmeter readings are recorded with the load connected. If the battery is good, the voltmeter reading should remain above 9.6 V. Many battery manufacturers recommend measuring twice:

the first 15 seconds of battery operation under load are used to release polarization EMF
the second 15 seconds - to obtain a more reliable assessment of the condition of the battery

A delay of 30 seconds between the first and second load cycles is required to give the battery a recovery time.

Figure: A tester of starting and charging characteristics of automobile batteries, produced by Bear Automotive, automatically turns the tested battery into operation under load for 15 seconds to remove the EMF polarization, then disconnects the load for 30 seconds to restore the battery and reconnects the load for 15 seconds. The tester displays information about the battery condition

Figure: Sun Electric VAT 40 tester (voltammeter, model 40) connected to battery for load testing. The operator, using the load current regulator, sets the discharge current value according to the ammeter reading, equal to half the nominal CCA battery current. The battery operates under load for 15 seconds and at the end of this time interval the battery voltage measured with the load connected must be at least 9.6 V

NOTE

Some testers measure the capacity of the battery to determine the state of charge and health of the battery. Follow the test procedure specified by the test equipment manufacturer.

If the battery fails the load test, recharge and retest. If the second check fails, the battery must be replaced.

Charging the battery

If the battery is severely discharged, it must be recharged. Charging the battery, in order to avoid damage due to overheating, is best done in standard charging mode. Refer to the figure for an explanation of the standard battery charging mode.

Figure: This battery charger is adjusted to charge the battery with a nominal charging current of 10 A. Charging the battery in standard mode, as in the above photo, does not affect the battery as much as boost mode, which does not exclude overheating of the battery and warpage of battery plates

Please be aware that charging a fully discharged battery pack may take eight hours or more. Initially, it is necessary to maintain the charging current at about 35 A for 30 minutes - in order to facilitate the start of the battery charging process. In the accelerated charging mode, the battery heats up and increases the risk of warping the battery plates. In boost charging mode, increased gassing (hydrogen and oxygen evolution) also occurs, which poses a health hazard and a fire hazard. The battery temperature should not exceed 125 ° F (52 ° C, battery hot to the touch). It is recommended, as a rule, to charge storage batteries with a charging current equal to 1% of the rated value of the CCA-current.

Boost Charge Mode - 15A Max
Standard charging mode - maximum 5A

It can happen to anyone!

Toyota owner disconnected the battery. After connecting the new battery, the owner noticed that the yellow airbag warning light on the dashboard came on and the radio was blocked. The owner purchased a used car from a dealer and did not know the secret four-digit code required to unlock the radio. Forced to look for a solution to this problem, he randomly tried three different four-digit numbers in the hope that one of them would work. However, after three unsuccessful attempts, the radio completely shut down.

The frustrated owner turned to the dealer. It cost more than three hundred dollars to fix the problem. To reset the airbag alarm, a special device was required. The radio had to be removed from the car and sent to another state, to an authorized service center, and on return, reinstalled in the car.

Therefore, before disconnecting the battery, be sure to coordinate this with the car owner - you must make sure that the owner knows the secret code for turning on the encoded radio, which is simultaneously used in the car security system. It may be necessary to use the radio memory backup device with the battery disconnected.

Figure: Here's a good idea. The technician made a backup power source for the memory from an old rechargeable flashlight and a cable with an adapter to the cigarette lighter socket. He simply connected the wires to the battery terminals of the rechargeable flashlight he had. The flashlight battery is more convenient to use than a regular 9-volt battery - in case someone wants to open the car door while the memory backup power supply is on the circuit. A 9-volt battery, which has a small capacity, in this case would be quickly discharged, while the capacity of the flashlight battery is large enough to provide the necessary memory power even when the interior lighting is turned on

Electromotive force

The electromotive force (EMF) of the battery E is the difference between its electrode potentials, measured with an open external circuit.

EMF of a battery consisting of n series-connected batteries.

It is necessary to distinguish between the equilibrium EMF of the battery and the nonequilibrium EMF of the battery during the time from opening the circuit to the establishment of an equilibrium state (the period of the transient process). EMF is measured with a high-resistance voltmeter (internal resistance of at least 300 Ohm / V). To do this, a voltmeter is connected to the terminals of the battery or battery. In this case, no charging or discharging current should flow through the accumulator (battery).

The equilibrium EMF of a lead battery, like any chemical current source, depends on the chemical and physical properties of the substances participating in the current-forming process, and does not depend at all on the size and shape of the electrodes, as well as on the amount of active masses and electrolyte. At the same time, in a lead-acid battery, the electrolyte is directly involved in the current-forming process on the battery electrodes and changes its density depending on the state of charge of the batteries. Therefore, the equilibrium EMF, which in turn is a function of the density

The change in the EMF of the battery from temperature is very small and can be neglected during operation.

Charge and discharge voltage

The potential difference at the pole terminals of the battery (battery) in the process of charging or discharging in the presence of current in the external circuit is usually called the voltage of the battery (battery). The presence of the internal resistance of the battery leads to the fact that its voltage during discharge is always less than EMF, and during charging it is always higher than EMF.

When charging the battery, the voltage at its terminals must be greater than its EMF by the amount of internal losses. At the beginning of the charge, there is a voltage jump by the amount of ohmic losses inside the battery, and then a sharp increase in voltage due to the polarization potential, caused mainly by a rapid increase in the density of the electrolyte in the pores of the active mass. Further, a slow increase in voltage occurs, mainly due to an increase in the EMF of the battery due to an increase in the density of the electrolyte.

After the main amount of lead sulfate is converted to PbO2 and Pb, the energy consumption increasingly causes the decomposition of water (electrolysis). The excess amount of hydrogen and oxygen ions appearing in the electrolyte further increases the potential difference between opposite electrodes. This leads to a rapid increase in the charging voltage, which accelerates the decomposition of water. The resulting hydrogen and oxygen ions do not interact with active materials. They recombine into neutral molecules and are released from the electrolyte in the form of gas bubbles (oxygen is released on the positive electrode, hydrogen on the negative one), causing the electrolyte to "boil".

If you continue the charging process, you can see that the increase in the density of the electrolyte and the charging voltage practically stops, since almost all of the lead sulfate has already reacted, and all the energy supplied to the battery is now spent only on the side process - the electrolytic decomposition of water. This also explains the constancy of the charging voltage, which is one of the signs of the end of the charging process.

After stopping the charge, that is, disconnecting the external source, the voltage at the terminals of the battery drops sharply to the value of its nonequilibrium EMF, or by the value of ohmic internal losses. Then there is a gradual decrease in the EMF (due to a decrease in the density of the electrolyte in the pores of the active mass), which continues until the concentration of the electrolyte in the volume of the battery and the pores of the active mass is completely equalized, which corresponds to the establishment of an equilibrium EMF.

When the battery is discharged, the voltage at its terminals is less than the EMF by the value of the internal voltage drop.

At the beginning of the discharge, the battery voltage drops sharply by the value of ohmic losses and polarization due to a decrease in the electrolyte concentration in the pores of the active mass, that is, concentration polarization. Further, with a steady (stationary) discharge process, the density of the electrolyte in the volume of the battery decreases, causing a gradual decrease in the discharge voltage. At the same time, there is a change in the ratio of the content of lead sulfate in the active mass, which also causes an increase in ohmic losses. In this case, the particles of lead sulfate (which has about three times the volume in comparison with the particles of lead and its dioxide, from which they were formed) close the pores of the active mass, which prevents the passage of the electrolyte into the depth of the electrodes. This causes an increase in concentration polarization, leading to a more rapid decrease in the discharge voltage.

When the discharge stops, the voltage at the battery terminals quickly rises by the amount of ohmic losses, reaching the value of the nonequilibrium EMF. A further change in the EMF due to the equalization of the electrolyte concentration in the pores of the active masses and in the volume of the battery leads to a gradual establishment of the value of the equilibrium EMF.

The battery voltage during its discharge is determined mainly by the temperature of the electrolyte and the strength of the discharge current. As mentioned above, the resistance of a lead-acid battery (battery) is negligible and in a charged state is only a few milliohms. However, at currents of a starter discharge, the strength of which is 4-7 times higher than the value of the nominal capacity, the internal voltage drop has a significant effect on the discharge voltage. The increase in ohmic losses with decreasing temperature is associated with an increase in the resistance of the electrolyte. In addition, the viscosity of the electrolyte sharply increases, which complicates the process of diffusion into the pores of the active mass and increases the concentration polarization (that is, increases the voltage loss inside the battery by reducing the concentration of the electrolyte in the pores of the electrodes). At a current of more than 60 A, the dependence of the discharge voltage on the current strength is practically linear at all temperatures.

The average value of the battery voltage during charging and discharging is determined as the arithmetic mean of the voltage values \u200b\u200bmeasured at regular intervals

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Battery EMF (electromotive force) is the difference in electrode potentials in the absence of an external circuit. The electrode potential is the sum of the equilibrium electrode potential. It characterizes the state of the electrode at rest, that is, the absence of electrochemical processes, and the polarization potential, which is defined as the potential difference of the electrode during charging (discharging) and in the absence of a circuit.

Diffusion process.

Due to the diffusion process, equalization of the electrolyte density in the cavity of the battery body and in the pores of the active mass of the plates, electrode polarization can be maintained in the battery when the external circuit is disconnected.

The diffusion rate depends directly on the temperature of the electrolyte; the higher the temperature, the faster the process takes place and can vary greatly in time, from two hours to a day. The presence of two components of the electrode potential during transient modes led to the division into equilibrium and non-equilibrium EMF of the battery. The equilibrium EMF of the battery is influenced by the content and concentration of ions of active substances in the electrolyte, as well as the chemical and physical properties of active substances. The main role in the magnitude of the EMF is played by the density of the electrolyte and the temperature has practically no effect on it. The dependence of the EMF on density can be expressed by the formula:

E \u003d 0.84 + p Where E - battery EMF (B) P - electrolyte density reduced to a temperature of 25 gr. C (g / cm3) This formula is true at a working density of the electrolyte in the range of 1.05 - 1.30 g / cm3. EMF cannot directly characterize the degree of rarefaction of the battery. But if you measure it at the terminals and compare it with the calculated one in terms of density, then it is possible, with a degree of probability, to judge the state of the plates and capacity. At rest, the density of the electrolyte in the pores of the electrodes and the cavity of the monoblock are the same and equal to the EMF at rest. When connecting consumers or a charge source, the polarization of the plates and the concentration of electrolyte in the pores of the electrodes change. This leads to a change in the EMF. During charging, the EMF value increases, and during discharge, it decreases. This is due to a change in the density of the electrolyte, which is involved in electrochemical processes.

The EMF of the battery is not equal to the voltage of the battery, which depends on the presence or absence of a load on its terminals.

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Battery electromotive force

Is it possible to accurately judge the state of charge of the battery by the EMF?

The electromotive force (EMF) of a battery is the difference between its electrode potentials, measured with an open external circuit:

Е \u003d φ + - φ–

where φ + and φ– are the potentials of the positive and negative electrodes, respectively, when the external circuit is open.

EMF of a battery consisting of n series-connected batteries:

In turn, the electrode potential in an open circuit in the general case consists of the equilibrium electrode potential, which characterizes the equilibrium (stationary) state of the electrode (in the absence of transient processes in the electrochemical system), and the polarization potential.

This potential is generally defined as the difference between the potential of an electrode during discharge or charge and its potential in an equilibrium state in the absence of current. However, it should be noted that the state of the battery immediately after turning off the charging or discharging current is not equilibrium due to the difference in the electrolyte concentration in the pores of the electrodes and the interelectrode space. Therefore, the electrode polarization remains in the battery for a rather long time even after switching off the charging or discharge current and characterizes in this case the deviation of the electrode potential from the equilibrium value due to the transient process, that is, mainly due to diffusion equalization of the electrolyte concentration in the battery from the moment of opening the external circuit to the establishment of equilibrium stationary state in the battery.

The chemical activity of the reagents collected in the electrochemical system of the battery, and, consequently, the change in the EMF of the battery depends very little on temperature. When the temperature changes from –30 ° C to + 50 ° C (in the operating range for the battery), the electromotive force of each battery in the battery changes by only 0.04 V and can be neglected during battery operation.

With an increase in the density of the electrolyte, the EMF increases. At a temperature of + 18 ° C and a density of 1.28 g / cm3, the battery (meaning one bank) has an EMF equal to 2.12 V. A battery of six cells has an EMF equal to 12.72 V (6 × 2.12 V \u003d 12 , 72 V).

The EMF cannot accurately judge the state of charge of the battery. The EMF of a discharged battery with a higher electrolyte density will be higher than the EMF of a charged battery, but with a lower electrolyte density. The EMF value of a working battery depends on the density of the electrolyte (the degree of its charge) and varies from 1.92 to 2.15 V.

When operating batteries, by measuring the EMF, a serious malfunction of the battery can be detected (closure of plates in one or more banks, breakage of connecting conductors between banks, and the like).

EMF is measured with a high-resistance voltmeter (the internal resistance of the voltmeter is less than 300 Ohm / V). During measurements, a voltmeter is connected to the terminals of the battery or battery. In this case, no charging or discharge current must flow through the accumulator (battery)!

*** Electromotive force (EMF) is a scalar physical quantity that characterizes the work of external forces, that is, any forces of non-electrical origin, acting in quasi-stationary DC or AC circuits. EMF, like voltage, in the International System of Units (SI) is measured in volts.

orbyta.ru

27.3. Electrochemical reactions in the battery. Electromotive force. Internal resistance. Self-discharge. Sulfation of plates

If you close the external circuit of a charged battery, an electric current will appear. In this case, the following reactions occur:

at the negative plate

at the positive plate

where e is the electron charge equal to

For every two molecules of acid consumed, four water molecules are formed, but at the same time, two water molecules are consumed. Therefore, as a result, only two water molecules are formed. Adding equations (27.1) and (27.2), we obtain the discharge reaction in the final form:

Equations (27.1) - (27.3) should be read from left to right.

When the battery is discharged, lead sulfate is formed on the plates of both polarities. Sulfuric acid is consumed by both positive and negative plates, while the positive plates have more acid consumption than the negative ones. The positive plates form two water molecules. The electrolyte concentration decreases when the battery is discharged, while it decreases to a greater extent in the positive plates.

If you change the direction of the current through the battery, then the direction of the chemical reaction will be reversed. The battery charging process begins. The charge reactions of the negative and positive plates can be represented by equations (27.1) and (27.2), and the overall reaction can be represented by equation (27.3). These equations should now be read from right to left. When charged, lead sulfate at the positive plate is reduced to lead peroxide, and at the negative plate to metallic lead. This produces sulfuric acid and the electrolyte concentration increases.

The electromotive force and voltage of the battery depend on many factors, of which the most important are the acid content in the electrolyte, temperature, current and direction thereof, and the degree of charge. The relationship between electromotive force, voltage and current can be recorded

dignity as follows:

at discharge

where E0 is a reversible EMF; Ep - EMF of polarization; R is the internal resistance of the battery.

Reversible EMF is the EMF of an ideal battery in which all types of losses are eliminated. In such a battery, the energy received during charging is fully returned during discharge. Reversible EMF depends only on the acid content in the electrolyte and temperature. It can be determined analytically based on the heat of formation of the reacting substances.

A real battery is in conditions close to ideal if the current is negligible and the duration of its passage is also short. Such conditions can be created by balancing the battery voltage with some external voltage (voltage standard) using a sensitive potentiometer. The voltage measured in this way is called the open circuit voltage. It is close to reversible EMF. Table 27.1 shows the values \u200b\u200bof this voltage, corresponding to the density of the electrolyte from 1,100 to 1,300 (referred to a temperature of 15 ° C) and a temperature from 5 to 30 ° C.

As can be seen from the table, at an electrolyte density of 1.200, which is typical for stationary batteries, and a temperature of 25 ° C, the battery voltage with an open circuit is 2.046 V. During the discharge process, the electrolyte density slightly decreases. The corresponding open circuit voltage drop is only a few hundredths of a volt. The open circuit voltage change caused by the temperature change is negligible and is rather of theoretical interest.

If a certain current flows through the battery in the direction of charge or discharge, the battery voltage changes due to an internal voltage drop and a change in the EMF caused by side chemical and physical processes at the electrodes and in the electrolyte. The change in the electromotive force of a battery caused by the indicated irreversible processes is called polarization. The main reasons for polarization in the battery are the change in the concentration of electrolyte in the pores of the active mass of the plates in relation to its concentration in the rest of the volume and the change in the concentration of lead ions caused by this. When discharged, acid is consumed; when charged, it is formed. The reaction takes place in the pores of the active mass of the plates, and the influx or removal of molecules and acid ions occurs through diffusion. The latter can take place only in the presence of a certain difference in electrolyte concentrations in the area of \u200b\u200bthe electrodes and in the rest of the volume, which is set in accordance with the current and temperature, which determines the viscosity of the electrolyte. A change in the electrolyte concentration in the pores of the active mass causes a change in the concentration of lead ions and EMF. During discharge, due to a decrease in the electrolyte concentration in the pores, the EMF decreases, and during charging, due to an increase in the electrolyte concentration, the EMF increases.

The electromotive force of polarization is always directed towards the current. It depends on the porosity of the plates, current and

temperature. The sum of the reversible EMF and the EMF of polarization, i.e. E0 ± Ep, is the EMF of a battery under current or a dynamic EMF. During discharge, it is less than the reversible EMF, and during charging, it is more. The battery voltage under current differs from the dynamic EMF only by the value of the internal voltage drop, which is relatively small. Therefore, the voltage of the battery while energized also depends on the current and temperature. The influence of the latter on the battery voltage during discharge and charge is much greater than with an open circuit.

If the battery is opened during discharge, its voltage will slowly rise to open circuit voltage due to continued diffusion of the electrolyte. If you open the battery while charging, the voltage will slowly decrease to the open circuit voltage.

Inequality of electrolyte concentrations in the area of \u200b\u200bthe electrodes and in the rest of the volume distinguishes the operation of a real battery from an ideal one. When charged, the battery behaves as if it contains a very dilute electrolyte, and when charged, it is very concentrated. A dilute electrolyte is constantly mixed with a more concentrated one, while some energy is released in the form of heat, which, provided the concentrations are equal, could be used. As a result, the energy given off by the battery during discharge is less than the energy received during charging. Energy loss occurs due to imperfection of the chemical process. This type of loss is the main one in the accumulator.

Internal resistance of the battery. Internal resistance consists of the resistances of the plate frame, active mass, separators and electrolyte. The latter accounts for most of the internal resistance. The resistance of the battery increases with discharge and decreases with charge, which is a consequence of changes in the concentration of the solution and the content of sul-

veil in active mass. The resistance of the battery is low and is noticeable only at a high discharge current, when the internal voltage drop reaches one or two tenths of a volt.

Self-discharge of the battery. Self-discharge is a continuous loss of chemical energy stored in the battery due to side reactions on the plates of both polarities, caused by accidental harmful impurities in the materials used or impurities introduced into the electrolyte during operation. Self-discharge is of the greatest practical importance, caused by the presence in the electrolyte of various metal compounds that are more electropositive than lead, for example copper, antimony, etc. Metals are precipitated on negative plates and form a multitude of short-circuited elements with lead plates. As a result of the reaction, lead sulfate and hydrogen are formed, which are released on the metal of the pollution. Self-discharge can be detected by slight gas evolution from the negative plates.

On positive plates, self-discharge also occurs due to the usual reaction between the base lead, lead peroxide and electrolyte, which results in the formation of lead sulfate.

Self-discharge of the battery always occurs: both with an open circuit and with discharge and charge. It depends on the temperature and density of the electrolyte (Fig. 27.2), and with an increase in the temperature and density of the electrolyte, self-discharge increases (loss of charge at a temperature of 25 ° C and an electrolyte density of 1.28 is taken as 100%). The loss of capacity of a new battery due to self-discharge is about 0.3% per day. Self-discharge increases with age.

Abnormal sulfation of the plates. Lead sulfate forms on the plates of both polarities with each discharge, as seen from the discharge reaction equation. This sulfate has

fine crystal structure and charging current is easily reduced to metallic lead and lead peroxide on plates of appropriate polarity. Therefore, sulfation in this sense is a normal phenomenon that forms an integral part of the battery's performance. Abnormal sulfation occurs when batteries are over-discharged, systematically undercharged, or left in a discharged state and inactivity for long periods of time, or when operating at an excessively high electrolyte density and high temperatures. Under these conditions, the fine crystalline sulfate becomes denser, the crystals grow, greatly expanding the active mass, and it is difficult to recover during charging due to the high resistance. When the battery is inactive, temperature fluctuations contribute to sulfate formation. As the temperature rises, the small crystals of sulfate dissolve, and as the temperature decreases, the sulfate slowly crystallizes out and the crystals grow. As a result of temperature fluctuations, large crystals are formed at the expense of small ones.

In sulfated plates, the pores are clogged with sulfate, the active material is squeezed out of the gratings, and the plates are often warped. The surface of sulfated plates becomes hard, rough, and when rubbed

the material of the plates between the fingers feels like sand. The dark brown positive plates become lighter and white sulphate spots appear on the surface. The negative plates become hard, yellowish-gray. The capacity of the sulfated battery decreases.

The incipient sulfation can be eliminated by a prolonged charge with a bark current. With strong sulfation, special measures are required to bring the plates to their normal state.

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Car battery parameters | All about batteries

Let's look at the main parameters of the battery that we need when operating it.

1. The electromotive force (EMF) of the battery is the voltage between the terminals of the battery when the external circuit is open (and, of course, in the absence of any leaks). In "field" conditions (in a garage) EMF can be measured with any tester, before that, remove one of the terminals ("+" or "-") from the battery.

The EMF of a battery depends on the density and temperature of the electrolyte and does not at all depend on the size and shape of the electrodes, as well as on the amount of electrolyte and active masses. The change in the EMF of the battery from temperature is very small and during operation it can be neglected. With an increase in the density of the electrolyte, the EMF increases. At a temperature of plus 18 ° C and a density of d \u003d 1.28 g / cm3, the battery (meaning one bank) has an EMF equal to 2.12 V (battery - 6 x 2.12 V \u003d 12.72 V). The dependence of the EMF on the density of the electrolyte with a change in density in the range of 1.05 ÷ 1.3 g / cm3 is expressed by the empirical formula

E \u003d 0.84 + d, where

d is the density of the electrolyte at a temperature of plus 18 ° C, g / cm3.

It is impossible to accurately judge the degree of discharge of the battery by the EMF. The EMF of a discharged battery with a higher electrolyte density will be higher than the EMF of a charged battery, but with a lower electrolyte density.

By measuring the EMF, you can only quickly detect a serious malfunction of the battery (shorting of plates in one or several banks, breakage of connecting conductors between banks, etc.).

2. The internal resistance of the battery is the sum of the resistances of the output clamps, inter-element connections, plates, electrolyte, separators and the resistance arising at the points of contact of the electrodes with the electrolyte. The larger the battery capacity (number of plates), the lower its internal resistance. With a decrease in temperature and as the battery discharges, its internal resistance increases. The battery voltage differs from its EMF by the amount of voltage drop across the internal resistance of the battery.

With a charge U3 \u003d E + I x RVN,

and at discharge UР \u003d Е - I х RВН, where

I is the current flowing through the battery, A;

RVN - battery internal resistance, Ohm;

E - battery EMF, V.

The voltage change on the storage battery during its charging and discharging is shown in Fig. 1.

Fig. 1. Changing the voltage of the storage battery during its charge and discharge.

1 - beginning of gas evolution, 2 - charge, 3 - discharge.

The voltage of the car generator, from which the battery is charged, is 14.0 ÷ 14.5 V. On the car, the battery, even in the best case, under completely favorable conditions, remains undercharged by 10 ÷ 20%. The fault is the work of the car generator.

The generator begins to deliver sufficient voltage for charging at 2000 rpm or more. Idling speed 800 ÷ 900 rpm. Driving style in the city: acceleration (less than a minute), braking, stop (traffic light, traffic jam - duration from 1 minute to ** hours). The charge goes only during acceleration and movement at rather high speeds. The rest of the time there is an intensive discharge of the battery (headlights, other consumers of electricity, alarm - around the clock).

The situation improves when driving outside the city, but not in a critical way. The duration of the trips is not so long (full battery charge - 12 ÷ 15 hours).

At point 1 - 14.5 V, gas evolution begins (electrolysis of water for oxygen and hydrogen), and water consumption increases. Another unpleasant effect during electrolysis is that the corrosion of the plates increases, so you should not allow a prolonged excess of the voltage of 14.5 V at the battery terminals.

The voltage of the car generator (14.0 ÷ 14.5 V) was chosen from compromise conditions - ensuring more or less normal battery charging with a decrease in gassing (water consumption decreases, fire hazard decreases, the rate of destruction of plates decreases).

From the above, we can conclude that the battery should be periodically, at least once a month, fully recharged with an external charger to reduce the sulfation of the plates and increase the service life.

The voltage of the storage battery when it is discharged by the starting current (IP \u003d 2 ÷ 5 C20) depends on the strength of the discharge current and the temperature of the electrolyte. Figure 2 shows the current-voltage characteristics of the 6ST-90 storage battery at various electrolyte temperatures. If the discharge current is constant (for example, IP \u003d 3 C20, line 1), the lower the battery temperature, the lower the battery temperature. To maintain a constant voltage during discharge (line 2), it is necessary to reduce the strength of the discharge current with decreasing battery temperature.

Fig. 2. Volt-ampere characteristics of the 6ST-90 battery at different electrolyte temperatures.

3. The capacity of the battery (C) is the amount of electricity that the battery gives up when discharging to the lowest allowable voltage. The capacity of the battery is expressed in Ampere-hours (Ah). The greater the strength of the discharge current, the lower the voltage to which the battery can discharge, for example, when determining the nominal capacity of the battery, the discharge is carried out with a current I \u003d 0.05С20 up to a voltage of 10.5 V, the electrolyte temperature should be in the range + (18 ÷ 27) ° С, and the discharge time is 20 hours. It is considered that the end of the battery life comes when its capacity is 40% of С20.

The capacity of the battery in starter modes is determined at a temperature of + 25 ° C and a discharge current of ЗС20. In this case, the discharge time to a voltage of 6 V (one volt per battery) must be at least 3 minutes.

When the battery is discharged with a current ЗС20 (electrolyte temperature -18 ° C), the battery voltage 30 s after the start of the discharge should be 8.4 V (9.0 V for maintenance-free batteries), and after 150 s not lower than 6 V. This current is sometimes called cold cranking current or starting current, it may differ from ЗС20 This current is indicated on the battery case next to its capacity.

If the discharge occurs at a constant current strength, then the capacity of the battery is determined by the formula

С \u003d I х t where,

I is the discharge current, A;

t - discharge time, h.

The capacity of a storage battery depends on its design, the number of plates, their thickness, the material of the separator, the porosity of the active material, the design of the plate lattice, and other factors. In operation, the battery capacity depends on the strength of the discharge current, temperature, discharge mode (intermittent or continuous), state of charge and deterioration of the battery. With an increase in the discharge current and the degree of discharge, as well as with a decrease in temperature, the capacity of the battery decreases. At low temperatures, the drop in the capacity of the storage battery with increasing discharge currents occurs especially intensively. At a temperature of -20 ° C, about 50% of the battery capacity remains at a temperature of + 20 ° C.

The most complete state of the battery is shown by its capacity. To determine the real capacity, it is enough to put a fully charged serviceable battery on discharge with a current of I \u003d 0.05 C20 (for example, for a battery with a capacity of 55 Ah, I \u003d 0.05 x 55 \u003d 2.75 A). The discharge should be continued until the battery voltage reaches 10.5 V. The discharge time should be at least 20 hours.

It is convenient to use car incandescent lamps as a load when determining the capacity. For example, to provide a discharge current of 2.75 A, at which the power consumption is P \u003d I x U \u003d 2.75 A x 12.6 V \u003d 34.65 W, it is enough to connect a 21 W lamp and a 15 W lamp in parallel. The operating voltage of incandescent lamps for our case should be 12 V. Of course, the accuracy of setting the current in this way is "plus or minus bast shoes", but for an approximate determination of the state of the battery, it is quite enough, as well as cheap and affordable.

When testing new batteries in this way, the discharge time may be less than 20 hours. This is due to the fact that they gain the nominal capacity after 3 ÷ 5 complete charge-discharge cycles.

The capacity of the battery can also be estimated using the load plug. The load plug consists of two contact legs, a handle, a switchable load resistance and a voltmeter. One of the possible options is shown in Fig. 3.

Fig. 3. Load fork option.

To test modern batteries with only output terminals available, 12 volt load plugs must be used. The load resistance is chosen so as to ensure the battery load with a current I \u003d ЗС20 (for example, with a battery capacity of 55 Ah, the load resistance must consume current I \u003d ЗС20 \u003d 3 x 55 \u003d 165 A). The load plug is connected parallel to the output contacts of a fully charged battery, the time is noted during which the output voltage drops from 12.6 V to 6 V. This time for a new, serviceable and fully charged battery should be at least three minutes at an electrolyte temperature of + 25 ° FROM.

4. Self-discharge of the battery. Self-discharge is called a decrease in the capacity of batteries with an open external circuit, that is, with inactivity. This phenomenon is caused by redox processes that spontaneously occur on both the negative and positive electrodes.

The negative electrode is particularly susceptible to self-discharge due to the spontaneous dissolution of lead (negative active mass) in a sulfuric acid solution.

Self-discharge of the negative electrode is accompanied by the evolution of hydrogen gas. The rate of spontaneous dissolution of lead increases significantly with an increase in the concentration of the electrolyte. An increase in the density of the electrolyte from 1.27 to 1.32 g / cm3 leads to an increase in the self-discharge rate of the negative electrode by 40%.

Self-discharge can also occur when the outside of the battery is dirty or flooded with electrolyte, water or other liquids, which create the possibility of discharge through the conductive film located between the battery poles or jumpers.

Self-discharge of batteries is highly dependent on the temperature of the electrolyte. The self-discharge decreases with decreasing temperature. At temperatures below 0 ° C, it practically stops with new batteries. Therefore, storage of batteries is recommended in a charged state at low temperatures (down to -30 ° C). All this is shown in Fig. 4.

Fig. 4. Dependence of battery self-discharge on temperature.

During operation, the self-discharge does not remain constant and sharply increases towards the end of the service life.

To reduce self-discharge, it is necessary to use the purest materials for the production of batteries, use only pure sulfuric acid and distilled water for electrolyte preparation, both during production and during operation.

Typically, self-discharge is expressed as a percentage of capacity loss over a specified period of time. Self-discharge of batteries is considered normal if it does not exceed 1% per day, or 30% of the battery capacity per month.

5. The shelf life of new batteries. Currently, car batteries are produced by the manufacturer only in a dry-charged state. The shelf life of batteries without exploitation is very limited and does not exceed 2 years (warranty storage period is 1 year).

6. The service life of automotive lead-acid storage batteries is at least 4 years, subject to the operating conditions established by the plant. In my practice, six batteries have served for four years, and one, the most durable, for eight years.

akkumulyator.reglinez.org

Battery electromotive force - EMF

electromotive, force, battery

Battery - Battery EMF - Electromotive force

The EMF of the battery not connected to the load is 2 Volts on average. It does not depend on the size of the battery and the size of its plates, but is determined by the difference between the active substances of the positive and negative plates. Within small limits, the emf can vary from external factors, of which the density of the electrolyte is of practical importance, i.e., the greater or lesser acid content in the solution.

The electromotive force of a discharged battery with a high density electrolyte will be greater than the emf of a charged battery with a weaker acid solution. Therefore, the degree of charge of a battery with an unknown initial density of the solution should not be judged on the basis of the instrument readings when measuring the emf without a connected load. Batteries have an internal resistance that does not remain constant, but changes during charging and discharging, depending on the chemical composition of the active substances. One of the most obvious factors in battery resistance is the electrolyte. Since the resistance of the electrolyte depends not only on its concentration, but also on the temperature, the resistance of the battery also depends on the temperature of the electrolyte. As the temperature rises, the resistance decreases. The presence of separators also increases the internal resistance of the elements. Another factor that increases the resistance of the elements is the resistance of the active material and the gratings. In addition, the state of charge affects the resistance of the battery. Lead sulphate, which forms during the discharge on both positive and negative plates, does not conduct electricity, and its presence significantly increases the resistance to the passage of electric current. Sulfate closes the pores of the plates when the latter are in a charged state, and thus prevents the free access of the electrolyte to the active material. Therefore, when the cell is charged, its resistance is less than in the discharged state.

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Electromotive Force - Battery - Great Encyclopedia of Oil and Gas, article, page 1

Electromotive force - battery

Page 1

The electromotive force of a battery consisting of two parallel groups of three batteries connected in series in each group is 4 5 V, the current in the circuit is 1 5 A, and the voltage is 4 2 V.

The electromotive force of the battery is 18 V.

The electromotive force of a battery consisting of three identical batteries connected in series is 4 2 V. The battery voltage when it is closed to an external resistance of 20 ohms is 4 V.

The electromotive force of a battery, consisting of three identical batteries connected in series, is equal to 4 2 V. The voltage of the battery when it is closed to an external resistance of 20 ohms is 4 V.

The electromotive force of a battery of three parallel-connected batteries is 1 5 V, the external resistance is 2 8 ohms, the current in the circuit is 0 5 A.

Ohm - m; U is the electromotive force of the battery, V; / - current strength, A; K - constant coefficient of the device.

Therefore, such a coating must necessarily reduce the electromotive force of the battery.

When connected in parallel (see Fig. 14), the electromotive force of the battery remains approximately equal to the electromotive force of one cell, but the capacity of the battery increases n times.

So, when n identical current sources are connected in series, the electromotive force of the resulting battery is n times greater than the electromotive force of a separate current source, however, in this case, not only the electromotive forces are added, but also the internal resistances of the current sources. Such inclusion is beneficial when the external resistance of the circuit is very high in comparison with the internal resistance.

The practical unit of electromotive force is called the volt and differs little from the electromotive force of Daniel's battery.

Note that the initial charge on the capacitor, and therefore the voltage across it, is created by the electromotive force of the battery. On the other hand, the initial deflection of the body is created by an external force. Thus, the force acting on a mechanical oscillatory system plays a role similar to the electromotive force acting on an electrical oscillatory system.

Note that the initial charge on the capacitor, and therefore the voltage across it, is created by the electromotive force of the battery. On the other hand, the initial deflection of the body is created by an externally applied force. Thus, the force acting on a mechanical oscillatory system plays a role similar to the electromotive force acting on an electrical oscillatory system.

Note that the initial charge of the capacitor and, therefore, the voltage across it is created by the electromotive force of the battery. On the other hand, the initial deflection of the body is created from the outside by an applied force. Thus, the force acting on a mechanical oscillatory system plays a role similar to the electromotive force acting on an electrical oscillatory system.

Pages: 1 2

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EMF formula

Here is the work of external forces, is the magnitude of the charge.

The voltage unit is V (volts).

EMF is a scalar quantity. The closed-loop EMF is the force for moving the same charge over the entire contour. In this case the current in the circuit and inside the current source will flow in opposite directions. The external work that creates the EMF must not be of electrical origin (Lorentz force, electromagnetic induction, centrifugal force, force arising in the course of chemical reactions). This work is needed to overcome the repulsive forces of current carriers inside the source.

If there is a current in the circuit, then the EMF is equal to the sum of the voltage drops in the entire circuit.

Examples of solving problems on the topic "Electromotive force"

Purpose of starter batteries
Theoretical foundations of the conversion of chemical energy into electrical energy
Battery discharge
Battery charge
Consumption of main current-forming reagents
Electromotive force
Internal resistance
Charge and discharge voltage
Battery capacity
Battery energy and power
Battery self-discharge

Purpose of starter batteries

The main function of the battery is to reliably start the engine. Another function is the energy buffer when the engine is running. Indeed, along with the traditional types of consumers, many additional service devices have appeared that improve driver comfort and traffic safety. The battery compensates for the energy deficit when driving in the urban cycle with frequent and long stops, when the generator cannot always provide the power output necessary to fully provide all connected consumers. The third working function is power supply when the engine is off. However, prolonged use of electrical appliances while parked with the engine off (or the engine idling) leads to deep discharge of the battery and a sharp decrease in its starting characteristics.

The battery is also designed for emergency power supply. In the event of a generator, rectifier, voltage regulator failure or a break in the generator belt, it must ensure the operation of all consumers necessary for safe movement to the nearest service station.

So, starter batteries must meet the following basic requirements:

Provide the discharge current necessary for the operation of the starter, that is, have a low internal resistance for minimal internal voltage losses inside the battery;

Provide the required number of attempts to start the engine with a set duration, that is, have the necessary reserve of energy for the starter discharge;

Have a sufficiently large power and energy with the smallest possible size and weight;

Have a reserve of energy to power consumers when the engine is off or in an emergency (reserve capacity);

Maintain the voltage required for the operation of the starter when the temperature drops within the specified limits (cold cranking current);

Maintain performance for a long time at elevated (up to 70 "C) ambient temperature;

Receive a charge to restore the capacity consumed for starting the engine and powering other consumers from the generator with the engine running (charge acceptance);

Does not require special training of users, maintenance during operation;

Have high mechanical strength in relation to operating conditions;

Maintain the specified performance characteristics for a long time during operation (service life);

Have a slight self-discharge;

Have a low cost.

Theoretical foundations of the conversion of chemical energy into electrical energy

A chemical current source is a device in which, due to the course of spatially separated redox chemical reactions, their free energy is converted into electrical energy. By the nature of the work, these sources are divided into two groups:

Primary chemical current sources or galvanic cells;

Secondary sources or electric accumulators.

Primary sources can only be used once, since the substances formed during their discharge cannot be converted into initial active materials. A fully discharged galvanic cell, as a rule, is not suitable for further work - it is an irreversible source of energy.

Secondary chemical current sources are reversible energy sources - after an arbitrarily deep discharge, their performance can be fully restored by charging. To do this, it is enough to pass an electric current through the secondary source in the direction opposite to that in which it flowed during the discharge. During the charging process, the substances formed during the discharge will turn into the original active materials. This is how the free energy of a chemical current source is repeatedly converted into electrical energy (battery discharge) and the reverse conversion of electrical energy into free energy of a chemical current source (battery charge).

The passage of current through electrochemical systems is associated with the ongoing chemical reactions (transformations). Therefore, there is a relationship between the amount of a substance that has entered into an electrochemical reaction and underwent transformations and the amount of electricity consumed or released during this, which was established by Michael Faraday.

According to the first Faraday's law, the mass of a substance that has entered into an electrode reaction or obtained as a result of its flow is proportional to the amount of electricity passed through the system.

According to the second Faraday's law, with an equal amount of electricity passed through the system, the masses of reacted substances are related to each other as their chemical equivalents.

In practice, a smaller amount of a substance undergoes electrochemical change than according to Faraday's laws - when the current passes, in addition to the main electrochemical reactions, parallel or secondary (side) reactions also occur, which change the mass of products. To take into account the influence of such reactions, the concept of current efficiency was introduced.

The current efficiency is that part of the amount of electricity passed through the system that falls on the share of the main considered electrochemical reaction

Battery discharge

The active substances of a charged lead-acid battery that take part in the current-forming process are:

On the positive electrode - lead dioxide (dark brown);

On the negative electrode - spongy lead (gray);

The electrolyte is an aqueous solution of sulfuric acid.

Some of the acid molecules in an aqueous solution are always dissociated into positively charged hydrogen ions and negatively charged sulfate ions.

Lead, which is the active mass of the negative electrode, partially dissolves in the electrolyte and oxidizes in solution to form positive ions. The excess electrons released in this case impart a negative charge to the electrode and begin to move along a closed section of the external circuit to the positive electrode.

Positively charged lead ions react with negatively charged sulfate ions to form lead sulfate, which has little solubility and therefore deposits on the surface of the negative electrode. In the process of discharging the battery, the active mass of the negative electrode is converted from spongy lead to lead sulfate with a change from gray to light gray.

Lead dioxide from the positive electrode dissolves in the electrolyte in a much smaller amount than the lead from the negative electrode. When interacting with water, it dissociates (decomposes in solution into charged particles - ions), forming tetravalent lead ions and hydroxyl ions.

Ions impart a positive potential to the electrode and, by attaching electrons that come through the external circuit from the negative electrode, are reduced to divalent lead ions

Ions interact with ions to form lead sulphate, which for the above reason is also deposited on the surface of the positive electrode, as was the case on the negative one. The active mass of the positive electrode during the discharge is converted from lead dioxide to lead sulfate with a change in its color from dark brown to light brown.

As a result of the battery discharge, the active materials of both the positive and negative electrodes are converted to lead sulfate. In this case, sulfuric acid is consumed for the formation of lead sulfate and water is formed from the released ions, which leads to a decrease in the density of the electrolyte during discharge.

Battery charge

In the electrolyte, both electrodes contain small amounts of lead sulfate and water ions. Under the influence of the voltage of the direct current source, in the circuit of which the charged battery is connected, a directed movement of electrons to the negative terminal of the battery is established in the external circuit.

Divalent lead ions at the negative electrode are neutralized (reduced) by the incoming two electrons, converting the active mass of the negative electrode into metallic spongy lead. The remaining free ions form sulfuric acid

At the positive electrode, under the action of a charging current, divalent lead ions give up two electrons, oxidizing into tetravalent ones. The latter, combining through intermediate reactions with two oxygen ions, form lead dioxide, which is released at the electrode. Ions and, just like at the negative electrode, form sulfuric acid, as a result of which the density of the electrolyte increases during charging.

When the processes of transformation of substances in the active masses of the positive and negative electrodes are over, the density of the electrolyte stops changing, which indicates the end of the battery charge. With further continuation of the charge, the so-called secondary process occurs - the electrolytic decomposition of water into oxygen and hydrogen. Standing out from the electrolyte in the form of gas bubbles, they create the effect of its intense boiling, which also serves as a sign of the end of the charging process.

Consumption of main current-forming reagents

To obtain a capacity of one ampere-hour when the battery is discharged, it is necessary that the following take part in the reaction:

4.463 g lead dioxide

3.886 g spongy lead

3.660 g sulfuric acid

The total theoretical consumption of materials for obtaining 1 Ah (specific consumption of materials) of electricity will be 11.989 g / Ah, and the theoretical specific capacity - 83.41 Ah / kg.

At a nominal battery voltage of 2 V, the theoretical specific material consumption per unit of energy is 5.995 g / Wh, and the specific energy of the battery will be 166.82 Wh / kg.

However, in practice, it is impossible to achieve full use of active materials participating in the current-forming process. Approximately half of the surface of the active mass is inaccessible to the electrolyte, since it serves as the basis for the construction of a volumetric porous framework that provides the mechanical strength of the material. Therefore, the actual utilization rate of the active masses of the positive electrode is 45-55%, and the negative 50-65%. In addition, a 35-38% sulfuric acid solution is used as an electrolyte. Therefore, the value of the real specific consumption of materials is much higher, and the real values \u200b\u200bof the specific capacity and specific energy are much lower than the theoretical ones.

Electromotive force

The electromotive force (EMF) of the battery E is the difference between its electrode potentials, measured with an open external circuit.

EMF of a battery consisting of n series-connected batteries.

It is necessary to distinguish between the equilibrium EMF of the battery and the non-equilibrium EMF of the battery during the time from opening the circuit to the establishment of an equilibrium state (the period of the transient process).

EMF is measured with a high-resistance voltmeter (internal resistance of at least 300 Ohm / V). To do this, a voltmeter is connected to the terminals of the battery or battery. Thus through the battery (battery) must not flow charging or discharging current.

The change in the EMF of the battery from temperature is very small and can be neglected during operation.

Internal resistance

The resistance provided by the battery to the current flowing inside it (charging or discharging) is commonly called the internal resistance of the battery.

The resistance of the active materials of the positive and negative electrodes, as well as the resistance of the electrolyte, change depending on the state of charge of the battery. In addition, the resistance of the electrolyte is highly temperature dependent.

Therefore, ohmic resistance also depends on the state of charge of the battery and the temperature of the electrolyte.

The polarization resistance depends on the strength of the discharge (charging) current and temperature and does not obey Ohm's law.

The internal resistance of one battery and even a battery consisting of several batteries connected in series is negligible and in a charged state is only a few thousandths of an Ohm. However, it changes significantly during the discharge.

The electrical conductivity of active masses decreases for a positive electrode by about 20 times, and for a negative one - 10 times. The electrical conductivity of the electrolyte also changes depending on its density. With an increase in the density of the electrolyte from 1.00 to 1.70 g / cm3, its electrical conductivity first increases to its maximum value, and then decreases again.

As the battery is discharged, the density of the electrolyte decreases from 1.28 g / cm3 to 1.09 g / cm3, which leads to a decrease in its electrical conductivity by almost 2.5 times. As a result, the ohmic resistance of the battery increases with the discharge. In the discharged state, the resistance reaches a value more than 2 times higher than its value in the charged state.

In addition to the state of charge, temperature has a significant effect on the resistance of batteries. With a decrease in temperature, the resistivity of the electrolyte increases and at a temperature of -40 ° C it becomes about 8 times higher than at +30 ° C. The resistance of the separators also sharply increases with decreasing temperature and in the same temperature range increases by almost 4 times. This is the determining factor in increasing the internal resistance of batteries at low temperatures.

Charge and discharge voltage

The potential difference at the pole terminals of the battery (battery) in the process of charging or discharging in the presence of current in the external circuit is called the voltage of the battery (battery). The presence of the internal resistance of the battery leads to the fact that its voltage during discharge is always less than EMF, and during charging it is always higher than EMF.

When charging the battery, the voltage at its terminals must be greater than its EMF by the amount of internal losses.

At the beginning of the charge, there is a voltage jump by the amount of ohmic losses inside the battery, and then a sharp increase in voltage due to the polarization potential, caused mainly by a rapid increase in the density of the electrolyte in the pores of the active mass. Further, a slow increase in voltage occurs, mainly due to an increase in the EMF of the battery due to an increase in the density of the electrolyte.

After the main amount of lead sulphate is converted to PbO2 and Pb, energy expenditure increasingly causes the decomposition of water (electrolysis). The excess amount of hydrogen and oxygen ions appearing in the electrolyte further increases the potential difference between opposite electrodes. This leads to a rapid increase in the charging voltage, which accelerates the decomposition of water. The resulting hydrogen and oxygen ions do not interact with active materials. They recombine into neutral molecules and are released from the electrolyte in the form of gas bubbles (oxygen is released on the positive electrode, hydrogen on the negative one), causing the electrolyte to "boil".

If you continue the charging process, you can see that the increase in the density of the electrolyte and the charging voltage practically stops, since almost all of the lead sulfate has already reacted, and all the energy supplied to the battery is now spent only on the side process - the electrolytic decomposition of water. This explains the constancy of the charging voltage, which is one of the signs of the end of the charging process.

When the battery is discharged, the voltage at its terminals is less than the EMF by the amount of the internal voltage drop.

At the beginning of the discharge, the battery voltage drops sharply by the value of ohmic losses and polarization caused by a decrease in the concentration of electrolyte in the pores of the active mass, that is, concentration polarization. Further, with a steady (stationary) discharge process, the density of the electrolyte decreases in the volume of the battery, causing a gradual decrease in the discharge voltage. At the same time, there is a change in the ratio of the content of lead sulfate in the active mass, which also causes an increase in ohmic losses. In this case, the particles of lead sulfate (which has about three times the volume in comparison with the particles of lead and its dioxide, from which they were formed) close the pores of the active mass, which prevents the passage of the electrolyte into the depth of the electrodes.

This causes an increase in concentration polarization, leading to a more rapid decrease in the discharge voltage.

When the discharge stops, the voltage at the battery terminals quickly rises by the value of ohmic losses, reaching the value of the nonequilibrium EMF. A further change in the EMF due to the equalization of the electrolyte concentration in the pores of the active masses and in the volume of the battery leads to a gradual establishment of the value of the equilibrium EMF.

The battery voltage during its discharge is determined mainly by the temperature of the electrolyte and the strength of the discharge current. As mentioned above, the resistance of a lead-acid battery (battery) is negligible and in a charged state is only a few milliohms. However, at currents of a starter discharge, the strength of which is 4-7 times higher than the value of the nominal capacity, the internal voltage drop has a significant effect on the discharge voltage. An increase in ohmic losses with decreasing temperature is associated with an increase in the resistance of the electrolyte. In addition, the viscosity of the electrolyte sharply increases, which complicates the process of diffusion into the pores of the active mass and increases the concentration polarization (that is, increases the voltage loss inside the battery by reducing the concentration of the electrolyte in the pores of the electrodes).

At a current of more than 60 A, the dependence of the discharge voltage on the current strength is practically linear at all temperatures.

The average value of the battery voltage during charging and discharging is determined as the arithmetic average of the voltage values \u200b\u200bmeasured at regular intervals.

Battery capacity

Battery capacity is the amount of electricity drawn from the battery when it is discharged to its set end voltage. In practical calculations, battery capacity is usually expressed in ampere-hours (Ah). The discharge capacity can be calculated by multiplying the discharge current by the discharge duration.

The discharge capacity for which the battery is designed and which is indicated by the manufacturer is called the nominal capacity.

In addition to it, an important indicator is also the capacity imparted to the battery when charging.

The discharge capacity depends on a number of design and technological parameters of the battery, as well as its operating conditions. The most important design parameters are the amount of active mass and electrolyte, thickness and geometric dimensions of battery electrodes. The main technological parameters affecting the battery capacity are the formulation of active materials and their porosity. Operating parameters - electrolyte temperature and discharge current - also have a significant impact on discharge capacity. The generalized indicator characterizing the efficiency of the battery is the utilization rate of active materials.

To obtain a capacity of 1 Ah, as indicated above, theoretically 4.463 g of lead dioxide, 3.886 g of spongy lead and 3.66 g of sulfuric acid are needed. The theoretical specific consumption of the active masses of the electrodes is 8.32 g / Ah. In real batteries, the specific consumption of active materials at a 20-hour discharge mode and an electrolyte temperature of 25 ° C is from 15.0 to 18.5 g / Ah, which corresponds to a utilization rate of active masses of 45-55%. Consequently, the practical consumption of the active mass exceeds the theoretical values \u200b\u200bby 2 or more times.

The following main factors influence the degree of use of the active mass and, consequently, the value of the discharge capacity.

Porosity of the active mass. With an increase in porosity, the conditions for diffusion of the electrolyte into the depth of the active mass of the electrode are improved and the true surface on which the current-forming reaction occurs increases. With an increase in porosity, the discharge capacity increases. The amount of porosity depends on the size of the lead powder particles and the formulation for the preparation of active masses, as well as on the additives used. Moreover, an increase in porosity leads to a decrease in durability due to an acceleration of the process of destruction of highly porous active masses. Therefore, the value of porosity is chosen by manufacturers taking into account not only high capacitive characteristics, but also ensuring the required durability of the battery in operation. Currently, the optimal porosity is considered to be in the range of 46-60%, depending on the purpose of the battery.

The thickness of the electrodes. With a decrease in the thickness, the unevenness of the loading of the outer and inner layers of the active mass of the electrode decreases, which contributes to an increase in the discharge capacity. For thicker electrodes, the inner layers of the active mass are used very little, especially when discharging with high currents. Therefore, with an increase in the discharge current, the differences in the capacity of batteries with electrodes of different thicknesses sharply decrease.

Porosity and rationality of the separator material design. With an increase in the porosity of the separator and the height of its ribs, the supply of electrolyte in the interelectrode gap increases and the conditions for its diffusion improve.

The density of the electrolyte. Affects the capacity of the battery and its lifespan. With an increase in the density of the electrolyte, the capacitance of the positive electrodes increases, and the capacitance of the negative ones, especially at negative temperatures, decreases due to the acceleration of the passivation of the electrode surface. Increased density also negatively affects battery life by accelerating corrosive processes on the positive electrode. Therefore, the optimal electrolyte density is set based on the combination of requirements and conditions in which the battery is operated. So, for example, for starter batteries operating in a temperate climate, the recommended working density of the electrolyte is 1.26-1.28 g / cm3, and for areas with a hot (tropical) climate, 1.22-1.24 g / cm3.

The strength of the discharge current with which the battery should be continuously discharged for a given time (characterizes the discharge mode). The discharge modes are conventionally divided into long and short. In long-term modes, the discharge occurs with low currents for several hours. For example, 5-, 10-, and 20-hour digits. With short or starter discharges, the current strength is several times the nominal capacity of the battery, and the discharge lasts for several minutes or seconds. With an increase in the discharge current, the discharge rate of the surface layers of the active mass increases to a greater extent than the deep ones. As a result, the growth of lead sulfate in the mouths of the pores occurs faster than in depth, and the pore becomes clogged with sulfate before its inner surface has time to react. Due to the termination of the diffusion of the electrolyte into the pore, the reaction in it stops. Thus, the higher the discharge current, the lower the battery capacity, and, consequently, the utilization factor of the active mass.

To assess the starting qualities of batteries, their capacity is also characterized by the number of intermittent starter discharges (for example, 10-15 s duration with 60 s intervals between them). The capacity that the battery gives up during intermittent discharges exceeds the capacity for continuous discharge with the same current, especially in the starter discharge mode.

Currently, in the international practice of evaluating the capacitive characteristics of starter batteries, the concept of "reserve" capacity is used. It characterizes the battery discharge time (in minutes) at a discharge current of 25 A, regardless of the nominal battery capacity. At the discretion of the manufacturer, it is allowed to set the value of the nominal capacity for a 20-hour discharge mode in ampere-hours or according to the reserve capacity in minutes.

Electrolyte temperature. With its decrease, the discharge capacity of the batteries decreases. The reason for this is an increase in the viscosity of the electrolyte and its electrical resistance, which slows down the rate of diffusion of the electrolyte into the pores of the active mass. In addition, with decreasing temperature, the processes of passivation of the negative electrode are accelerated.

The temperature coefficient of the capacitance a shows the change in capacitance in percent with a temperature change of 1 ° C.

During the tests, the discharge capacity obtained during the long-term discharge mode is compared with the value of the nominal capacity determined at an electrolyte temperature of +25 ° C.

The electrolyte temperature when determining the capacity in a long-term discharge mode in accordance with the requirements of the standards should be in the range from +18 ° С to +27 ° С.

The parameters of the starter discharge are estimated by the duration of the discharge in minutes and the voltage at the beginning of the discharge. These parameters are determined on the first cycle at +25 ° C (check for dry-charged batteries) and on subsequent cycles at temperatures of -18 ° C or -30 ° C.

The degree of charge. With increasing state of charge, all other things being equal, the capacity increases and reaches its maximum value when the batteries are fully charged. This is due to the fact that with an incomplete charge, the amount of active materials on both electrodes, as well as the density of the electrolyte, do not reach their maximum values.

Battery energy and power

The battery energy W is expressed in Watt-hours and is determined by the product of its discharge (charging) capacity by the average discharge (charging) voltage.

Since the capacity of the battery and its discharge voltage change with a change in the temperature and discharge mode, with a decrease in temperature and an increase in the discharge current, the battery energy decreases even more significantly than its capacity.

When comparing chemical current sources that differ in capacity, design and even in the electrochemical system, as well as when determining the directions of their improvement, the indicator of specific energy is used - energy per unit mass of the battery or its volume. For modern lead-acid starter maintenance-free batteries, the specific energy at a 20-hour discharge rate is 40-47 Wh / kg.

The amount of energy given off by the battery per unit of time is called its power. It can be defined as the product of the discharge current and the average discharge voltage.

Battery self-discharge

Self-discharge is called a decrease in the capacity of batteries with an open external circuit, that is, with inactivity. This phenomenon is caused by redox processes that spontaneously occur on both the negative and positive electrodes.

The negative electrode is particularly susceptible to self-discharge due to the spontaneous dissolution of lead (negative active mass) in a sulfuric acid solution.

The presence of impurities of various metals on the surface of the negative electrode has a very significant (catalytic) effect on an increase in the rate of self-dissolution of lead (due to a decrease in the overvoltage of hydrogen evolution). Almost all metals found in the form of impurities in battery raw materials, electrolyte and separators, or introduced in the form of special additives, contribute to an increase in self-discharge. Once on the surface of the negative electrode, they facilitate the conditions for the evolution of hydrogen.

Some of the impurities (salts of metals with variable valence) act as carriers of charges from one electrode to another. In this case, metal ions are reduced on the negative electrode and oxidized on the positive one (such a self-discharge mechanism is attributed to iron ions).

Self-discharge of the positive active material is caused by the reaction.

2PbO2 + 2H2SO4 -\u003e PbSCU + 2H2O + О2 T.

The rate of this reaction also increases with increasing electrolyte concentration.

Since the reaction proceeds with the release of oxygen, its rate is largely determined by oxygen overvoltage. Therefore, additives that reduce the potential for oxygen evolution (for example, antimony, cobalt, silver) will increase the rate of the lead dioxide self-dissolution reaction. The self-discharge rate of the positive active material is several times lower than the self-discharge rate of the negative active material.

Another reason for the self-discharge of the positive electrode is the potential difference between the material of the current collector and the active mass of this electrode. The galvanic microelement resulting from this potential difference converts the lead of the current collector and lead dioxide of the positive active mass into lead sulfate when current flows.

Self-discharge can also occur when the outside of the battery is dirty or flooded with electrolyte, water or other liquids, which create the possibility of discharge through an electrically conductive film located between the pole terminals of the battery or its jumpers. This type of self-discharge does not differ from the usual discharge with very small currents with a closed external circuit and is easily eliminated. To do this, keep the surface of the batteries clean.

Self-discharge of batteries is highly dependent on the temperature of the electrolyte. The self-discharge decreases with decreasing temperature. At temperatures below 0 ° C, it practically stops with new batteries. Therefore, we recommend storing batteries in a charged state at low temperatures (down to -30 ° C).

During operation, the self-discharge does not remain constant and sharply increases towards the end of the service life.

A decrease in self-discharge is possible due to an increase in the overvoltage of oxygen and hydrogen evolution on the battery electrodes.

For this, it is necessary, firstly, to use the purest possible materials for the production of batteries, to reduce the quantitative content of alloying elements in battery alloys, to use only

pure sulfuric acid and distilled (or close to it in purity with other purification methods) water for the preparation of all electrolytes, both during production and during operation. For example, due to the decrease in the content of antimony in the alloy of current taps from 5% to 2% and the use of distilled water for all technological electrolytes, the average daily self-discharge is reduced by 4 times. Replacing antimony with calcium can further reduce the self-discharge rate.

The addition of organic substances - self-discharge inhibitors - can also contribute to a decrease in self-discharge.

The use of a common cover and hidden inter-element connections significantly reduces the rate of self-discharge from leakage currents, since the likelihood of galvanic coupling between far-apart pole terminals is significantly reduced.

Sometimes self-discharge refers to the rapid loss of capacity due to a short circuit inside the battery. This phenomenon is explained by the direct discharge through the conductive bridges formed between the opposite electrodes.

Application of envelope separators in maintenance-free batteries

eliminates the possibility of short circuits between opposite electrodes during operation. However, this possibility remains due to possible equipment malfunctions during mass production. Typically, this defect is detected in the first months of operation and the battery must be replaced under warranty.

Typically, self-discharge is expressed as a percentage of capacity loss over a specified period of time.

Self-discharge is also characterized by the current standards by the voltage of the starter discharge at -18 ° C after testing: inactivity for 21 days at a temperature of +40 ° C.

Battery(element) - consists of positive and negative electrodes (lead plates) and separators separating these plates, installed in the body and immersed in an electrolyte (sulfuric acid solution). The accumulation of energy in the battery occurs during the course of a chemical oxidation reaction - reduction of electrodes.

Accumulator battery consists of 2 or more sections (batteries, cells) connected in series and / or in parallel to provide the required voltage and current.It is capable of accumulating, storing and giving off electricity, ensuring engine start-up, as well as powering electrical appliances when the engine is not running.

Lead Acid Battery - storage battery, in which the electrodes are made mainly of lead, and the electrolyte is a solution of sulfuric acid.

Active mass- this is a constituent part of the electrodes, which undergoes chemical changes when an electric current passes during charge-discharge.

Electrode - a conductive material capable of producing an electric current upon reaction with an electrolyte.

Positive electrode (anode) -an electrode (plate) whose active mass in a charged battery consists of lead dioxide (PbO2).

Negative electrode (cathode) -an electrode, the active mass of which in a charged battery consists of spongy lead.

Electrode gridserves to hold the active mass, as well as to supply and withdraw current to it.

Separator - material used to isolate electrodes from each other.

Pole leadsserve to supply the charging current and to return it under the total voltage of the battery.

Lead - (Pb) is a chemical element of the fourth group of the periodic system of D. I. Mendeleev, serial number 82, atomic weight 207.21, valency 2 and 4. Lead is a bluish-gray metal, its specific gravity, in solid form, is 11.3 g / cm 3 decreases during melting depending on temperature. The most ductile among metals, it rolls well to the thinnest sheet and is easily forged. Lead is easily machined and belongs to low-melting metals.

Lead (IV) oxide (lead dioxide) PbO 2 is a dark brown heavy powder with a subtle characteristic smell of ozone.

Antimonyis a silver-white metal with a strong luster, crystalline structure. In contrast to lead, it is a hard metal, but very brittle and easily crushed into pieces. Antimony is much lighter than lead, its specific gravity is 6.7 g / cm 3. Water and weak acids do not affect antimony. It dissolves slowly in strong hydrochloric and sulfuric acids.

Cell plugs cover the cell holes in the battery cover.

Central ventilation capserves to close the gas outlet in the battery cover.

Monoblockis a polypropylene battery case, divided by partitions into separate cells.

Distilled watertopped up to replace battery losses due to water decomposition or evaporation. Use only distilled water to top up batteries!

Electrolyte is a solution of sulfuric acid in distilled water, which fills the free volumes of the cells and penetrates into the pores of the active mass of electrodes and separators.

It is capable of conducting electric current between electrodes immersed in it. (For central Russia with a density of 1.27-1.28 g / cm3 at t \u003d + 20 ° С).

Sedentary electrolyte:To reduce the risk of electrolyte spilled from the battery, agents are used to reduce its fluidity. Substances can be added to the electrolyte that make it gel. Another way to reduce the mobility of the electrolyte is to use glass mats as separators.

Open battery - an accumulator with a plug with a hole through which distilled water is added and gaseous products are removed. The opening can be provided with a ventilation system.
Sealed battery - an accumulator that is normally closed, but has a device that allows gas to escape when the internal pressure exceeds a set value. Usually, additional filling of electrolyte into such a battery is impossible.
Dry-charged battery - a storage battery stored without electrolyte, the plates (electrodes) of which are in a dry charged state.

Tubular (armor) plate - a positive plate (electrode), which consists of a set of porous tubes filled with active mass.

Safety valve - part of the vent plug, which allows gas to escape in the event of excessive internal pressure, but does not allow air to enter the accumulator.

Ampere hour (Ah)is a measure of electrical energy equal to the product of current strength in amperes and time in hours (capacity).

Battery voltage - potential difference between the terminals of the battery during discharge.
Battery capacity - the amount of electrical energy given off by a fully charged battery when it is discharged before reaching the final voltage.

Internal resistance - resistance to current through the element, measured in Ohms. It consists of the resistance of the electrolyte, separators and plates. The main component is the electrolyte resistance, which changes with temperature and sulfuric acid concentration.

Electrolyte density - ethen the characteristic of a physical body, equal to the ratio of its mass to the occupied volume. It is measured, for example, in kg / l or g / cm3.

Battery life - period of useful battery life under specified conditions.
Gas release - gassing during electrolysis of the electrolyte.

Self-discharge - spontaneous loss of capacity by the battery at rest. The self-discharge rate depends on the material of the plates, chemical impurities in the electrolyte, its density, the purity of the battery and the duration of its operation.

Battery EMF(electromotive force) is the voltage across the pole terminals of a fully charged storage battery when the circuit is open, i.e., when there are no charge or discharge currents at all.

Cycle - one sequence of charge and discharge of the cell.

The formation of gases on the electrodes of the lead battery. It is especially abundant in the final phase of charging a lead-acid battery.

Gel batteries - these are sealed lead-acid batteries (not sealed, since a small release of gases does occur when the valves are opened), closed, completely maintenance-free (not refilled) with gel-like acid electrolyte (Dryfit and Gelled Electrolite-Gel technologies).

AGM technology (Absorbed Glass Mat) - fiberglass absorbent pads.

Energy return - the ratio of the amount of energy given up when the battery is discharged to the amount of energy required to charge to its original state under certain conditions. The energy efficiency for acid batteries under normal operating conditions is 65%, and for alkaline batteries 55 - 60%.
Specific energy - the energy given off by the battery during the discharge per unit of its volume V or mass m, i.e. W \u003d W / V or W \u003d W / m. The specific energy of acid batteries is 7-25, nickel-cadmium 11-27, nickel-iron 20-36, silver-zinc 120-130 W * h / kg.

Short circuit in batteries occurs when electrically connecting plates of different polarity.