If you close the external circuit of a charged battery, an electric current will appear. In this case, the following reactions take place:
at the negative plate
at the positive plate
Where e - the charge of an electron is
For every two molecules of acid consumed, four water molecules are formed, but at the same time two water molecules are consumed. Therefore, in the end, only two water molecules are formed. Adding equations (27.1) and (27.2), we obtain the final discharge reaction:
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 the positive and negative plates, while the positive plates consume more acid than the negative ones. At the positive plates, two water molecules are formed. The electrolyte concentration decreases when the battery is discharged, while it decreases to a greater extent at 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 will begin. The charge reactions at the negative and positive plates can be represented by equations (27.1) and (27.2), and the total reaction can be represented by equation (27.3). These equations should now be read from right to left. When charging, lead sulfate at the positive plate is reduced to lead peroxide, at the negative plate - into metallic lead. In this case, sulfuric acid is formed and the concentration of the electrolyte 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 its direction, and the degree of charge. The relationship between electromotive force, voltage and current can be written
san as follows:
at discharge 
Where E 0 - reversible EMF; E p - EMF of polarization; R - 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 when discharging. The reversible EMF depends only on the acid content in the electrolyte and temperature. It can be determined analytically from the heat of formation of the reactants.
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 the reversible emf. In table. 27.1 shows the values of this voltage, corresponding to the density of the electrolyte from 1.100 to 1.300 (refer to a temperature of 15 ° C) and a temperature of 5 to 30 ° C.
As can be seen from the table, at an electrolyte density of 1.200, which is common for stationary batteries, and a temperature of 25 ° C, the battery voltage with an open circuit is 2.046 V. During the discharge, the density of the electrolyte decreases slightly. The corresponding voltage drop in an open circuit is only a few hundredths of a volt. The change in open circuit voltage caused by temperature change is negligible and is of more theoretical interest.
If a certain current passes through the battery in the direction of charge or discharge, the battery voltage changes due to an internal voltage drop and a change in EMF caused by side chemical and physical processes at the electrodes and in the electrolyte. The change in the EMF of the battery, caused by these irreversible processes, is called polarization. The main causes of polarization in the battery are the change in the electrolyte concentration in the pores of the active mass of the plates in relation to its concentration in the rest of the volume and the resulting change in the concentration of lead ions. 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 acid molecules and ions occurs through diffusion. The latter can take place only if there is a certain difference in electrolyte concentrations in the region of the 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 polarization EMF, i.e. E 0 ± E P , represents the EMF of the battery under current or dynamic EMF. When discharged, it is less than the reversible emf, and when charged, it is greater. 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 an energized battery also depends on 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 circuit is opened while discharging, the battery voltage will slowly increase to the open circuit voltage due to continued diffusion of the electrolyte. If you open the battery circuit while charging, the battery voltage will slowly decrease to the open circuit voltage.
The inequality of electrolyte concentrations in the area of the 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 contained a very dilute electrolyte, and when charged, it behaves as if it contains a very concentrated one. A dilute electrolyte is constantly mixed with a more concentrated one, while a certain amount of energy is released in the form of heat, which, provided that 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 the imperfection of the chemical process. This type of loss is the main one in the battery.
Battery internal resistanceTorah. The internal resistance is made up 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 during discharge and decreases during charging, which is a consequence of changes in the concentration of the solution and the content of sulphate.
veil in the active mass. The resistance of the battery is small and noticeable only at a large discharge current, when the internal voltage drop reaches one or two tenths of a volt.
Battery self-discharge. Self-discharge is the 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. Of greatest practical importance is self-discharge caused by the presence in the electrolyte of various metal compounds that are more electropositive than lead, such as copper, antimony, etc. Metals are released on negative plates and form many short-circuited elements with lead plates. As a result of the reaction, lead sulfate and hydrogen are formed, which is released on the contaminated metal. Self-discharge can be detected by slight outgassing at the negative plates.
On the positive plates, self-discharge also occurs due to the normal reaction between 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 (the loss of charge at a temperature of 25 ° C and an electrolyte density of 1.28 is taken as 100%). The capacity loss of a new battery due to self-discharge is about 0.3% per day. As the battery ages, self-discharge increases.
Abnormal plate sulfation. Lead sulfate is formed on plates of both polarities with each discharge, as can be seen from the discharge reaction equation. This sulfate has
fine crystalline structure and charging current is easily restored into lead metal and lead peroxide on plates of the appropriate polarity. Therefore, sulfation in this sense is a normal phenomenon that is an integral part of battery operation. Abnormal sulfation occurs when batteries are over-discharged, systematically undercharged, or left in a discharged state and inactive for long periods of time, or when they are operated with excessively high electrolyte density and at high temperatures. Under these conditions, fine crystalline sulfate becomes denser, crystals grow, greatly expanding the active mass, and are difficult to recover when charged due to high resistance. If the battery is inactive, temperature fluctuations contribute to the formation of sulfate. As the temperature rises, small sulfate crystals 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 grids, and the plates often warp. 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 spots of sulfate appear on the surface. Negative plates become hard, yellowish gray. The capacity of the sulfated battery is reduced.
Beginning sulfation can be eliminated by a long charge with a light current. With strong sulfation, special measures are necessary to bring the plates back to normal.
The active substances of the positive and negative plates have certain potentials relative to the electrolyte. The difference between these potentials determines the emf of the battery, which does not depend on the amount of active substance in the plates. The emf of the battery depends mainly on the density of the electrolyte, this dependence is determined by the empirical formula:
where d is the electrolyte density in the pores of the active mass of the plates. The battery voltage during charging is greater than the EMF value by the value of the internal voltage drop:
U З \u003d E + I З ∙ r 0,
where r 0 is the internal resistance of the battery, and when discharging, respectively:
U R \u003d E - I R ∙ r 0.
A discharged lead battery has a density of d = 1.17, then E = 0.85 + 1.17 = 2.02 V. A charged battery has d = 1.21, then E = 0.85 + 1.21 = 2, 06 V => EMF of a discharged battery with a disconnected load differs little from the EMF of a charged battery. When the battery is charging, its charge voltage is 2.3 - 2.8 V. The discharge voltage is approximately 1.8 V.
Lead battery capacity
The nominal capacity is determined with a ten-hour discharge to a voltage of 1.8 V, at an electrolyte temperature of 25°C. The nominal capacity of a lead battery is 36 Ah. This capacity corresponds to the discharge current I P \u003d Q / 10 \u003d 3.6 A.
If you change the discharge current I P and the temperature of the electrolyte, then its capacity will also change. An increase in ambient temperature contributes to an increase in capacity, but at a temperature of 40 ° C, the positive plates warp and the self-discharge of the battery increases sharply, therefore, for normal operation of the battery, a temperature of + 35 ° C - 15 ° C should be maintained.
The nominal capacity at a temperature of 25°C and a ten-hour discharge is determined by the formula:
where P t is the utilization factor of the battery active mass, %;
T is the actual temperature of the electrolyte during discharge.
Types of lead-acid batteries
Stationary batteries are marked with the letters C, SK, SZ, SZE, SN and others:
C - stationary battery;
K - a battery that allows a short-term discharge;
Z - battery in a closed version;
E - ebonite vessel;
H - battery with smeared plates.
The number that is placed after the letter indicates the number of the battery:
S-1 - 36 A / h;
S-4 - 4 x 36 A / h;
and others...
Types of alkaline batteries
Marking N-Zh (Nickel - Iron), N-K (Nickel - Cadmium), S - C (Silver - Zinc). The electromotive force (EMF) of N–L batteries is: E Z = 1.5 V; E R = 1.3 V. The EMF of H-K batteries is: E Z = 1.4 V; E P \u003d 1.27 V. The average charge voltage is U Z \u003d 1.8 V; discharge U P = 1 V.
POWER SYSTEMS
General provisions
Stationary automation and communication equipment in railway transport is powered from direct current sources with rated voltages, for example, 24, 60, 220 V, etc. Sources with a rated voltage of 24 V are used to power transistor equipment, signaling circuits, automation relay circuits, etc. .; sources with a rated voltage of 60 V - for automatic telephone exchanges, telegraph switching equipment; sources with a voltage of 220 V - for powering communication equipment, turnout motors, etc. Current sources having a certain rated voltage are usually made in the form of independent equipment that is part of the general complex of the power supply installation of a communication house, an EC post or another facility where centralized power supplies are located.
The main power supply systems include autonomous, buffer, batteryless and combined power systems (Fig. 2.1). The autonomous system is designed to power portable and stationary automation and communication equipment, and the rest - to power stationary equipment.
Rice. 2.1. Structural diagram of power supply systems
Autonomous power system
The power supply system from primary elements is mainly used to ensure the operation of portable equipment (radio stations, measuring equipment, etc.). To power stationary equipment, an autonomous power supply system is used in places where there are no AC mains. The battery power system according to the "charge-discharge" method (Fig. 2.2) is designed for cases where power from AC networks is supplied irregularly. The essence of this power supply method is that for each voltage gradation there is a separate rectifier and two (or more) batteries . The equipment is powered from one battery, and the other is charged from the rectifier or is in reserve charged. As soon as the battery is discharged to a certain state, it is disconnected and connected to a rectifier for charging, and a charged battery is connected to power the equipment. When working according to this method, batteries are most often charged in a constant current mode. The capacity of the batteries is determined based on the duration of the power supply of the equipment for 12-24 hours, so the batteries are very bulky and require specially equipped large rooms for their installation. The service life of such batteries is 6-7 years, since deep and frequent charge and discharge cycles lead to the rapid destruction of the plates. The need to constantly monitor the charging and discharging processes leads to high operating costs.
Fig.2.2. Scheme of the battery power system according to the "charge - discharge" method:
F - feeder; ShPT - AC bus; ЗШ - charging tires; RSh-discharge tires; 1, 2, 3 - battery groups
These shortcomings, along with the low efficiency of the installation (30-45%) limit the use of this mode. The advantages of the method include the absence of voltage ripple on the load and the possibility of using various current sources for charging.
Buffer power system
With such a power supply system in parallel with the rectifier USD and the load is connected to the battery GB(Fig. 2.3). In the event of an AC failure or rectifier failure, the battery continues to power the load without power interruption. The battery provides reliable backup of electrical energy sources, and, in addition, together with the power filter, it provides the necessary smoothing of the ripple. With a buffer power system, three modes of operation are distinguished: medium current, pulsed and continuous charging.
In medium current mode(fig. 2.4) rectifier US, connected in parallel with the battery GB, provides a constant current I in regardless of the change in current I n in the load R n. When the load current I n is small, the rectifier supplies the load and charges the battery with current I 3, and when the load current is high, the rectifier, together with the battery, which is discharged with current I p, supplies the load. During charging, the voltage on each battery of the battery increases and can reach 2.7 V, and during discharge it decreases to 2 V. To implement this mode, simple rectifiers without automatic adjustment devices can be used. The rectifier current is calculated based on the amount of electrical energy (ampere-hours) consumed to power the load during the day. This value should be increased by 15-25% to compensate for the losses that always exist when charging and discharging batteries.
The disadvantages of the mode include: the inability to accurately determine and set the required rectifier current, since the actual nature of the load current change is never exactly known, which leads to undercharging or overcharging of batteries; short battery life (8-9 years) caused by deep charge and discharge cycles; significant voltage fluctuations on the load, since the voltage on each battery can vary from 2 to 2.7 V.
In pulse charging mode(Fig. 2.5) the rectifier current changes abruptly depending on the voltage on the battery GB. At the same time, the rectifier USD provides power to the load R n together with the battery G IN or feed the load
Figure 2.3 - Scheme of the buffer power supply system
Figure 2.4 - Average current mode:
a - scheme; b – current diagram; c – dependences of currents and voltages on time; I Z and I R - respectively, the charge and discharge currents of the battery
Figure 2.5 - Pulse recharge mode:
a - scheme; b - diagram of currents and voltages; c, d – dependences of currents and voltages on time
and recharges the battery. The maximum rectifier current is set slightly higher than the current that occurs at the hour of the greatest load, and the minimum load current I V max is less than the minimum load current I n.
Let us assume that in the initial position the rectifier gives the minimum current. The battery pack is discharging and the battery voltage drops to 2.1 V per cell. Relay R releases the armature and shunts the resistor R with contacts . The current at the output of the rectifier increases stepwise to the maximum. From this point on, the rectifier powers the load and charges the battery. During the charging process, the voltage on the battery increases and reaches 2.3 V per cell. Relay trips again R, and the rectifier current drops to the minimum; the battery starts to drain. Then the cycles are repeated. The duration of the maximum and minimum rectifier current time intervals varies in accordance with the change in the current in the load.
The advantages of the mode include: simplicity of the system for regulating the current at the output of the rectifier; small limits of voltage change on the battery and on the load (from 2.1 to 2.3 V per cell); increase in battery life up to 10-12 years due to less deep charge and discharge cycles. This mode is used to power automation devices.
In continuous charge mode(fig. 2.6) load R n is powered completely by the rectifier US. Charged battery GB receives from the rectifier a small direct charging current, compensating for self-discharge. To implement this mode, it is necessary to set the voltage at the rectifier output at the rate of (2.2 ± 0.05) V for each battery and maintain it with an error of no more than ± 2%. At the same time, the charging current for acid batteries I p \u003d (0.001-0.002) C n and for alkaline batteries I p \u003d 0.01 C N. Therefore, for you-
Figure 2.6 - Continuous charge mode:
a - scheme; b – current diagram; c - dependence of currents and voltages on time
To complete this mode, rectifiers must have accurate and reliable voltage stabilization devices. Failure to do so will result in batteries being overcharged or deep discharged and sulphated.
The advantages of the mode include: rather high efficiency of the installation, determined only by the rectifier (η = 0.7÷0.8); long battery life, reaching 18-20 years due to the absence of charge and discharge cycles; high voltage stability at the output of the rectifier; lower operating costs due to the possibility of automation and simplification of battery maintenance.
Batteries are normally in a charged state and do not require continuous monitoring. The absence of charge and discharge cycles and a properly selected boost current reduce sulfation and allow you to increase the periods between recharges and control discharges.
The disadvantage of the mode is the need to complicate the supply devices due to the elements of stabilization and automation. The mode is used in devices for powering communication equipment.
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, the electrolyte density equalization in the cavity of the battery case and in the pores of the active mass of the plates, the electrode polarization can be maintained in the battery when the external circuit is turned off.
The diffusion rate directly depends 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 in transient conditions led to the division into equilibrium and non-equilibrium battery emf.
On the equilibrium battery emf 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 practically does not affect it. The dependence of EMF on density can be expressed by the formula:
Where E is the battery emf (V)
P - electrolyte density reduced to a temperature of 25 gr. C (g/cm3) This formula is valid for electrolyte operating density in the range of 1.05 - 1.30 g/cm3. EMF cannot characterize the degree of rarefaction of the battery directly. But if you measure it at the conclusions and compare it with the calculated density, then you can, with a certain degree of probability, 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 rest EMF. When connecting consumers or a charge source, the polarization of the plates and the electrolyte concentration in the pores of the electrodes change. This leads to a change in the EMF. When charging, the EMF value increases, and when discharged, it decreases. This is due to a change in the density of the electrolyte, which is involved in electrochemical processes.
Battery(element) - consists of positive and negative electrodes (lead plates) and separators separating these plates, installed in a housing and immersed in an electrolyte (solution of sulfuric acid). The accumulation of energy in the battery occurs during the course of a chemical reaction of oxidation - reduction of the electrodes.
Accumulator battery consists of 2 or more series or (and) parallel sections (batteries, cells) connected to each other to provide the required voltage and current.It is able to accumulate, store and distribute electricity, provide engine start, and also power electrical appliances when the engine is not running.
Lead Acid Battery- a battery in which the electrodes are made mainly of lead, and the electrolyte is a solution of sulfuric acid.
active mass- this is an integral part of the electrodes, which undergoes chemical changes during the passage of electric current during the charge-discharge.
Electrode A conductive material capable of producing an electric current when reacting 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 whose active mass in a charged battery consists of spongy lead.
Electrode grid serves to hold the active mass, as well as to supply and remove current to it.
Separator - material used to isolate electrodes from each other.
Pole terminals serve to supply the charging current and to return it under the total voltage of the battery.
Lead -(Pb) - 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 the temperature. The most ductile among metals, it rolls well to the thinnest sheet and is easily forged. Lead is easily machined and is one of the fusible metals.
Lead(IV) oxide(lead dioxide) PbO 2 is a dark brown heavy powder with a subtle characteristic smell of ozone.
Antimony is a silver-white metal with a strong luster, crystalline structure. In contrast to lead, it is a hard metal, but very brittle and easily broken 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 slowly dissolves in strong hydrochloric and sulfuric acids.
Cell plugs cover the cell openings in the battery cover.
Central ventilation plug serves to block the gas outlet in the battery cover.
Monoblock- this is a polypropylene battery case, divided by partitions into separate cells.
Distilled water added to the battery to compensate for its losses as a result of water decomposition or evaporation. Only distilled water should be used to top up batteries!
Electrolyte is a solution of sulfuric acid in distilled water, which fills the free volumes of 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=+20°C).
Slow-moving electrolyte: To reduce the danger from the electrolyte spilled out of the battery, agents are used that reduce its fluidity. Substances can be added to the electrolyte that turn it into a gel. Another way to reduce electrolyte mobility is to use glass mats as separators.
open battery- a battery with a plug with a hole through which distilled water is added and gaseous products are removed. The hole can be provided with a ventilation system.
closed accumulator- an accumulator that is closed under normal conditions, but has a device that allows gas to be released when the internal pressure exceeds a set value. Usually, additional filling of electrolyte into such a battery is not possible.
Dry charged battery- a rechargeable battery stored without electrolyte, the plates (electrodes) of which are in a dry charged state.
Tubular (shell) plate- 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 case of excessive internal pressure, but does not allow air to enter the accumulator.
Ampere hour (Ah)- this 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 until the final voltage is reached.
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 resistance of the electrolyte, which changes with temperature and sulfuric acid concentration.
Electrolyte density - e then 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- the useful life of the battery under given conditions.
Outgassing- gas formation in the process of electrolyte electrolysis.
self-discharge- spontaneous loss of battery capacity 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 at the pole terminals of a fully charged battery in an open circuit, that is, in the absence of charge or discharge currents.
Cycle- one sequence of charge and discharge of the element.
The formation of gases on the electrodes of a lead battery. It is especially abundantly released in the final phase of the charge of a lead battery.
Gel batteries- these are sealed lead-acid batteries (not sealed, because a small release of gases does occur when the valves are opened), closed, completely maintenance-free (not topped up) with a gel-like acid electrolyte (Dryfit and Gelled Electrolite-Gel technologies).
AGM Technology(Absorbed Glass Mat) - absorbent fiberglass pads.
Energy return- the ratio of the amount of energy given off when the battery is discharged to the amount of energy required to charge it to its original state under certain conditions. The energy return 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 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.
If you close the external circuit of a charged battery, an electric current will appear. In this case, the following reactions take place:
at the negative plate
at the positive plate
Where e - the charge of an electron is
For every two molecules of acid consumed, four water molecules are formed, but at the same time two water molecules are consumed. Therefore, in the end, only two water molecules are formed. Adding equations (27.1) and (27.2), we obtain the final discharge reaction:
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 the positive and negative plates, while the positive plates consume more acid than the negative ones. At the positive plates, two water molecules are formed. The electrolyte concentration decreases when the battery is discharged, while it decreases to a greater extent at 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 will begin. The charge reactions at the negative and positive plates can be represented by equations (27.1) and (27.2), and the total reaction can be represented by equation (27.3). These equations should now be read from right to left. When charging, lead sulfate at the positive plate is reduced to lead peroxide, at the negative plate - into metallic lead. In this case, sulfuric acid is formed and the concentration of the electrolyte 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 its direction, and the degree of charge. The relationship between electromotive force, voltage and current can be written
san as follows:
at discharge 
Where E 0 - reversible EMF; E p - EMF of polarization; R - 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 when discharging. The reversible EMF depends only on the acid content in the electrolyte and temperature. It can be determined analytically from the heat of formation of the reactants.
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 the reversible emf. In table. 27.1 shows the values of this voltage, corresponding to the density of the electrolyte from 1.100 to 1.300 (refer to a temperature of 15 ° C) and a temperature of 5 to 30 ° C.
As can be seen from the table, at an electrolyte density of 1.200, which is common for stationary batteries, and a temperature of 25 ° C, the battery voltage with an open circuit is 2.046 V. During the discharge, the density of the electrolyte decreases slightly. The corresponding voltage drop in an open circuit is only a few hundredths of a volt. The change in open circuit voltage caused by temperature change is negligible and is of more theoretical interest.
If a certain current passes through the battery in the direction of charge or discharge, the battery voltage changes due to an internal voltage drop and a change in EMF caused by side chemical and physical processes at the electrodes and in the electrolyte. The change in the EMF of the battery, caused by these irreversible processes, is called polarization. The main causes of polarization in the battery are the change in the electrolyte concentration in the pores of the active mass of the plates in relation to its concentration in the rest of the volume and the resulting change in the concentration of lead ions. 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 acid molecules and ions occurs through diffusion. The latter can take place only if there is a certain difference in electrolyte concentrations in the region of the 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 polarization EMF, i.e. E 0 ± E P , represents the EMF of the battery under current or dynamic EMF. When discharged, it is less than the reversible emf, and when charged, it is greater. 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 an energized battery also depends on 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 circuit is opened while discharging, the battery voltage will slowly increase to the open circuit voltage due to continued diffusion of the electrolyte. If you open the battery circuit while charging, the battery voltage will slowly decrease to the open circuit voltage.
The inequality of electrolyte concentrations in the area of the 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 contained a very dilute electrolyte, and when charged, it behaves as if it contains a very concentrated one. A dilute electrolyte is constantly mixed with a more concentrated one, while a certain amount of energy is released in the form of heat, which, provided that 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 the imperfection of the chemical process. This type of loss is the main one in the battery.
Battery internal resistanceTorah. The internal resistance is made up 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 during discharge and decreases during charging, which is a consequence of changes in the concentration of the solution and the content of sulphate.
veil in the active mass. The resistance of the battery is small and noticeable only at a large discharge current, when the internal voltage drop reaches one or two tenths of a volt.
Battery self-discharge. Self-discharge is the 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. Of greatest practical importance is self-discharge caused by the presence in the electrolyte of various metal compounds that are more electropositive than lead, such as copper, antimony, etc. Metals are released on negative plates and form many short-circuited elements with lead plates. As a result of the reaction, lead sulfate and hydrogen are formed, which is released on the contaminated metal. Self-discharge can be detected by slight outgassing at the negative plates.
On the positive plates, self-discharge also occurs due to the normal reaction between 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 (the loss of charge at a temperature of 25 ° C and an electrolyte density of 1.28 is taken as 100%). The capacity loss of a new battery due to self-discharge is about 0.3% per day. As the battery ages, self-discharge increases.
Abnormal plate sulfation. Lead sulfate is formed on plates of both polarities with each discharge, as can be seen from the discharge reaction equation. This sulfate has
fine crystalline structure and charging current is easily restored into lead metal and lead peroxide on plates of the appropriate polarity. Therefore, sulfation in this sense is a normal phenomenon that is an integral part of battery operation. Abnormal sulfation occurs when batteries are over-discharged, systematically undercharged, or left in a discharged state and inactive for long periods of time, or when they are operated with excessively high electrolyte density and at high temperatures. Under these conditions, fine crystalline sulfate becomes denser, crystals grow, greatly expanding the active mass, and are difficult to recover when charged due to high resistance. If the battery is inactive, temperature fluctuations contribute to the formation of sulfate. As the temperature rises, small sulfate crystals 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 grids, and the plates often warp. 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 spots of sulfate appear on the surface. Negative plates become hard, yellowish gray. The capacity of the sulfated battery is reduced.
Beginning sulfation can be eliminated by a long charge with a light current. With strong sulfation, special measures are necessary to bring the plates back to normal.
