BATTERY TECHNOLOGY HANDBOOK Second Edition edited by H. A. KIEHNE Technical Consultant Breckerfeld, Germany MARCEL MARCEL DEKKER, INC. ⃝ Extensive information on battery technology ⃝ Preview your personal ' Download bag' of the files papers with detailed insights into battery technology. Download Citation on ResearchGate | Battery Technology Handbook | This book discusses batteries in various applications as rechargeable secondary.
|Language:||English, Portuguese, Hindi|
|ePub File Size:||30.68 MB|
|PDF File Size:||12.42 MB|
|Distribution:||Free* [*Registration needed]|
This practical reference remains the most comprehensive guide to the fundamental theories, techniques, and strategies used for battery operation and design. but batteries are the best choice for most applications. .. Pb–acid batteries are a relatively old technology that maintain 40–45% of the in bq20zxx product family, Texas Instruments Inc., raudone.info [ 84] Ehrlich, G.M. () Lithium ion batteries, in Handbook of Batteries (eds D. For today, we'll focus on batteries for portable energy storage. •Drag feet on carpet. •Pet a cat .. Handbook of Batteries 3e, Eds Linden and Reddy. Rate effects.
It could be through conference attendance, group discussion or directed reading to name just a few examples.
We provide a free online form to document your learning and a certificate for your records. Already read this title? Stay on CRCPress. Exclusive web offer for individuals on print book only. Preview this Book. Battery Technology Handbook 2nd Edition H. Select Format: Add to Wish List. Close Preview. Toggle navigation Additional Book Information. Summary This practical reference remains the most comprehensive guide to the fundamental theories, techniques, and strategies used for battery operation and design.
It includes new and revised chapters focusing on the safety, performance, quality, and enhancement of various batteries and battery systems. From automotive, electrochemical, and high-energy applications to system implementation, selection, and standardization, the Second Edition presents expert discussions on electrochemical energy storage, the advantages of battery-powered traction, the disposal and recycling of used batteries, hazard prevention, and the chemistry and physics of lithium primary batteries.
Share this Title. Recommend to Librarian. My thanks also to Expert Verlag, the original publisher. Berndt 1. Kiehne 2. Preuss 3. Kiehne 4. Willmes 6. Franke 7. Stahl 8. Sassmannhausen and E. Nann 9. Dustmann The Solar Generator General Requirements and Selection of Chargers E. Wehrle Will Kiehne Kiehne, D. Spahrbier, D. Sprengel, and W.
Raudzsus Kiehne and W. Tuphorn Jacobi Energy Density Knudsen Contributors Dr. Berndt Kronberg, Germany Dr. Franke Ennepetal, Germany Dr. Kiehne Breckerfeld, Germany N. Nann Brilon, Germany Dr.
Sassmannhausen Brilon, Germany Dr. Spahrbier Kelkheim, Germany Dr. Stahl Berlin, Germany Dipl. Wehrle Eschbach Germany Dipl. Will Erlangen, Germany Dipl. It can universally be applied and easily be converted into light, heat or mechanical energy. A general problem, however, is that electrical energy can hardly be stored. Capacitors allow its direct storage, but the quantities are small, compared to the demand of most applications.
In general, the storage of electrical energy requires its conversion into another form of energy. In batteries the energy of chemical compounds acts as storage medium, and during discharge, a chemical process occurs that generates energy which can be drawn from the battery in form of an electric current at a certain voltage.
Linden’s Handbook of Batteries, Fourth Edition
For a number of battery systems this process can be reversed and the battery recharged, i. As a consequence, two different battery systems exist: Primary batteries that are designed to convert their chemical energy into electrical energy only once. Secondary batteries that are reversible energy converters and designed for repeated discharges and charges.
They are genuine electrochemical storage systems. There is no clear border between them, and some primary battery systems permit charging under certain conditions. Usually, however, their rechargeability is limited. Rechargeable batteries usually are the choice in such applications, since primary batteries would be too expensive for the required rather high capacity. The second part Chapters 15 to 19 regards batteries mainly in portable applications and concerns smaller capacities.
When the battery is discharged, chemical compounds of higher energy content are converted by this reaction into compounds of lower energy content.
Usually the released energy would be observed as heat. Thus the generation or consumption of energy that is connected to the cell reaction is directly converted into an electric current. This is achieved in the electrochemical cell, sketched in Fig. A positive and a negative electrode are immersed in the electrolyte and the reacting substances the active material usually are stored within the electrodes, sometimes also in the electrolyte, if it participates in the overall reaction.
During discharge, as shown in Fig. This direct conversion of the current into chemical energy characterizes batteries and fuel cells.
Figure 1. S N red and S P ox are the components of the negative and the positive electrode respectively. Fuel cells are also based on an electrochemical cell as shown in Fig. Therefore, fuel cells cannot directly be compared with batteries.
The arrangement shown in Fig. Chemical reactions do not occur and the physical structure of the electrodes is not affected. But the amount of stored energy per weight or volume is comparatively small. In batteries such a double layer also exists, and the large surface area of the active material gives rise to a high double layer capacitance when impedance measurements are made.
The real battery capacity, however, is much higher and based on chemical reactions. In the following, a brief survey is given of the most important rules. For details and derivations, the reader is referred to textbooks of electrochemistry or fundamental books on batteries e. Thermodynamic or equilibrium parameters describe the system in equilibrium, when all reactions are balanced. This means that these parameters represent maximum values that only can be reached under equilibrium.
Kinetic parameters appear when the reaction occurs. Kinetic parameters include mass transport by migration or diffusion that is required to bring the reacting substances to the surface of the electrode.
The thermodynamic parameters describe the possible upper limit of performance data. The thermodynamic parameters of an electrochemical reaction are 1. Enthalpy of reaction DH represents the amount of energy released or absorbed. Entropy of reaction DS characterizes the reversible energy loss or gain connected with the chemical or electrochemical process. DS, is called reversible heat effect. DS can be positive or negative. Otherwise, T? DS contributes additional heat cf. Uo describes the generated electrical energy kJ.
Thermodynamic parameters describe the fundamental values of a battery, like the equilibrium voltage and the storage capability. Some examples are listed in Table 1. Section 1. Thermodynamic quantities like DH and DG depend on the concentrations or more accurately activities of the reacting components, as far as these components are dissolved. Table 1. The difference between these values and those observed in practice Column 9 is caused by kinetic parameters.
H2 SO4 , 2? Depends on acid concentration cf. Combination of Eq. The lead-acid battery may be taken as an example: When this value is inserted into Eq. The special battery systems, listed in the lines 11 and 12 in Table 1. The dependence of the equilibrium voltage on the concentration of dissolved components is given by the Nernst equation Eq.
It is independent of the present amount of lead, lead dioxide or lead sulfate, as long as all three substances are available in the electrode. The result of this equation is plotted in Fig. In battery practice, mostly the approximation is used: Actually not the true equilibrium voltage but only the open circuit voltage can be measured with lead-acid batteries. Due to the unavoidable secondary reactions of hydrogen and oxygen evolution and grid corrosion, mixed potentials are established at both electrodes, which are a little different from the true equilibrium potentials cf.
But the differences are small and can be ignored. The discrepancy between the theoretical value and that in practice Column 9 is caused by all the passive components that are required in an actual cell or battery. In battery practice, hydrogen reference electrodes are not used. Instead, a number of reference electrodes are used, e. In lithium ion batteries with organic electrolyte the electrode potential is mostly referred to that of the lithium electrode cf.
Chapter This means that electron transfer has to be forced into the desired direction, and mass transport is required to bring the reacting substances to the electrode surface or carry them away. The overvoltage, caused by electrochemical reactions and concentration deviations on account of transport phenomena.
The ohmic voltage drops, caused by the electronic as well as the ionic currents in the conducting parts including the electrolyte. The sum of both is called polarization, i. Overvoltage can only be separated by special electrochemical methods. Usually the reaction path consists of a number of reaction steps that precede or follow the actual charge transfer step as indicated in Fig.
The slowest partial step of this chain is decisive for the rate of the overall reaction. As a consequence, overvoltages, or even limitations of the reaction rate, often are not caused by the electron-transfer step itself, but by preceding or following steps. Some of these steps include mass transport, since the reaction would soon come to a standstill, if ions would no longer be available at the surface of the electrode or if reaction products would not be cleared away and would block the reacting surface.
In a number of electrode reactions, the reaction product is dissolved. This applies, for example, to some metal electrodes, like zinc, lithium, cadmium, and also to lead. Furthermore, chemical reactions may precede or follow the electron transfer step.
Double-lined arrows mark the charging reaction. A corresponding number of electrons is removed from the electrode as negative charge. The discharging reaction at the positive electrode proceeds in a similar manner: During charging of the battery, these reactions occur in the opposite direction, as indicated by the double-line arrows in Fig. The electrochemical reaction, the transfer step, can only take place where electrons can be supplied or removed, which means that this conversion is not possible on the surface of the lead sulfate, as lead sulfate does not conduct electric current.
If the product of the discharge is highly soluble, during discharge the electrode will to a large extent be dissolved and will lose its initial structure.
This leads to problems during recharge because the redeposition of the material is favored where the concentration of the solution has its highest value. As a consequence, the structure of the electrode will be changed as demonstrated in the upper row of Fig. Connected to the shape change is a further drawback of the high solubility, namely the tendency that during recharging the precipitated material forms dendrites that may penetrate the separator and reach the opposite electrode, thus gradually establishing a short circuit.
Zinc electrodes are therefore not used in commercial secondary batteries, with the exception of the rechargeable alkaline zinc manganese dioxide battery RAM 6 which is a battery of low initial cost, but also limited cycle life. The metallic lithium electrode is another example where cycling causes problems due to its high solubility that causes shape change cf.
Chapter 18 and the lithium-ion system in Fig. Extremely low solubility of the reaction products leads to a more or less dense covering layer lower row in Fig. Thus only a thin layer of the active material reacts. To encounter such a passivation, the active material in technical electrodes, e. The advantage of the low solubility is that the products of the reaction are precipitated within the pores of the active material, close to the place of their origin, and the structure of the electrode remains nearly stable.
This mechanism is illustrated in Fig. Here the reaction product is not dissolved, but the nickel ions are oxidized or reduced while they remain in their crystalline structure that of course undergoes certain changes. When the nickel electrode is charged oxidized , these protons have to leave the crystal lattice. Otherwise, local space charges would immediately bring the reaction to a standstill.
Here oxidation and reduction occur within the solid state, and it depends on the potential of the electrode how far the material is oxidized.
Float charging at a comparatively low voltage, as it is normal for standby applications, does not preserve full capacity and requires regular equalizing charges or corresponding oversizing of the battery. During discharge, lithium ions are intercalated into the oxide from Ref. Another reaction mechanism that in a certain aspect resembles to the above one characterizes lithium-ion batteries cf.
The course of the cell reaction is illustrated in Fig. These positive electrodes intercalate the lithium when discharged, i. As a consequence, the problems caused by solution of a metallic lithium electrode as indicated in Fig. Electron Transfer The electron transfer reaction denotes the central reaction step where the electrical charge is exchanged cf.
EA actually depends on temperature, but often can approximately be treated like a constant.
In electrode reactions, n? F between mass transport and current i; U is the electrode potential; and cj the concentration of the reacting substance that releases or absorbs electrons. Electron transfer, however, does not occur in only one direction: Thus, Eq. T where addend 1 describes the anodic reaction e.
Electron transfer according to Eq. This leads to the usual form of Eq. T where io is the exchange current density that characterizes the dynamic equilibrium, as shown in Fig. The resulting current is represented in Fig. According to Eq. In practice polarization is always determined. The reaction of the lead electrode is inserted as an example. Electrode Polarization Polarization has been introduced as the deviation of the actual voltage from equilibrium by Eq.
Polarization of the single electrode in a battery is a very important parameter. Tafel Lines If the potential is shifted far enough from the equilibrium value, in Eq. The constant b represents the slope of the Tafel line and means the potential difference that causes a current increase of one decade. Tafel lines are important tools when reactions are considered that occur at high overvoltages, since such a linearization allows quantitative considerations.
They are often used with lead-acid batteries, since polarization of the secondary reactions hydrogen evolution and oxygen evolution is very high in this system cf. This dependence is described by the Arrhenius equation, which already has been introduced as Eq.
The logarithmic form of Eq. Very often the approximation holds true that a temperature increase of 10 K or C doubles the reaction rate. In electrochemical reactions, this means that the equivalent currents are doubled, which denotes a quite strong temperature dependence. A temperature increase of 20 K means a current increase by a factor of 4; a rise in temperature of 30 K corresponds to a factor of 8.
Transport of the reacting species is achieved by two mechanisms: F qx zj? Addend 1 of the right-hand part of this equation describes transport by diffusion that always equalizes concentration differences. When transport by diffusion of reacting neutral particles like that of O2 in the internal oxygen cycle Fig. If cj reaches zero, a further increase of the current is not possible.
Such a situation is called a diffusion limiting current, which according to Eq. Addend 2 in the right-hand part of Eq. It is characterized by the transference number. In binary salt solutions they are fairly close to 0. For diluted solutions of sulfuric acid given in Ref. For potassium hydroxide true for a wide concentration range given in Ref. This is one reason to aim at conducting salts with large anions cf. The dashed curve shows the equilibrium voltage according to the Nernst equation. Flooded traction cell with tubular plates Ah at 5-hour rate.
The dashed curve at the top represents the changing equilibrium voltage due to the gradually decreasing acid concentration, according to the Nernst equation Eq. If all the partial-reaction steps were fast enough, i. So, with increasing load, the dischargeable share of the capacity is more and more reduced by the impact of kinetic parameters, and the current amount that can be drawn from the battery is markedly reduced, although the end-of-discharge voltage is lowered with increased load.
Mainly acid depletion at the electrode surface reduces the rate of the reaction. Furthermore, some of the undischarged material may be buried underneath the growing PbSO4 layer. This layer grows very fast at high loads, resulting in a thin but compact covering layer that prevents further discharge very early. It is related to the thermodynamic equilibrium parameters of the concerned reaction, and is strictly connected with the amount of material in electrochemical equivalents that reacts.
Thus, the reversible heat effect does not depend on discharge or recharge rates. When the cell reaction is reversed, the reversible heat effect is reversed too, which means it gets the opposite sign. Thus, energy loss in one direction means energy gain when the reaction is reversed, i.
Ucal is a hypothetical voltage that includes the reversible heat effect, and is used instead of the equilibrium voltage for caloric calculations. Combination with Eqs. This heat is called the Joule effect; it always means loss of energy. Strictly speaking, the negative absolute value should be used in Eq. Then the Joule effect reads according to Eq. For heat effects this is not relevant, since heat generation is proportional to polarization. Wh, or as work per time unit: Strictly speaking, Eq.
The thermodynamic data that determine the equilibrium values are listed in Table 1. The table also Table 1. Section 5. Assumed internal resistance 4. Sections 1. This is illustrated in Fig.
In a vented lead-acid battery heat effects during charging are caused by the charging reaction itself and by water decomposition that accompanies the charging process at an increasing rate with increasing cell voltage. The charging reaction is a very fast one which means that overvoltage is small. At an assumed internal resistance of 4. The reversible heat effect, on the other hand, is determined by the amount of converted material formula mass that is proportional to current and amounts to 0. Most of the energy that is employed for water decomposition escapes from the cell as energy content of the generated gases.
This energy consists of the two components: Both shares are proportional to the amount of decomposed water, which again is only determined by the current i as the product Ucal? The portion of heat that remains within the cell is generated by Joule heating and determined by polarization of the water-decomposition reaction, i.
As an example Fig. The current-limited initial step of charging is followed by a constant-voltage period at 2. Equalizing charging up to 2. Lead-acid with tubular positive plates Varta PzS , Ah. Heat-generation values referred to Ah of nominal capacity. The sum of the whole charging period amounts to Internal resistance 4. The center part of Fig. Only when the voltage approaches the 2.
The broken horizontal line marks the average voltage during this initial step. When subsequently the cell voltage remains at 2. During the equalizing step, nearly all the current is used for water decomposition on account of the progressively reduced charge acceptance. During discharge, water decomposition again can be neglected because of the reduced cell voltage. At the bottom of Fig. The distribution between reversible heat effect, charging, and water decomposition is marked by different patterns of the areas concerned.
The value above each block is the total heat generation in Wh. The heat is mainly generated by the Joule effect, on account of the high current and the rather high internal resistance of 4. But the reversible heat effect also contributes noticeably to heat generation, on account of the converted active material.
When the internal oxygen cycle is established, almost all the overcharging current is consumed by the internal oxygen cycle center bar in the graph. The bar on the right corresponds to a vented battery. Internal resistance assumed as 0. When 2. During the equalizing step, gas evolution required for mixing of the electrolyte dominates. During discharge, due to the small overvoltage, heat generation is also small, and further reduced by the reversible heat effect that now causes cooling.
Heat generation in a valve-regulated lead-acid battery VRLA battery is mainly determined by the internal oxygen cycle that characterizes this design. It means that the overcharging current is almost completely consumed by the internal oxygen cycle formed by oxygen evolution at the positive electrode and its subsequent reduction at the negative electrode cf.
As a consequence, the cell voltage in total means polarization that produces heat. For this reason, overcharging of valveregulated lead-acid batteries must be controlled much stronger than that of vented ones to avoid thermal problems. The charging behavior of a valve-regulated type is shown in Fig. In the center of Fig. The sum of charging current and internal oxygen cycle represents the charging current hydrogen evolution and grid corrosion equivalents are not considered, since they are two orders of magnitude smaller than that of the internal oxygen cycle.
Actually, the current would slightly be increased by heating of the battery. This increase also is not considered in Fig. The bottom part of Fig. At the beginning, the reversible heat effect dominates heat generation due to the high amount of material that is converted.
The relation between the reversible heat effect and Joule heating is determined by the internal resistance of the battery. Internal resistance 0. Heating of the battery during charging is not considered. Heat generation: This applies, for example, to Fig.
When the charging voltage is reached, the current decreases and this applies also to heat generation due to the reversible heat effect and Joule heating, while heat generation by the internal oxygen cycle remains constant, according to the constant cell voltage which actually would slightly be increased by heating up.
But the total heat generation is largely determined by the internal oxygen cycle, especially during the equalizing step that in Fig. Actually, an even larger heat generation is to be expected, since, as already mentioned, the calculation did not consider the heat increase within the cell during charging that again would increase the rate of the internal oxygen cycle.
Due to the uncertain thermodynamic data, these calculations are only rough approximations, but correspond with practical experience. During the initial two sections of the charging period, slight cooling is observed on account of the reversible heat effect that consumes heat at a constant rate proportional to the current. With increasing cell voltage, Joule heating is increased, and when the charging voltage exceeds 1.
In total The main reason is that the reversible heat effect generates additional heat during discharge, while it compensates for heat generation during charging. The voltage curve at the top shows the gradual increase of charging voltage with charging time. The generated heat is calculated as an average value for different sections of this curve.
The numbers beside the charging curve are the average voltages V per cell for the corresponding section. The numbers are the heat in kJ for comparison, converted to Ah of nominal capicity.
Charging with constant current I5 5 hour rate until 1. Discharge also with I5. In the top part, sections are shown that were used to calculate the average heat generation, shown in the bottom part. So the number for this section is written below the zero line. When the charging process approaches completion, nearly all the current is used for the internal oxygen cycle, which causes much heat generation. Since this cycle can attain extremely fast rates, the situation is very dangerous in regard to thermal runaway.
Altogether The charging factor for the lead-acid battery in Fig. For comparison, all values are converted to Ah of nominal capacity. Rapid charging methods, as described in Section 13, are always based on this principle. Equation 42 points out that heat generation and heat dissipation are parameters of equal weight, which means that possibilities to dissipate heat are to be considered as thoroughly as the problem of heat generation. The rate of the temperature change is determined by the heat capacity of the battery CBatt.
Heat dissipation increases with a growing temperature difference DT between the battery and its surroundings, and a stable temperature of the battery is reached at a certain DT when heat generation balances heat dissipation, i. If heat generation within the battery increases faster with increasing battery temperature than heat dissipation, such a thermal balance is not reached and temperature increase continues unlimited.
For the emission of heat these ways are sketched in Fig. A corresponding situation with all the arrows reversed would apply for heat absorption from a warmer surroundings. Three mechanisms are involved in this heat exchange: Heat radiation.
Heat transport by a cooling or heating medium. Usually they occur in combination. The bottom surface usually is in contact with the basis that attains the same temperature as the battery itself, except the battery is equipped with cooling channels in the bottom.
The upper surface usually is of little importance for heat exchange, since the lid has no direct contact to the electrolyte, and the intermediate layer of gas hinders heat exchange because of its low heat conductivity cf. Moreover, in monobloc batteries the cover often consists of more than one layer. Cooling through the terminal occasionally has been applied with submarine batteries which are equipped with massive copper terminals The fourth power of T in Eq.
If the temperature of the surroundings is higher, a corresponding amount of heat would be absorbed. The size of the exposed surface referred to capacity depends largely on size and design of the battery.
According to these values, heat dissipation by radiation can be expected in the Table 1. But the estimation shows that radiation is fairly effective and thus a hot surface in its neighborhood will considerably heat up a battery.
It shows that heat conductivity is fairly high for materials that are used within the battery, like the various metals or water. When metal is used as container, the temperature drop across its wall can be neglected.
For plastic materials l is in the order of 0: Thus heat conductivity even through a plastic container wall is fairly high, and the temperature measured at the sidewall usually represents a good approximation of the average cell temperature.
This is no longer true at very high loads. For example, during high rate discharges about 6 minutes of discharge duration , temperature differences up to 15 K have been observed between the center and the surface in Ah monoblocs of lead-acid batteries Corresponding spacing of battery blocks should always be observed. Comparison between Eq. Consequently, uniform radiation conditions should be observed when a battery is installed.
Heated surfaces in the neighborhood e.
Handbook for Stationary Lead-Acid Batteries
Forced Cooling and Heat Management Proper heat management of a battery is not only intended to avoid a too high temperature, rather it is of the same importance to keep all cells of a battery within a range of temperatures that is as small as possible.
Then charging and discharging performance of the individual cells would no longer be uniform, and premature failure of the cells that are in an unfavorable location might cause premature failure of the whole battery. Forced cooling, however, is required for large and compact batteries, especially when they are loaded heavily or cycled. Therefore, forced cooling systems have mainly been developed for electric vehicles, to prevent overheating and to attain uniform temperature in the inner and outer cells in larger batteries It uses air as coolant that is blown by a fan through channels formed by the spacing of the cells or monoblocs within the battery.
More effective coolants are mineral oil and water. A widespread method for forced cooling uses pockets of plastic material that are arranged between the cells or blocks along the sidewalls and are passed by the cooling medium, usually water. Other battery designs provide special passages for the coolant within the cells or monoblocs. In secondary batteries, it depends on charge or discharge which of the two electrodes is the anode or cathode.
The opposite applies to the positive electrode: When used with secondary batteries, the terms anode and cathode always apply to the discharging situation. The cells in such a pack are often selected to have uniform capacity to prevent premature failure by deep discharging of single cells. This term usually concerns materials in the positive and negative electrode, but may also include certain components of the electrolyte, like sulfuric acid in lead-acid batteries.
Furthermore, some battery systems exist where the battery is stored separately cf. The active material suffers chemical conversion on charge and discharge, and thereby often changes its volume. This may require special design features: Volume for expansion must be provided when the volume of the active material grows. Often the current conductor simultaneously acts as a support for the active material. In some systems the container of the cell is made of metal and often simultaneously acts as terminal.
When a number of electrodes are connected in parallel within the cell, corresponding connecting parts like pole bridges are required. Additives, like carbon or metal powder, sometimes are required to improve the conductivity within the active material, especially in thick layers.
Otherwise, a short circuit is formed that discharges the battery. On the other hand, the ionic current through the electrolyte should be hindered as little as possible.
Polymer electrolytes can also be regarded as ion-conducting separators. In lead-acid batteries it is a must to use glass, rubber, or plastics on account of the high cell voltage that would destroy all metals.
The advantage of a plastic container is that no insulation is required between adjacent cells. A general drawback of plastic materials is their permeability for gasses, water vapor, and volatile substances. In vented batteries with liquid electrolyte it has to prevent creeping of the electrolyte, which especially is observed for batteries with alkaline electrolyte.
With sealed batteries, the post seal, furthermore, has to prevent the escape of hydrogen, and also has to prevent the intake of oxygen from the surroundings. Special techniques have been developed for the different battery systems. The openings in such vent plugs are small to minimize water loss by diffusion of the water vapor.
Valves allow only the escape of gas and are required in valve-regulated leadacid batteries for the escape of hydrogen, but are also used in most other sealed batteries to prevent damage of the cell in the case of a too high internal pressure when the battery is abused, e.
Rechargeable button cells in general have a rupture vent breaking point embossed into their metallic cell container that opens on a preset overpressure before the cell explodes.
Safety features that prevent overpressure sometimes are also employed in primary batteries. But often this equilibrium voltage cannot exactly be measured even not at an open circuit , since the electrode process is not quite reversible, as in the case of the nickel electrode or since secondary reactions cause a slight deviation cf.
Then the open cell voltage OCV actually is measured. In some battery systems, like lead-acid, the OCV can be used for a rough determination of the state of charge. Other systems again, e. A further term is the nominal voltage of a cell or battery that approximates the voltage of a system for its characterization cf. It is caused by the positive electrode, and can be ascribed to crystallization overvoltage Initial voltage minima are also known Figure 1.
The broken curve denotes the equilibrium or open circuit voltage. They are referred to as initial voltage delay and are caused by protecting layers at the negative electrode. Discharge current. Voltage limit, i. Any comparison of capacity data must always consider these parameters. The upper curve represents a low-rate discharge; the lower curve stands for a high-rate discharge. The broken curve in Fig.
In other systems, where the electrolyte is not involved in the cell reaction, e. The difference between the actual discharge curves and Uo means polarization according to Eq. This difference usually increases with progressing discharge on account of the gradual increase of the internal resistance. The depth of discharge DOD is an important parameter in regard to the number of cycles that can be reached with rechargeable batteries.
For lead-acid batteries, deep discharges that are continued beyond the recommended maximum DOD can reduce the service life dramatically cf. Section 4. For various applications, nominal or rated capacities are often referred to different discharge durations, termed as C20, C10, or C5.
But they soon became common practice also with lead-acid batteries and other rechargeable battery systems. The factor m has no relation to discharge time; m is a pure number. For this reason, discharge rates in terms of multiples of the capacity cannot be converted into discharge duration.
To evaluate the discharge duration, the discharge curves must be known. In regard to the dimensions, Eq. It is also used in the IEC standard for secondary cells and batteries containing alkaline or other nonacid electrolytes For primary batteries two discharge methods are in use: Constant current discharge, Constant resistance discharge.
Some test houses prefer method 1. Method 2 prevails also in the IEC standard for primary batteries. Depending on manufacturer, the capacity of primary batteries may be given in terms of a service output duration that is gained by a test that simulates the concerned application e. This duration in hours or days is proportional to the available capacity under these conditions.
An exception are button cells to operate, for example, wristwatches. Their capacity is given in mAh. Capacity measurements often are carried out at a constant-current load, and the energy output is calculated by multiplying the measured capacity with the discharge voltage.
Therefore either the exact integration according to Eq. Corresponding terms are. Initial discharge voltage: Average discharge voltage or mean discharge voltage: Midpoint discharge voltage: In the negligent use of such terms it is often also called energy density or gravimetric density, although a density should always be referred to a volume.
The energy density is of special interest for batteries designed to power portable equipment. In such applications the size of the battery is in general of a higher priority than its weight.
With most battery systems the internal resistance increases when the end of discharge is approached, because of reduced conductivity of the formed compounds.
Mostly the direct-current method is applied, where the terminal voltage is compared at two different loads. The battery is loaded with the current i1 for a few seconds and the voltage U1 results.
Equation 56 implies that the overvoltage is comparatively small compared to the ohmic voltage drop. For batteries with aqueous electrolyte the internal resistance can be determined by this method only for the discharge, but not during charging because of the high overvoltage of the gassing reactions. When a preceding diffusion process limits the transport of reacting particles and a limiting current according to Eq.
Equation 56 would then lead to the Ri??. The short-circuit current is of interest especially for larger stationary batteries, since it stands for the maximum current that could be supplied by the battery for a short period of time. Its value helps in estimating the size of a fuse that might operate with the battery. The short-circuit current is determined according to Eq.
It represents a dynamic parameter that decreases quickly with proceeding discharge. During the last decades, the ohmic part of the impedance gained importance as a possibility to check the situation of valveregulated lead-acid batteries that otherwise could be determined only by discharging.
The impedance is mostly measured at a frequency of Hz. The resulting values are only used for comparison to detect faulty cells within a string. Established methods and devices are on the market. One reason can be the gradual reduction of the oxidation state in the positive electrode, e. The two electrochemical reactions compensate each other, and gradual discharge is the consequence. The fast rise of this curve indicates the low overvoltage that characterizes this reaction.
It means that high discharge rates can be achieved at low overvoltage or polarization values. The high overvoltage, marking the hydrogen evolution reaction, is expressed by the gradual rise of the corresponding current curve.
The result of this balance is the mixed potential UM in Fig. As typical for an equilibrium potential, no external current appears. But the mixed potential is Figure 1. The dashed curve represents reduced hydrogen overvoltage. The position of the mixed potential is largely determined by the faster one of the two reactions, while the rate of the reaction, the self-discharge in this example, is determined by the slower reaction.
This is illustrated by comparison of the continuous and broken curves in Fig. The continuous curve represents a fairly high hydrogen overvoltage; the broken curve depicts the case when hydrogen can be evolved more easily. The position of UM is only slightly changed between the two examples, but the rate of the self-discharge, the current i2, grows to a multiple of i1 in the second case.
In this way both electrodes are gradually discharged. The cathode material itself, if soluble to a certain extent like Ag2O, may also lead to the formation of electron conducting Ag bridges if reduced by cell components, like an unsuitable separator Then the delivered capacity is reduced by an increased voltage drop, although the electrodes are still fully charged, i. The extent of this loss depends on system, construction, and storage conditions, like temperature.
In general, there is a distinct difference in capacity loss during storage between primary batteries and secondary batteries.
The latter usually suffer faster selfdischarge. Practical values for primary batteries at ambient temperature, that also include the apparent self-discharge, are in the range of 0.
The low value applies, for example, to lithium batteries with a high-quality seal. The true chemical or electrochemical self-discharge is much smaller. Also secondary battery systems exhibit a broad range of different rates of selfdischarge.
Their values, however, are based on a 1-month period in contrast to primary systems 1-year period. The free enthalpy DG should be large to achieve a high cell voltage Eq.
The equivalent weight mole weight per exchanged electron of the reacting components should be as low as possible to gain a high energy output per weight. Some examples of such a choice are listed in the matrix of Table 1. However, Eq. Comparison of this value with the two last columns in Table 1. Often it is only the medium for electrode reactions and ionic conductivity and does not appear in the cell reaction e.
A great number of battery systems employ aqueous electrolyte, like the primary systems in Lines 1 to 4 in Table 1. Their advantage is a high conductivity of acid and alkaline solutions at room temperature, and, furthermore, that quite a number of suitable electrode reactions occur in such solutions.
The disadvantage of aqueous electrolytes is the comparatively low decomposition voltage of the water that amounts to 1. Lithium as active material would heavily react with water. Batteries with lithium electrodes therefore have to use nonaqueous inorganic electrolytes, like thionyl chloride Line 6 in Table 1. A general disadvantage of organic electrolytes is the conductivity that at least is one order of magnitude below that of aqueous electrolytes. It must be compensated by narrow spacing of thin electrodes.
Furthermore, interaction between the electrolyte and the active material is unavoidable at the high cell voltage as will be shown in Chapter High load batteries are the two examples, in Lines 11 and 12 of Table 1. Technical details of these batteries and their application, however, are subjects of later chapters. It is the oldest secondary system, widely used, and well known. It is characterized by the fact that lead is used in both electrodes as the active material.
These values depend on acid concentration cf. The comparatively high cell voltage, as a result of the high potential of the positive electrode and the low potential of the negative electrode, gives rise to a number secondary reactions that occur at electrode potentials within the cell voltage.
Oxygen evolution at the positive electrode 2? H2 Both together mean water decomposition 2? Furthermore, at an electrode potential below 1. As a further problem, at the high potential of the positive electrode all metals are destroyed by oxidation. This applies also for lead that in principle starts to corrode at the potential of the negative electrode in the form of the discharge reaction Pb PbSO4.
As a consequence, the following unwanted reactions are always present in a lead-acid battery: Oxygen evolution at the positive electrode. Oxygen reduction at the negative electrode. Hydrogen evolution at the negative electrode. Grid corrosion. The horizontal axis shows the potential scale referred to the standard hydrogen electrode, the range of the negative electrode on the left, the range of the positive electrode on the right. In the center, a range of about 1. The rates of these reactions are indicated by current potential curves.
The two hatched columns represent the equilibrium potentials of the negative and positive electrodes. Their dependence on acid concentration is indicated by the width of these columns. The charging and discharging reactions are represented by the broken curves. They are very steep, since these reactions are fast, and occur at a high rate even at a small deviation from the equilibrium potential. When, however, this deviation from the equilibrium potential exceeds a certain value, the two curves show a steep increase.
This means that hydrogen as well as oxygen generation gain in volume enormously at correspondingly low and high polarization. At an electrode potential below this minimum, corrosion increases due to destabilization of the protecting layer. Above this minimum, the corrosion rate follows the usual exponential increase with increasing electrode potential.
The origin of the horizontal scale is the equilibrium potential of the hydrogen electrode. The rate of oxygen reduction according to Eq. In conventional batteries with liquid electrolyte, this limiting current is very small, since the diffusion rate of dissolved oxygen is very slow and its solubility is small.
As a consequence the equivalent of oxygen reduction is limited to a few mA per Ah of nominal capacity and thus is hardly noticed in battery practice. But in valve-regulated lead-acid batteries it is a fast reaction that characterizes overcharging cf. But hydrogen evolution is unavoidable, since its equilibrium potential is about mV more positive than that of the negative electrode.
For this reason, hydrogen evolution always occurs, even at the open circuit voltage, and a mixed potential is formed according to Fig. When the electrode is polarized to more negative values, hydrogen evolution is increased according to the curves shown in Fig. Polarization to more positive values than the equilibrium potential reduces hydrogen evolution, but simultaneously means discharge of the electrode. In valve-regulated lead-acid batteries cf. In the mixed potential of Fig. Hydrogen evolution is extremely hindered at the lead surface.
This is pointed out in Fig. In this semilogarithmic plot, the hydrogen evolution curves represent Tafel lines Section 1. The position of its Tafel line is far to the left, and hydrogen evolution at a faster rate than 0. Of all metals only mercury shows a similar hindrance of hydrogen evolution.
At nickel, copper, and antimony, hydrogen is evolved at the rate of 2, 0. At the lead surface, this value that approximately corresponds to the rate of self-discharge at open-circuit voltage is about six orders of magnitude smaller compared to hydrogen evolution at the other metals.
Selfdischarge by hydrogen evolution is noticed in the lead-acid battery despite of this small rate only because of the large surface area of the active material of about m2 per Ah of nominal capacity. Extreme hindrance of an electrochemical reaction is always endangered to be released by contaminants. Thus hydrogen evolution on the lead surface would enormously be increased by the precipitation of traces of other metals, like those shown in Fig.
Such a contamination shifts the Tafel line more to the right and annuls or at least aggravates the exceptional situation of lead. This concerns mainly the active material and the grid in the negative electrode, but also all the other components of the cell, since critical substances may be leached out and migrate to the negative electrode where they are precipitated when their equilibrium potential is more positive than that of lead.
In the near future, this question may gain in importance, since due to the growing recycling efforts of all materials, secondary lead has increasingly to be used also for the active material in batteries.
Secondary lead, however, may contain quite a number of additives which in their entirety determine the hydrogen evolution rate 28 , and it is an economical question how far the various smelters can purify the lead at an acceptable price. This corresponds to 1.
Additives like organic expanders are often considered as a possibility to increase hydrogen over-voltage and reduce so hydrogen evolution. The corresponding reaction, already mentioned as Eq. But oxygen evolution at the open circuit potential is small and therefore selfdischarge due to oxygen evolution usually is not noticed.
But oxygen evolution increases more rapidly with increasing potential than hydrogen evolution, and the slope of the corresponding Tafel line is steeper. For this reason, considerable rates of oxygen evolution are observed at a higher potential of the positive electrode. SHE in the acid solution. Thus it is always possible at the negative electrode, and oxygen is immediately reduced when it reaches the surface of the negative electrode.
Thus the rate of this reaction is largely determined by the rate of oxygen transport to the negative electrode surface, and forms a limiting current according to Eq. Data that determine this transport are shown in Table 1. The resulting ratio is Transport rate in the air 0: In the latter case, oxygen has to permeate in the dissolved state only the thin wetting layer on the surface.As a consequence, the structure of the electrode will be changed as demonstrated in the upper row of Fig.
Store batteries on racks or on pallets, not on the floor. The Ampere-hour capacity of the system is the same as that of the individual batteries. The immobilization of the electrolyte has a side-effect of enormous practical importance: Secondary batteries that are reversible energy converters and designed for repeated discharges and charges.
When this value is inserted into Eq. VanLandingham Check that the vent-plugs or manifolds are firmly in position. The free enthalpy DG should be large to achieve a high cell voltage Eq.