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.

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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., [ 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.

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.

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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.

Battery Technology Handbook

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. Then the potential of the wetting layer may be shifted to more positive values and the cathodic corrosion protection may be lost and corrosion occur as indicated in Fig. The continual oxygen reduction at the wetted lead parts has the following results: The potential of the metallic parts is uniform.

As a result, three different zones can be observed: Zone 1: Zone 2: The situation is aggravated as the acid in the wetting layer at lug and group bar becomes diluted by the generated water. Furthermore, corrosion roughens the surface and that again increases current and voltage drop. Thus the progressing corrosion accelerates the process.

Zone 3 in Fig. So lead can only be oxidized. It depends mainly on the used alloys whether serious corrosion actually occurs in Zone 2. The selection of welding alloys is also important.

In Fig. Underneath the porous lead dioxide that constitutes the active material, a dense layer, also of lead dioxide, covers the grid surface. This layer is formed by corrosion and protects the grid. However, lead dioxide and lead cannot exist beside each other for thermodynamic reasons, and a thin layer of less oxidized material PbOx is always formed between the grid and the lead dioxide. The existence of lead oxide PbO in this layer has been determined; the existence of higher oxidized species is assumed, but their structure is not yet known exactly.

This intermediate layer is the main reason why periodical charges are required with leadacid batteries during prolonged storage periods, since at open circuit this layer gradually grows by further oxidation of the lead, while PbO2 is reduced.

But cracks are formed when the oxide layer exceeds a given thickness, on account of the growth in volume when lead becomes converted into lead dioxide. Underneath the cracks, the corrosion process starts again and again. An assumed penetration rate of 0.

Spine of a positive tubular plate. New plate: Aged plate: Since grid material is converted into lead dioxide, a slight increase of the actual capacity is often observed. The reduced cross-section in Fig. Advanced corrosion of the grid can cause an intolerable voltage drop at high discharge rates.

General parameters that determine this corrosion current are. Electrode potential. Surface area of the grid.

In general, pure lead has the lowest corrosion rate. A survey of alloys that especially are applied in valve-regulated lead-acid batteries is given in Ref.

The grid surface that is exposed to the active material and thereby to the electrolyte can vary between about cm2 in a tubular electrode and about cm2 per Ah of nominal capacity in a thin punched grid as are used in cylindrical VRLA batteries. Based on these values, currents between 0. Table 4 in Ref.

Types of Alloys The unavoidable corrosion of the positive grid and other parts of conducting material has far-reaching consequences for the lead-acid battery, and is the reason that alloys play such an important role: Grid corrosion also is a water-consuming reaction which is of great importance in valve-regulated lead-acid batteries cf.

Selection of appropriate alloying additives is important to reach the desired service life. Alloying constituents are released when the grid material is transformed into lead dioxide.

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If not absorbed by the active material, they are leached out of the positive plate and can reach the negative electrode by diffusion. As it is not possible to match all these requirements at their optimum with one alloy, a number of different grid alloys are in use.

A survey on metallurgical properties of lead alloys for batteries is given in Ref. In Table 1. Furthermore, a high antimony content stabilizes the active material in the positive electrode and improves the cycle performance of the battery. The disadvantage of a high antimony content is the increase of waterdecomposition rate with service time, caused by antimony released from the positive grid by corrosion and precipitated on the surface of the negative electrode.

Hydrogen can be evolved much more easily at the antimony-contaminated lead surface as indicated in Fig. Copper and sulfur are such additives. The most effective additive is selenium. Starting, lighting, ignition SLI means starter battery for motor cars.

In valve-regulated lead-acid batteries they are substituted by antimony-free alloys. As a consequence, the hydrogen evolution rate is low and remains practically unaltered during the whole service life of the battery.

One disadvantage of lead-calcium alloys is their tendency to grid growth, a further demerit of the reduced stability of the capacity, especially when deep discharge cycles are performed A passivating layer of lead sulfate was often observed between the grid and the active material, and was assumed to be the reason for capacity loss.

More recent investigations 40 showed that this capacity loss is a much more general problem, mainly obvious in batteries with antimony-free grid alloys, but depends also on a number of other parameters like the charge current rate and the history of the battery Tin additions are used in a wide range between 0.

Low calcium alloys have been introduced during recent years. Their main advantage is reduced corrosion attack, and thereby reduced grid growth. Silver addition is claimed to increase the creep resistance and improve the corrosion behavior A disadvantage of these alloys is their softness.

So they cannot be manufactured by the conventional casting process, because they could not be handled in the usual equipment, like pasting machines. Rather they depend on continuous processes that combine grid manufacture with pasting. Lead-tin alloys gained much importance as a remedy against corrosion problems of the conducting elements in valve-regulated lead-acid batteries.

But they are also applied for grids, where a tin addition of 0. Cylindrical cells of valve-regulated design Gates, now Hawker with punched grids employ lead for both grids with a small tin addition A similar alloy, basically corresponding the Astag alloy, but using tin as a further additive, is applied under the name Astatin It contains 1.

With this alloy, cycling is possible owing to the antimony content. On the other hand, antimony release is rather low because the intermetallic compound SbCd is formed between antimony and cadmium that keeps antimony within the positive plate. A disadvantage of this alloy is its high cadmium content, because of the toxicity of cadmium. This does not concern the battery, but might cause problems for remelting of these batteries when they are recycled after service.

For oxygen reduction the situation is different: In valve-regulated lead-acid batteries, oxygen transport occurs through the gaseous phase and is very fast cf. A strong relation between these reactions is given by the primary rule: Charging itself shows no differences for both designs: At the beginning, charge acceptance is high and the charging process is automatically limited by the charging device.

Later charge acceptance is reduced more and more, and with increasing cell voltage, secondary reactions gain in importance. Thus the zero point of the horizontal axis is the open circuit voltage of the cell, i. Furthermore, the current now is plotted in a logarithmic scale, and the current voltage curves of oxygen and hydrogen evolution are represented by Tafel lines according to Section 1.

The values are based on model calculations by U. Teutsch 46, At open circuit, i. The latter would be equivalent to a selfdischarge of the negative of 1. The corrosion behavior is represented in Fig. This is a rough approximation, but it corresponds to the practical experience and describes the always observed minimum of corrosion at 40 to 80 mV above the open-circuit potential of the positive electrode. At lower potentials the protecting layer of PbO2 is destabilized.

At polarization values more positive than this minimum, the corrosion rate increases also exponentially in respect to polarization.

The slope of this Tafel line is mV per decade, which is in accordance with the general experience in practice. Oxygen reduction is limited by the slow transport rate. It is expressed by a horizontal line, which means that the oxygen reduction rate is independent from the potential of the negative electrode. At a very small polarization, which corresponds to a correspondingly low cell voltage, decrease of this reaction is to be expected, due to the reduced oxygen evolution at the positive electrode, as indicated by the broken curve.

Current voltage curves as current equivalents in a semilogarithmic scale versus polarization, referred to the open circuit potential of the negative and positive electrodes. Values based on generalized data.

The curves for charge and discharge are not drawn, since they would be represented by nearly vertical lines due to their low polarization. The inserted double arrows show the situation that results at a voltage of 2.

Overcharging at 2. At the positive electrode, such a polarization causes oxygen evolution equivalent to a current of Together with hydrogen evolution equivalent to At the positive electrode it causes oxygen evolution equivalent to a current of 8.

At the negative electrode oxygen reduction 2 mA and hydrogen evolution 8. A nearly stoichiometric ratio of hydrogen and oxygen escapes from the cell, and a corresponding water loss is observed The deviation from stoichiometry is caused by corrosion and oxygen reduction. When, for example, hydrogen evolution is more hindered, the corresponding Tafel line is shifted downwards, and polarization of both electrodes also would be shifted to more negative values.

Such an addition increases the hydrogen evolution rate and shifts the polarization of both electrodes to more positive values. Valve-Regulated Batteries In valve-regulated lead-acid batteries VRLA batteries , the limiting situation no longer exists for oxygen reduction, since the electrolyte is immobilized next section and fast transport of oxygen occurs via the gaseous phase cf.

Furthermore, the cell is closed by the valve. As a consequence, the internal oxygen cycle characterizes the overcharging situation, i. Since the transport of oxygen and its reduction are fast, the partial pressure of oxygen is kept fairly low. The reaction at the positive electrode is thus reversed at the negative electrode. In lead-acid batteries, however, it can only be approximated, since hydrogen evolution at the negative electrode and also grid corrosion at the positive electrode are always present as secondary reactions at a certain rate.

Hydrogen that is generated at the negative electrode must escape from the battery. In lead-acid batteries, however, this cycle is unavoidably accompanied by hydrogen evolution and corrosion of the positive grid. Hydrogen is not oxidized within the cell, but has to escape through the valve. Corrosion consumes oxygen that remains in the cell as PbO2. This can be achieved by two methods: Application of absorbent-glass-mat separators AGM that are soaked by the acid so that liquid acid is not left within the cell.

This liquid can be removed or is lost according to Eq.

For most applications, the differences between the two immobilization methods are marginal. When batteries of the same size and design are compared, the internal resistance of the gel battery is slightly higher, mainly due to the conventional separator that is required with gelled electrolyte, since the gel itself does not prevent the penetration of lead dendrites that can cause short circuits between the electrodes.

As a consequence, AGM batteries are preferred for high load applications, because of the possibility to achieve a very low internal resistance. In general they are superior in cycle applications, and tall gel cells can be operated also in upright position, while with tall AGM batteries, operation in horizontal position usually is recommended to limit the height of the separator to about 30 cm.

In gelled electrolyte, most of the oxygen must surround the separator. This is one of the reasons that the maximum rate of the internal oxygen cycle is lower in gel cells.

Another reason may be that a certain portion of the surface is masked by the gel. A charging current that exceeds this maximum causes gas escape as in a vented battery. The more limited maximum rate of the internal oxygen cycle in gel batteries even offers the advantage that gel batteries are less sensitive to thermal runaway when overcharged at a too high voltage. The immobilization of the electrolyte has a side-effect of enormous practical importance: At the end of discharge, the concentration of the acid is lower, but more-or-less uniform, except the share localized below the electrodes that had not been included into the convection due to its higher density.

Complete uniformity of all the acid would be reached only after a prolonged period of time by diffusion. Thus in the bottom part disintegration of the active material in the positive electrode and its sulfatation in the negative electrode are results that will cause premature failure of the battery.

During such overcharging periods the heavy gassing produces bubbles that ascend within the electrolyte and so cause mixing. More effective mixing is achieved by forced acid agitation with the aid of inserted air-lift pumps.

Such applications are automatic guided transport vehicles where only intermediate boost charges are possible that do not fully recharge the battery. In stationary applications, VRLA batteries perform superior as standby batteries for wind and solar energy generation. These batteries cannot be recharged regularly and properly, since the required energy often is not available. Thus oxygen evolution is completely compensated for by oxygen reduction, and the current equivalents for oxygen evolution and oxygen reduction equal each other.

Current voltage curves as in Fig. This is indicated by the lower double arrow in Fig. Also in the valve-regulated design, the sum of hydrogen evolution and oxygen reduction at the negative electrode in electrochemical equivalents must equal the sum of oxygen evolution and grid corrosion at the positive electrode, and in the example of Fig.

At the positive electrode, such a polarization causes oxygen evolution equivalent to a current of 37 mA and corrosion equivalent to 3. At the negative electrode oxygen reduction 37 mA and hydrogen evolution 3. The result, shown in Fig.

The polarization of the negative electrode is small and the hydrogen evolution rate is close to the self-discharge rate at open circuit. Compared to the vented design in Fig. This is a question of balance between hydrogen evolution and grid corrosion. Characteristics as in Fig. Zero point of polarization: Three important statements can be derived from Fig. The polarization of the negative and positive electrodes is determined by the balance between hydrogen evolution and grid corrosion, expressed in current equivalents.

Water loss is equivalent to the hydrogen evolution rate, which together with corrosion results in 2? In regard to water loss, attention has to be drawn to the fact that water loss in VRLA batteries can only be determined by measurement of the escaped hydrogen. Due to the fugazity of hydrogen such measurements have to be carried out very thoroughly and it has to be observed that only suitable tubing and sealing materials are used.

Otherwise, the permeation of hydrogen disturbs the results too much. Water loss cannot be determined by loss of weight, since oxygen that is consumed by corrosion remains in the cell, and only the low-weight hydrogen escapes.

Balance Between Hydrogen Evolution and Grid Corrosion Standby batteries in stationary applications are continuously overcharged at a comparatively low voltage for two reasons: The overcharging current should be as low as possible to minimize hydrogen evolution and corrosion and thus water loss. The gap between charging and discharging voltage should be as small as possible to allow uninterrupted power supply without additional switching equipment. To emphasize the problem that may arise, in Fig.

As a consequence, the ratio between hydrogen evolution and grid corrosion is shifted correspondingly. But then the polarization of the cell 2. This, however, represents a very critical situation of the negative electrode, since the slightest further increase of hydrogen evolution or a reduction of the cell voltage would cause positive polarization and this means discharge of the negative electrode. This is clearly to be seen at the lower overcharging voltage of 2. At this overcharging voltage, balance between hydrogen evolution and grid corrosion can no longer be achieved, since the self-discharge rate of the negative electrode at zero polarization exceeds grid corrosion at mV of positive polarization.

As a consequence, the polarization of the negative electrode is shifted to a positive value. Thus the surplus of hydrogen evolution is compensated by a selfdischarge current equivalent to 1.

The discharge of the negative electrode equivalent to 1. The balance between hydrogen evolution and grid corrosion can also be disturbed by the intake of oxygen which may be caused by a not properly closing valve or a leakage in the sealings of the container. Due to its easy access to the negative electrode, oxygen would be reduced and form an additional anodic current with the consequence that the potential of the negative electrode is shifted to more positive values.

When the amount of oxygen exceeds a certain limit, the potential of the negative electrode will even be shifted to positive values and its gradual discharge would be the result, as described in the preceding section for the unbalanced cell.

The problem of discharged negative electrodes has repeatedly been reported in the United States, and it mainly appeared with batteries designed for long service life cf.

The reason may be that in such batteries highly corrosion-resistant alloys are combined with negative electrodes that evolve too much hydrogen. As mentioned in Section 1. One possibility is the installation of a small catalyst within the cell The effect of such a catalyst is illustrated in Fig.

Its principle is that the direct recombination Figure 1. The continuous line indicates the relation when both electrodes are discharged according to Eq. The broken line represents the relation when only the negative electrode is discharged Eq. Characteristic data as in Fig. Thus the corrosion rate is also slightly reduced, and the amount of hydrogen that must escape from the cell equals this reduced corrosion rate.

Thus the catalyst is effective in several aspects: It stabilizes the potential of the negative electrode at a more negative polarization. Water loss is reduced to the rate of the reduced corrosion, since oxygen and hydrogen that directly are recombined remain as water in the cell. Such catalysts have been in practical use since and experience has proved the above statements Such recombination plugs are aimed to recombine as much as possible of the generated hydrogen and oxygen gases to reduce water loss.

High recombination rates are required and thermal problems are the main concern, since the recombination generates much heat. The here described catalyst in the VRLA battery has only to disturb the internal oxygen cycle and cause a small gap between oxygen evolution and the amount of oxygen that reaches the negative electrode. They belong to a whole family of secondary batteries that are based on aqueous, but alkaline electrolyte, usually diluted potassium hydroxide.

A further common feature of these battery systems is that they employ the nickel-hydroxide electrode as the positive one. Some of their basic features will be described in the following. The positive electrode, the nickel-hydroxide electrode, already has been mentioned and its reaction mechanism sketched in Fig. Thus 2? NiOOH as the charged state more precisely should be written u?

NiO2 v? As a consequence of its complex reaction, the nickel-hydroxide electrode is not quite reversible, and does not attain a true equilibrium potential, and thermodynamic data and values are only approximate. From the discharge equation can be deduced that 0. In sealed batteries, despite their lower content of electrolyte, the density change caused by discharge does not exceed 0. Thus a concentration change with progressing discharge as shown in Fig.

Actually, after charging, values between 1. But on open circuit the cell voltage decreases to less than 1. Comparison with the value of DG shows that the reversible heat effect amounts to During charging, water decomposition can only be critical as a heat source, when the cell voltage considerably exceeds 1.

But in the sealed version, the internal oxygen cycle can cause serious thermal problems cf. The drawing corresponds to Fig. The upper one represents the potential referred to the hydrogen electrode in the same solution HESS. The lower scale represents the electrode potential referred to the standard hydrogen electrode SHE.

The equilibrium potential of the cadmium electrode is about 20 mV more positive than that of the hydrogen electrode. As a consequence, self-discharge by hydrogen evolution, as described for the lead electrode in Fig.

The equilibrium potential of the nickel-hydroxide electrode is slightly above that of water decomposition. In this respect the situation resembles that of the leaddioxide electrode, but the much lower value of this potential allows the use of nickel as conducting element, since corrosion of this metal can be neglected, at least under normal conditions. For this reason, corrosion is not shown in Fig. Only in foam electrodes with an extremely large surface area of the substrate, nickel corrosion may slightly disturb the current balance in sealed cells.

For the x-axis, i. The upper one represents the potential Uh referred to the hydrogen electrode in the same solution HESS ; the lower scale Uo is referred to the standard hydrogen electrode SHE for 1. Compared to the situation of the lead-acid battery, illustrated in Fig. As in lead-acid batteries, oxygen evolution cannot be avoided at the positive electrode, it already occurs at open circuit and is increased with a more positive polarization of the positive electrode. The reduction itself at the nickel substrate of the cadmium electrode is faster than in lead-acid batteries.

In practice, overcharging at the 5 hour rate is possible for many of the sealed designs without hydrogen evolution. As already mentioned, self-discharge of the cadmium electrode caused by hydrogen evolution does not occur. Corrosion of the nickel substrate in the positive electrode is negligible, and thus not shown in Fig. Formation of a gel, as described in Section 1.

As mentioned above, corrosion almost does not occur in the nickel-hydroxide electrode and its current connectors, and at the cadmium electrode hydrogen evolution can be avoided. The overcharging situation corresponds to that shown in Fig. Thus only the internal oxygen cycle is left, and the battery does not need a valve. Hydrogen oxidation at the nickel-hydroxide electrode occurs faster compared to the reaction rate at a lead-dioxide surface in acid electrolyte, but it is still a very slow reaction, and thus an internal hydrogen cycle of an acceptable rate is not established.

Hydrogen gas, if formed at the negative electrode, increases the internal pressure until the valve opens or the cell bursts. Hydrogen, however, cannot be generated as long as the negative electrode retains its potential above the hydrogenequilibrium potential, which means that the potential of the negative electrode has to be kept quite close to its equilibrium value c.

The potential of the negative electrode must be stable, i. The overcharge current must not exceed the maximum oxygen transport rate, so that all the oxygen that has been generated at the positive electrode reaches the negative electrode fast enough to be reduced.

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This utilizable capacity of the positive electrode is synonymous with the capacity of the cell. The capacity of the negative electrode is represented by the lower block.

Its capacity is oversized, compared to the positive electrode. To match the design conditions shown in Fig. Some of the active material in the negative electrode must remain discharged as Cd OH 2 when the positive electrode is fully charged and the cell is sealed. This share of capacity is called charge reserve.

After the battery cell is sealed, this layout remains largely unchanged under normal operating conditions. When the battery is charged, the two dotted blocks in Fig. When the positive electrode reaches the state of full charge, oxygen evolution occurs instead of the charging reaction, and this oxygen is transported to the negative electrode and reduced. When the overcharging current only generates oxygen at the positive electrode that subsequently is reduced at the negative electrode, no current is left for further charging of the negative electrode.

Thus, the charge reserve remains in its discharged state for an unlimited period of overcharge. Such a charge reserve would not make sense in a valve-regulated lead-acid battery, since it would increase the rate of the inherent hydrogen evolution and so even aggravate the situation and further disturb the balance, described in Section 1.

Therefore a certain amount of electrolyte is not required between the electrodes, rather they can be narrowly spaced. A cylindrical cell is contained in a drawn stainless steel can, which is the cathode connection. The positive electrode mixture is a compressed paste of manganese dioxide with carbon powder added for increased conductivity. The paste may be pressed into the can or deposited as pre-molded rings.

The hollow center of the cathode is lined with a separator, which prevents contact of the electrode materials and short-circuiting of the cell. The separator is made of a non-woven layer of cellulose or a synthetic polymer.

The separator must conduct ions and remain stable in the highly alkaline electrolyte solution. The negative electrode is composed of a dispersion of zinc powder in a gel containing the potassium hydroxide electrolyte. The zinc powder provides more surface area for chemical reactions to take place, compared to a metal can.

This lowers the internal resistance of the cell. To prevent gassing of the cell at the end of its life, more manganese dioxide is used than required to react with all the zinc. Also, plastic-made gasket is usually added to increase leakage resistance. The cell is then wrapped in aluminium foil, a plastic film, or rarely, cardboard, which acts as a final layer of leak protection as well as providing a surface on which logos and labels can be printed.

When describing AAA, AA, C, sub-C and D size cells , the negative electrode is connected to the flat end, and the positive terminal is the end with the raised button. This is usually reversed in button cells, with the flat ended cylindrical can being the positive terminal. If not, turn the ignition key to the auxiliary position.

Disconnect the earth-connector first. This is normally the negative on modern vehicles. This can result in the loss of memory settings; please refer to the vehicle handbook. Disconnect the live-connector second. If a CMS is used, the connector will still remain live after it has been disconnected.

To prevent the connector shorting against the car, place an insulator such as a rubber glove over the connector. Remove the hold-down clamps. Preparation of a Battery for Fitting Check that the battery has the correct polarity for the vehicle. Check that the battery has the correct height for the vehicle. If a battery is too high, it can short out on the bonnet or the bottom of a seat, or it can damage the bonnet.

It is good practice to place the old and new battery side by side to compare polarities, hold-downs and performance-levels. Some batteries have hold-downs at both the sides and ends. Only the ones used for securing the battery on the vehicle need to be checked. Check that the battery is clean and dry.

Check that the vent-plugs or manifolds are firmly in position. Check that the battery has a voltage above If not, charge the battery or use another that has a voltage above Ensure the 2 terminal caps are still fitted at this stage. Preparation of the Vehicle Clear away any items on the battery-tray which might damage the battery. Placing a heavy battery on a piece of sharp grit can puncture the bottom of the battery. Check that the connectors, the hold-down clamps and the tray are clean and corrosion-free.

If there is any corrosion, hot water will instantly remove this. If there is severe corrosion which might affect the stability of the battery or has affected other parts of the engine compartment, have the vehicle checked by an authorised distributor. Check that the alternator drive-belt tension is correct. Refer to the vehicle handbook or service manual. It is recommended that the electrical system, and particularly the charging system, of the vehicle be checked to make sure it is operating correctly.

Installing the Battery Fit and tighten the hold-down clamps. These should be tight enough to secure the battery and not allow it to move. Connect the live-connector first to the correct battery-terminal normally the positive after removing the terminal cap.

Connect the earth-connector to the other terminal after removing the terminal cap. Place the 2 terminal caps on the old battery that has been removed from the vehicle to avoid the possibility of short-circuits.

Replace onto the new battery any components that have been taken from the old battery such as exhaust tubes, vent-elbows, terminal covers, removable hold-down strips widgets etc. The use of petroleum-jelly Vaseline is not necessary on modern polypropylene batteries, but there is no disadvantage in using it.

Smear lightly on the terminals. It is still recommended for hard-rubber batteries. Do not use grease. Remove the CMS. Charging the battery on the vehicle is not recommended. Refer to Section F for information about removing the battery from the vehicle. Do not charge on constant current chargers or boost chargers. There are no removable vent-plugs or manifolds. The battery is able to vent gases through breathing holes, and so it is not strictly sealed.

A new, unused battery with a voltage below See Section B. General Procedure for All Types of Chargers This section gives common information for all types of chargers. The sections below give details for different types of charger. Check the electrolyte-levels in all the cells.The state of charge can easily be controlled by measurement of the internal H2 pressure.

High load batteries are the two examples, in Lines 11 and 12 of Table 1. For the negative electrode, hydrogen is used as active material instead of cadmium. Alloying constituents are released when the grid material is transformed into lead dioxide. Therefore, adequate cooling of the storage tank is required during charging.