The operating principle of the LAB, following the Gladstone and Tribe scheme, is as follows:

Negative electrode

$\text{Pb}+{\text{HSO}}_4^{-}\iff {\text{PbSO}}_4+{\text{H}}^{+}+2{\text{e}}^{-}({\phi }^{\text{o}}=-0.358\text{V)}$

Positive electrode

${\text{PbO}}_2+3{\text{H}}^{+}+{\text{HSO}}_4^{-}+2{\text{e}}^{-}\iff {\text{PbSO}}_4+2{\text{H}}_2\text{O}\text{(}{\phi }^{\text{o}}=1.690\text{V)}$

Cell

${\text{PbO}}_2+\text{Pb}+2{\text{H}}_2{\text{SO}}_4\iff 2{\text{PbSO}}_4+2{\text{H}}_2\text{O}\text{(}{U}^{\text{o}}=2.048\text{V)}$

where φ^o and U^o represent the electrode potential (φ) and the cell voltage (U), respectively, for the standard state (concentration ≈ activity = 1), that is, approximately $c_{{\text{H}}_2{\text{SO}}_4}=1\text{mol}/\text{L}$. In practice, higher sulfuric acid concentrations are used: 5.0–6.3 mol/L, 33–38% acid strength, and 1.24–1.28 g/cm³ specific gravity, so that practical cell voltages are higher. During discharge the sulfuric acid concentration is reduced, and the cell voltage is decreased in accordance with the Nernst equation. An empirical equation for the dependence of the open circuit cell voltage (U_o) on sulfuric acid concentration (in practice generally measured as density) is as follows:

$U_{\text{o}}\left(\text{V}\right)\approx 0.86+\text{sulfuric}\text{acid}\text{density}(\text{g}/{\text{cm}}^3)$

The LAB cell voltage (> 2 V) is higher than the decomposition voltage of water (> 1.23 V), which is a major component of the electrolyte (aqueous sulfuric acid). Therefore, the following corrosion reactions (local element reactions) are thermodynamically favorable and should lead to self-discharge of the active masses of the electrodes:

Negative electrode

$\text{Pb}\Rightarrow {\text{Pb}}^{2+}+2{\text{e}}^{-}$

${\text{H}}_2{\text{SO}}_4+2{\text{e}}^{-}\Rightarrow {\text{H}}_2+{\text{SO}}_4^{2-}$

$\text{total}\text{}:\text{Pb}+{\text{H}}_2{\text{SO}}_4={\text{PbSO}}_4+{\text{H}}_2$

Positive electrode

${\text{PbO}}_2+3{\text{H}}^{+}+{\text{HSO}}_4^{-}+2{\text{e}}^{-}\Rightarrow {\text{PbSO}}_4+2{\text{H}}_2\text{O}$

${\text{H}}_2\text{O}\Rightarrow 1/2{\text{O}}_2+2{\text{H}}^{+}+2{\text{e}}^{-}$

$\text{total}\text{}:{\text{PbO}}_2+{\text{H}}_2{\text{SO}}_4\Rightarrow {\text{PbSO}}_4+1/2{\text{O}}_2+{\text{H}}_2\text{O}$

The reaction rates of these self-discharge processes, however, are very low due to the high overpotentials for the gas evolution reactions. For self-discharge data see Section 5.2.3.2.

The above gas evolution reactions are equivalent to the overcharge reactions at the negative and positive electrodes, respectively:

Negative electrode

$2{\text{H}}^{+}+2{\text{e}}^{-}\Rightarrow {\text{H}}_2$

Positive electrode

${\text{H}}_2\text{O}\Rightarrow 1/2{\text{O}}_2+2{\text{H}}^{+}+2{\text{e}}^{-}$

As a result of this tendency to produce gases the LAB has, historically, not been sealed. The cell is equipped with a screw lid, which is gas- and partially liquid-permeable. Such cells are referred to as “flooded cells.”

Much effort has been expended in the past to find a way to close the cell without an unacceptable increase in pressure. This challenge was resolved with the aid of the oxygen recombination cycle: if oxygen developed at the positive electrode is able to reach the negative electrode, then the main overcharge reaction at the negative electrode is an oxygen reduction:

$1/2{\text{O}}_2+2{\text{H}}^{+}+2{\text{e}}^{-}\Rightarrow {\text{H}}_2\text{O}$

This is the result of the more positive potential of the oxygen reduction as compared with that of hydrogen evolution.

The reactions in the overcharge phase are in this case

Positive electrode

${\text{H}}_2\text{O}\Rightarrow 1/2{\text{O}}_2+2{\text{H}}^{+}+2{\text{e}}^{-}$

Negative electrode

$1/2{\text{O}}_2+2{\text{H}}^{+}+2{\text{e}}^{-}\Rightarrow {\text{H}}_2\text{O}$

and therefore the overall cell reaction is zero.

For practical use of this phenomenon, the oxygen must be transported from the positive electrode to the negative electrode. Diffusion of oxygen through the liquid phase is rather slow so the mass transport depends on immobilization of the electrolyte on inert separator materials with high surface areas. Small gas channels are created within the separator material, and these allow the oxygen transport to take place through the gas phase.

As a result, the LAB can be closed but not hermetically sealed. If the oxygen evolution reaction at the positive electrode becomes much higher than the oxygen reduction at the negative, a gas pressure is built up in the cell. A gas valve integrated into the cell lid is able to release excess pressure to the external atmosphere. Batteries comprising cells operating the oxygen recombination cycle are therefore called valve-regulated lead–acid (VRLA) batteries or sealed lead–acid (SLA) batteries.

Silica is the principal absorbent material used for the immobilization of the electrolyte. In one case the silica is porous, with an agglomerate diameter of 10–250 μm. The silica forms a gel with the electrolyte so that this VRLA battery variant is known as the gel type. In another case silica glass fibers with a diameter of 0.6–6 μm, arranged in a paper-like glass mat with a thickness of 1–4 mm are used. The VRLA battery with this glass mat separator is called absorbent glass mat (AGM) type.

5.2.2 Design

As previously mentioned, there are many electrode designs for the LAB. For stationary industrial applications with high lifetime requirement, the Planté-type negative electrode is used; a high-surface-area compact lead active mass is formed electrochemically. The so-called tubular type electrode (Figure 5.1) is used for the positive.

For automotive and transportation applications where power, weight, and costs are limiting factors, grid-type electrodes (Figure 5.2) are mostly used. The grid (cast, punched, or expanded) is pasted with precursor materials, which are then cured and formed into the final active masses.

Normally the positive and negative grids are kept apart by a separator and are stacked together to form a cell block, which is housed in a plastic compartment equipped with a cell plug. Normally six such compartments are contained in a battery monobloc case so that the battery has an open circuit voltage just above 12 V. In most cases the container has a central volume where the gases collected from all six cells are vented through a porous flame arrestor frit, which retains acid fumes and protects against ignition by external sparks (see Figure 5.3).

In some cases the plates will be spiral-wound (see Figure 5.4).

The spiral-wound configuration brings with it a high mechanical stability for the cell, and this allows the use of pure lead (99.999%), which has an inherently low mechanical stability but high corrosion resistance. In this way a high operating lifetime can be achieved.

The prismatic configuration shown in Figure 5.3 can be used for

∙ flooded-type cells,

∙ gel-type cells,

∙ AGM-type cells,

whereas the spiral-wound type is used mainly for AGM-type cells alone.

Currently automotive batteries are usually 12 V (six cell) monobloc units of either the flooded or the AGM design. Capacities (measured at the 20 h rate) range from about 25 to 110 Ah for cars, depending on size and the demands of electric accessories, and up to about 250 Ah for trucks. For heavy-duty commercial vehicles in Europe, the SLI battery nominal voltage has been established as 24 V (primarily driven by the cold cranking power demand), which is realized by two 12 V batteries of typically 150 Ah or higher capacity connected in series. In contrast, North American trucks historically used two 12 V units connected in parallel to enhance cranking power and capacity.

5.3.1 Lead

Inhalation and ingestion are the primary exposure routes that result in lead being taken up into the body. Absorption through the skin and/or scalp does not occur to a significant extent. The amount of exposure to lead is most commonly assessed by measuring the concentration of lead in blood. Noninvasive methods for measuring the lead content of bone (the repository for most of the lead in the body) have been developed but are complex and primarily applied in research settings. High levels of lead in the blood of adults (> 500 μg/L) can result in weakness, memory loss, and difficultly in concentration along with impairment of kidney and reproductive system function. The maximum allowable levels of lead in the blood of workers are generally 400–500 μg/L, with many industries working to lower voluntary limits of 300–400 μg/L. Lower levels of lead in blood are known to adversely impact the intellectual development of children, and a threshold for this effect has yet to be identified. As a result, blood lead “reference concentrations” for children and pregnant women have been adopted to maintain blood lead levels below 50–100 μg/L. Blood lead reference concentrations can be exceeded in the vicinity of battery manufacturing and recycling facilities or, more commonly, as a result of high levels of lead in paint, soil, food, water, or artisan ceramic-ware used in food preparation.

5.3.2 Sulfuric acid

Sulfuric acid, H₂SO₄, is highly corrosive, and eye contact can cause permanent blindness; swallowing damages internal organs that can lead to death. First aid treatment calls for flushing the skin for 10–15 min with large amounts of water to cool the affected tissues and to prevent secondary damage.

Environmental and safety problems with LABs could occur during production, use, and disposal/recycling.

5.3.3 Production process

During the production process both the lead oxide and grid processing, and the plate processing, are sources for environmental problems related to lead. Therefore strong regulations exist for air, soil, water, and disposal requirements for LAB production in industrial countries. In developing countries especially in older factories these high environmental standards are not well enforced, which may cause emission problems even on a global level. For example, a 2010 study documents that ~ 30% of airborne lead particulates in parts of California are being transported from Asia (Ewing et al., 2010). Fortunately the LAB is the most recycled of all modern industrial products (see below).

5.3.4 Use

During use there are mainly three hazard problems

• chemical hazards,

• fire and explosion hazards,

• electrical hazards.

The chemical hazard is mainly related to sulfuric acid, which could be spilled especially from flooded batteries.

The fire and explosion hazards are due to the possibility of oxygen/hydrogen gas mixtures occurring. During charge/overcharge hydrogen and oxygen are generated, and this mixture can explode if any spark is present. Such an explosion can occur, e.g., during a jump-start, if the supporting battery is connected incorrectly to the car battery, because in the connection process arcing can take place.

The electrical hazard occurs when sufficient cells are connected in series to create a voltage of 60 V or more.

5.3.5 Disposal/recycling

In industrial countries ≥ 95% of spent LABs are recycled and ≥ 80% of the lead that is need for LAB production comes from recycled lead. These are good values but not applicable for all countries. The average recycling efficiency in China, the world's largest LAB manufacturer, amounts to only to 80–85% and only 32% of the refined lead comes from recycling (IPE, 2011).

The incorrect disposal of LABs in landfill remains an issue in Asia and is a direct threat to the health and safety of the population in that part of the world.

Life-cycle assessments show that LABs have, either on a per kilogram or per watt-hour capacity basis, the lowest production energy, carbon dioxide emissions, and criteria pollutant emissions (Sullivan and Gaines, 2010) in comparison with other battery types. This is mainly caused by the high recycling rates and the limited temperatures needed during battery material production.

As described already in Section 5.2.2, the flooded SLI battery is designed to deliver short-time high power discharges and should have low cost but is not suitable for regular deeper cycling.

The standard flooded battery, however, cannot fulfill the demand of the micro-HEV when regenerative braking is in use (see Section 5.5). The AGM battery is better suited for the micro-HEV application as it provides much longer lifetime and a more consistent DCA. Problems with sulfation during PSoC operation can be reduced by the incorporation of carbon additives on the negative plate.

The gel battery has very long cycle life but the power capability is too low for the cold cranking function. Spiral-wound AGM batteries (see Figure 5.4) provide high power together with remarkable cycle life at manufacturing cost significantly above those of prismatic AGM. Neither the Gel battery nor the spiral-wound AGM battery have yet been used in micro-HEVs. Spiral-wound AGM batteries may, however, be considered as candidate for applications with extreme demands toward both power and cycling, e.g., mild hybrid traction batteries: Recent research by Exide has demonstrated that the addition of carbon to the negative active mass can also improve the high-rate PSoC life of the spiral-wound AGM battery markedly.

Automotive AGM batteries are about 1.5–2 times more expensive than the flooded SLI batteries of identical capacity and cranking performance. Therefore, enhanced flooded batteries (EFBs) have been developed with deep cycle life below the AGM battery values but similar shallow cycling performance and durability and at distinctly lower cost, only 20–40% above their conventional flooded counterparts.

The EFB design strategy is as follows:

∙ Higher positive paste density for longer cycle life but, simultaneously, the pore structure must be optimized to maintain the cold cranking performance;

∙ Use of a nonwoven scrim on the positive or both electrodes (replacing the pasting paper in continuous platemaking processes) to further enhance cycle life and also reduce electrolyte stratification;

∙ Additives (mostly carbon) for the negative active mass to reduce PSoC problems;

∙ Optimized grid structure and thinner plates together with more electrodes per cell block that minimize internal resistance and thus improve both voltage quality during automated restart and DCA.

With these improvements, several EFB designs have demonstrated to achieve almost equivalent cycle life as AGM at similarly high battery weight (2–3 kg above SLI) but significantly lower cost. Other OEMs (original equipment manufacturers = car manufacturers) have focused on weight-optimized EFB designs that are just good enough for the shallow cycling demands of microhybrid vehicles. First in Japan, and more recently in Europe, carbon additives have been used to allow further weight reduction at identical microhybrid service life. A few years after broad market introduction of microhybrid technology it can be seen that AGM batteries will be mostly restricted to premium car or commercial vehicle applications that add significant deeper cycling requirements to the microhybrid duty cycle, while the different grades of EFB can satisfy the majority of mass-market microhybrid needs.

The DCA of most EFBs and AGM batteries exceeds the acceptance criteria of the draft EN 50342-6 standard. However, further improvement of DCA remains a challenge in order to improve real-world fuel economy as well as to avoid undercharge issues in modern power supply systems with ever more demanding transient loads.

5.4.2 LABs with added carbon

In recent years different concepts have been developed for using carbon in LABs. The carbon in lead–acid technology offers the possibility of matching growing demands to microhybrid batteries with cost- and weight-efficient LABs. Moreover, it has been proposed to use this technology to address more demanding future automotive applications, such as mild HEV.

There are in general three concepts for using carbon:

 Carbon mixed homogeneously with the negative active material (NAM),

 Carbon on the grid side of the active mass (in place of the lead alloy),

 Carbon on the outside of the active mass (adjacent to the electrolyte).

Although the mechanisms by which various forms of carbon in or on the negative plate are able to improve high-rate PSoC performance are not fully understood, the main effects are thought to be:

 Capacitive contribution,

 Extension of the electrochemical active surface area, which reduces the actual current density (mA/cm²), that is, reduces polarization of the negative electrode,

 Physical effects, for example, for stabilizing and even increasing the AM porosity via additional nucleation sites and for impeding the growth of PbSO₄ crystals.

Optimization of the beneficial effects imposes different requirements on the types of carbon used. For example, for capacitive contribution the carbon should have a large surface area (small particle size), high double layer and pseudo capacitance, and high conductivity. The physical effects do not need electronic conductivity but are thought to operate most effectively with large carbon particles (in contrast to the capacitive process). It is not surprising, therefore, that combinations of more than one type of carbon have proved to be particularly effective.

5.4.2.1 Carbon mixed homogeneously with the NAM

Carbon is used as an additive for the NAM with the main effects to increase the surface area (see Figure 5.10) and to stabilize the structure (physical effect).

This approach can lead to a remarkable improvement of the DCA and of the cycle life even under PSoC conditions (see “Depth of discharge and dynamic charge acceptance” section). But the influence of the carbon on lifetime depends on the concentration and the type of carbon used (see Figure 5.11).

The type and the amount of the carbon addition to the negative active mass must be well coordinated with the organic expander used; otherwise, water loss increases, and cold-cranking performance decreases.

5.4.2.2 Carbon on the grid side of the active mass (in place of the lead alloy)

In this case a porous carbon material acts as a current collector, which is impregnated with a slurry of lead oxides and then formed to sponge lead on the negative plate as normal. Because of the porous structure, the resultant negative plate has an enormous surface-area advantage over conventional lead–acid grid structures. This results in much-improved active material utilization and enhanced fast-recharge capability. The foam structure, which encapsulates the active mass, can also lead to a higher lifetime (Figure 5.12) in high-rate charge applications.

This approach is being developed by both Firefly International Energy (with Microcell™ composite foam) and ArcActive Ltd. (with reticulated vitreous carbon that has been activated by an electric arc).

5.4.2.3 Carbon on the outside of the active mass (adjacent to the electrolyte)

In comparison to supercapacitors, the power of LAB is relatively low and limited by the negative electrode. To increase the power capability of the LAB the lead negative electrode is combined with a supercapacitor carbon electrode in the UltraBattery® (Figure 5.13).

In this design the capacitor electrode and the negative VRLA battery plate work in parallel, that is, the total current of the combined negative plate consists of the capacitor current and the VRLA negative-plate current. In this way the capacitor electrode can act as a buffer to share current with the lead plate and thus prevent it from being discharged and charged at high rates. With this strategy in place high cycle numbers (four times higher than for a conventional VRLA battery) can be achieved in high-rate PSoC operation (Furukawa, 2014).

5.5.1 Micro-HEVs (14 V systems)

To date, the microhybrid application, where the throughput and power demands are modest, is dominated by LABs, either EFB or AGM. It has been demonstrated that both technologies can support shallow cycling throughput exceeding 500 times nominal capacity. Even during more substantial PSoC cycling, for example, at 17.5% DoD (prEN 50342-6 levels M2/M3), more than 250 times nominal capacity are achieved by several EFB and all automotive AGM products. The deeper cycling requirements in the latter case would already exceed the demand of the typically shallow, microhybrid duty cycle, which gives rise to weight-optimized EFB solutions found in many passenger cars, particularly small cars or vehicles with manual transmission (SBA S0101:2006, prEN 50342-6 level M1).

The DCA of all LABs strongly depends on their short-term cycling history. Figure 5.14 illustrates a run-in experiment that simulates, without experimental acceleration, microhybrid battery operation under a set of difficult or worst-case conditions: engine after-run and key offloads discharge the battery before and during many, also long, parking events, idle-off loads and the like. Under these conditions, the DCA of typical flooded, enhanced flooded, and AGM batteries settles around 0.2 A/Ah. This is a run-in effect and should not be misinterpreted as aging: The battery can fulfill all power-supply system functions for years at this low DCA level. Actually the high initial DCA is an effect of the initial capacity test that was performed with the test samples; if this is omitted, DCA will be much closer to the run-in level right from the beginning of service. Some conventional flooded batteries show significantly lower DCA below 0.1 A/Ah (for example, poor flooded curve in Figure 5.14), which would not consistently support the stop/start functionality in urban traffic. Consequently, such low-DCA batteries will not be allowed to carry the stop/start label defined in prEN50342-6. Conversely, several innovative additives to the NAM or the electrolyte have been demonstrated to keep the DCA of EFBs around or above 0.5 A/Ah (for example, see the EFG gen-2 curve in Figure 5.14), usually at somewhat elevated gassing and water consumption levels. It can be expected that by optimization of material compositions and perhaps alternator operating strategies, such high DCA levels can be achieved robustly without deteriorating battery service life in warm and hot climates. EFBs with high DCA improve real-world fuel economy, reduce the risk of car breakdowns due to undercharge and sulfation, and offer further lead-weight reduction opportunities. In theory, similar concepts can be applied to 12 V AGM batteries. However, demand from vehicle OEMs regarding DCA or weight optimization of AGM appears to be weak, as the market for automotive AGM batteries becomes progressively constricted to premium applications with substantially deeper cycling requirements beyond just microhybridization.

In most customer vehicles, DCA will be somewhat higher than the worst-case results illustrated in Figure 5.14. Nevertheless, the alternator in modern vehicles can supply 150–250 A during deceleration, which by far exceeds the DCA of any known LAB technology: Even at 1 A/Ah, the typical starter battery sizes between 50 and 80 Ah would realize only a third to half of the brake energy recuperation in an optimized 14 V power supply system (Figure 5.15). As lithium-ion batteries do not share the DCA weakness described above, they have been suggested as an alternative starter battery technology that would, in addition, provide a significant weight reduction. However, their market penetration is low up to now due to some technical issues (e.g., limitations of cold-cranking and cold-charging power, incompatibility with engine bay package locations owing to heat intolerance) and very significant on-cost. As a consequence, several car makers have already introduced or are developing dual storage solutions that combine the robust lead–acid base starter battery with a high-power storage device that provides consistently high DCA (Warm et al., 2014) and protects the LAB against excessive cyclic aging (Schindler et al., 2012). It appears that lithium iron-phosphate and lithium titanate batteries are the most promising cell types for these add-on solutions because they can operate robustly within the voltage window above the open circuit voltage (OCV) of LABs and below the maximum automotive system voltage around 15 V, that is, they avoid the need for a dc/dc converter between both storage devices. In the longer run, it is certainly possible that lithium-ion technology makes further progress toward a complete drop-in replacement of the lead–acid starter battery.

5.5.2 Mild-HEVs and 48 V systems

The power limitations of the 14 V alternators can be overcome by introducing a higher voltage level for generator and battery, for example, 7–12 kW on 48 V nominal system voltages, which would allow avoiding certain cost-intensive high-volt safety measures that would be required above 60 V. A 48 V system enables recuperation of more energy during typical drive cycles than would be consumed in electric ancillaries and hence is typically designed as a mild hybrid that consumes excess electricity in propulsion functions, sometimes in combination with aggressive engine downsizing (see Section 5.6.4). Actually, the older mild hybrid systems that typically operated in the 120–150 V range are tend to be replaced by either full hybrids or 48 V systems. In addition to their powertrain functionality, 48 V systems are expected to enable additional electric functions (e.g., chassis comfort) that would exceed the power capability of 14 V systems. As the on-cost associated with the introduction of a second voltage distribution in vehicles is substantial, most market introduction scenarios assume a business case combined of a reduction in CO₂ emissions and new electric comfort functions in premium cars.

So far, all 48 V industrialization projects assume the use of high-power lithium-ion batteries or of supercapacitors where the focus is exclusively on high-power ancillaries. Recently, several experimental 48 V vehicles have been equipped with spiral-wound or prismatic AGM batteries that use innovative carbon additives in the NAM to enhance DCA. A string of 21 or 22 cells, that is, a battery nominal voltage of 42 or 44 V, fits into the 48 V system voltage specifications. Some examples are the LC Super Hybrid vehicle demonstration project funded by the UK government and Kia/Hyundai concept vehicles at Geneva and Paris motor shows in 2014. Because of higher battery weight, DCA run-in degradation, lower energetic charge/discharge efficiency, and inherent cycle life limitations, it is difficult for lead/carbon AGM batteries to outpace existing lithium-ion solutions technically. Nevertheless, several LAB developers believe they can offset these technical deficiencies by low material cost and applying mature automotive battery production processes, particularly in applications that are not primarily driven by CO₂ saving requirements but by new electric functions. When it comes to robust delivery of power at very low and very high temperatures, the lead–acid chemistry has an inherent advantage over lithium-ion.

5.5.3 EV applications

For a few “light” pure electric transportation applications such as golf carts, airport passenger transportation vehicles, and the like, LABs (often VRLA) are used. In the future there may be a class of battery electric automobile, such as the neighborhood EV, for which the limited range and relatively short cycle life are sufficiently offset by the low first cost of a lead–acid design, but for all vehicles with a range between charges of over 100 miles or 160 km, lithium-ion batteries will be needed.

5.6.1 The addition of carbon

The addition of certain types of carbon in a variety of configurations has been shown to provide a very significant improvement in the performance of LABs that are intended for use in the high-rate PSoC duty that is the characteristic of HEVs. Although the parent technology has a very long history, this special exploitation of carbon is relatively new, and it is likely that the optimization process will yield rewards for some time to come. See more about this topic in Section 5.4.2.

5.6.2 Bipolar batteries

Bipolar cell configurations are used especially for fuel cells but also sometimes in batteries that are intended to provide high voltage and high power. In the bipolar design the geometric electrode surface, and therefore the capacity, is limited by the cross-section of the bipolar stack. The current path and cell weight can be minimized, and the current density is very homogeneous across the electrode (Saakes et al., 2005). Although there have been many attempts to develop this design of battery, they have all failed commercially, mainly because of corrosion of the bipolar plate and as a result of insufficient cell sealing. The only non-lead electronically-conducting bipolar plate materials with high corrosion stability that have shown promise are vitreous carbon, fluoride-doped tin dioxide, titanium suboxides (Magneli phases), or lead-infiltrated ceramic material.

Despite the challenges of this system, several companies continue to seek the final breakthrough.

5.6.3 Grid design

The current distribution across a flat plate electrode is nonuniform, especially at high rates. The equipotential curves of a state-of-the-art electrode with a central lug (Figure 5.16) show that, due to high potential losses, the lower part of the electrode is hardly involved at all during high rate discharge.

A consequence of this finding is that it is advisable to use shorter electrodes or to use two lugs (one at the top of the cell and one at the bottom) on each plate when high-current operation is required. The so-called Rholab battery (Figure 5.17), which incorporated the latter principle, was used successfully as a much cheaper alternative, to replace the original nickel–metal-hydride battery in a Honda Insight.

A twin-tab design is not simple to manufacture, but the importance of high power, particularly during charge events, dictates that development of a grid design to match the needs of HEV batteries is likely to continue.

5.6.4 Batteries for downsize and for boost

As shown in Table 5.1 mild HEVs, full HEVs, and plug-in hybrid electric vehicles (PHEVs) show in relation to micro-HEVs higher fuel savings/CO₂ emission reductions, but at much higher specific costs ($/% CO₂ saving).

One approach for having higher fuel savings at low specific costs is to reduce the size of the engine of conventional cars and making good the consequent loss of power by the addition of an enlarged turbocharger, electric supercharger, and a 48 V system (see Figure 5.18).

Using the same concept but without downsizing the engine is another approach to achieving higher car performances, but not to quite as high a fuel savings as the first approach.

The Advanced Lead–Acid Battery Consortium (ALABC) conducted two relevant projects:

 Downsize project: 2 L Ford Focus with a radically downsized (1.0 L) three-cylinder gasoline engine, enlarged turbocharger, electric supercharger, and a 220 F supercapacitor for energy storage.

 Boost project: 1.4 L TSI Passat, without downsizing but with enlarged turbocharger, electric supercharger, and a 42 V/24 Ah Exide Orbital lead–carbon battery for energy storage.

Based on these projects first approximations for savings and costs by super hybrids have been done:

 OEM on-cost: €750–1500,

 CO₂ emission reduction: 15–30%,

 Specific OEM cost: €50–60/% CO₂ emission reduction.