Elsevier

Journal of Power Sources

Volume 174, Issue 1, 22 November 2007, Pages 49-53
Journal of Power Sources

Short communication
Lead–acid battery chemistry adapted for hybrid electric vehicle duty

https://doi.org/10.1016/j.jpowsour.2007.06.065Get rights and content

Abstract

Lead–acid batteries that are intended for traditional tasks such as SLI or deep cycle duty do not perform well in hybrid electric vehicles (HEVs). With the benefit of a few straightforward modifications in design, however, batteries deploying lead–acid chemistry can satisfy the performance requirements of power-assist HEVs in both bench- and in-vehicle tests.

Introduction

The lead–acid battery community anticipates a serious challenge during the next few years as its largest market, i.e. for SLI batteries, faces the prospect that conventional 12 V batteries will be substantially replaced by batteries designed for vehicles with a markedly higher electrical power requirement. The crux of the challenge is that neither conventional 12 V SLI batteries nor present generation deep-cycle batteries are able to perform the duty required by the new, high-power, automobile systems for an acceptable life. Batteries for these high-power systems will operate from a partial-state-of-charge baseline and will be discharged, and particularly re-charged, at extraordinarily high rates (albeit within a small range of state-of-charge). Within such duty, the life-limiting mechanism appears to involve the progressive accumulation of lead sulphate on the negative plate. This failure mode appears as a result of the very high rates of recharge and persists because the battery is not routinely returned to a full state-of-charge in the required duty. Partial-state-of-charge operation does bring one benefit, however, in that, at intermediate states-of-charge, charge-acceptance can be extremely high.

In order to offer an acceptable life in such applications, conventional designs of VRLA batteries must be revised. The battery must be able to sustain the negative plate charge reaction at very high rates, overcoming diffusion limitations (leading to reduced lead sulphate solubility etc.) which would otherwise lead to the onset of secondary reactions, such as hydrogen evolution, and charge inefficiency.

There are two straightforward design modifications that offer the potential to redeem this situation and to allow the lead–acid battery to perform successfully in the high-rate partial-state-of-charge (HRPSoC) routine demanded in hybrid electric vehicles [1]: The provision of an appropriate grid design allows the plates in the battery to accept the high charge rates required; and the incorporation of elevated concentrations of carbon (a few wt% instead of the traditional 0.2 wt%) alleviates the tendency for sulphate to accumulate, and appears to offer the route to a long operating life in the HRPSoC regime.

This paper provides an early indication of the successful operation of lead–acid batteries that incorporate these features, both in the laboratory and in hybrid electric vehicles on the road.

Section snippets

Grid design

The potential distribution (and hence the current density) across the plates of a battery of monopolar design is never uniform (Fig. 1). The current density gradients across the plate become steeper as the rates (of charge and discharge) at which the plate is being used increase. At the very high rates that are normal in hybrid electric vehicle duty, current can be so concentrated in the region adjacent to the current take-off lug that much of the plate (and much of the active material) is

Composition of the negative active mass

When conventional lead–acid batteries are exposed to HRPSoC duty they generally fail quickly as a result of the accumulation of lead sulphate on the negative plate. There is a concentration of the lead sulphate at the top of the plates, as described above. There is also a concentration of sulphate at the surface of the plates and this may have to do with diffusional limitations in the liquid phase. It has become clear that this second type of sulphate accumulation can be relieved to a

Micro–mild operation

A 36 V pack consisting of six 6 V, 25 Ah spiral-wound cells containing elevated levels of carbon materials in their negative plates has been evaluated in a Ford Focus mild hybrid operated on a dynamometer [6]. The vehicle has been run repeatedly through a 1 min operating cycle that exposes the battery to a sequence with a maximum discharge of around 200 A and maximum charge of over 25 A, whilst maintaining the string in a ±1% SOC window, in this case around 40% SOC. At first, the voltage during

Conclusions

The behaviour of lead–acid batteries that have been designed for SLI or for deep cycle use is not a reliable indicator of how other forms of lead–acid-based batteries might function at high rates from a partial-state-of-charge. In fact, when designed specifically for purpose, batteries that make use of the lead–acid chemistry are quite capable of providing useful service in HRPSoC regimens. In the medium hybrid application (offering regenerative braking, and power-assist but little or no

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