High rate partial-state-of-charge operation of VRLA batteries

The world market for 12 V SLI batteries currently stands at around US$ 12 billion. The lack of a serious challenge from other battery types has allowed lead–acid products to serve this market exclusively, with minimal demand for product improvement through research and development, and a sharp competition has, over time, cut sales prices to commodity levels. The electrochemical storage of energy in automobiles now faces the possibility of a major change, in the form of the proposed 36/42 V electrical systems for vehicles that remain primarily powered by internal combustion engines, and of the hybrid electric vehicle. The duty cycle for these two applications sees the battery held at a partial-state-of-charge (PSoC) for most of its life and required to supply, and to accept, charge at unprecedented rates. The remarkable advances achieved with VRLA battery technology for electric vehicles during the past 8–10 years will be of only passing value in overcoming the challenges posed by high rate PSoC service in 36/42 V and HEV duty. This is because the failure modes seen in PSoC are quite different from those faced in EV (deep cycle) use. The replacement of the 12 V SLI will not take place rapidly. However, if the applications which take its place are to be satisfied by a lead–acid product (probably VRLA), rather than by a battery of a different chemistry, a program of development as successful as that mounted for deep cycle duty will be required. The present phase of the Advanced Lead–Acid Battery Consortium (ALABC) R&D program has begun to shed light on those aspects of the function of a VRLA battery which currently limit its life in high rate PSoC duty. The program is also pursuing the several technologies which show promise of overcoming those limits, including multiple tab plate design, mass transport facilitation and minor component (both beneficial and detrimental impurity) management. This paper presents a brief review of the changes which are taking place in lead–acid battery technology in response to the challenges of the new duty cycle.