Evaluation of lead/acid batteries under simulated electric-vehicle duty: development of design parameters on the basis of SFUDS performance

Abstract

As manufacturers move to meet the expanding demand in batteries for electric-vehicle (EV) applications, there is a need to develop and apply test schemes that provide a true measure of battery performance. The Simplified Federal Urban Driving Schedule (SFUDS) is one of several such duty profiles that have been derived from extensive studies of urban vehicle duty. Accurate implementation of the SFUDS is, however, difficult because the load is specified in terms of power and is varied every few seconds. This necessitates a sophisticated control strategy, combined with high-speed monitoring. In our laboratories, these requirements have been met by a digital measuring and control system in which all functions are handled by a microprocessor. SFUDS testing reveals battery performance to be critically dependent on the specific power capability. In particular, maximum vehicle driving range depends primarily on the proportion of current-generating materials that are present. For optimum performance, lead/acid batteries for EV service should be designed for minimum unit weight and maximum power output. In this way, the average rate of discharge is minimized and the battery voltage remains longer above the cutoff value. From these observations, it is suggested that the next generation of EV batteries will probably resemble present-day automotive (thin plate) batteries rather than the heavier (thick plate) units that are currently used in motive-power applications. They will also need to incorporate improved negative plates which are better able to withstand repetitive high-rate cycling. The latter is the defining feature of EV duty because it places severe demands on both the positive and negative plates.