The status of high-power batteries for hybrid electric vehicles

Abstract

This paper reviews the status and prospects of high-power batteries intended for application in power trains for hybrid electric vehicles. It is based on a survey of a d v a d batteries conducted by the author for the Califomia Air Resources Board in the first half of 1999. Hybrid electric vehicle (HEV) concepts and technology are still evolving, as are the associated requirements for energy storage. The most important requirements atablished or implied for several generic HEV typa are discussed to help judge the applicability of candidate batteries. On that basis, high power versions of nickel-metal hydride, lead acid, and lithium ion batteries meet the performance requirements for the power assidregeneration BEV application. Nickel-metal hydride, lithium ion and lithium polymer batteries appear suited for hybrid electric vehicles that have significant driving range on battery power alone. The status of these technologies and their prospects for availability w i t h the nexl 3-5 years are discussed. The emergence of hybrid electric vehicles promises viable markets for advanced batteries (as well as for electric power conversion, control and motor technologies) in electric transportation Hybrid electric vehicles especially those with significant battery-only driving range also might become stepping stones in the commercial introduction of electric vehicles that take advantage of the remarkable advances in battery technolog achieved over the past five years. Introduction The successful introduction of Toyota’s PRIUS hybrid vehicle in Japan and the announcements by Ronda and Toyota of plans to introduce their hybrid vehicles in the U.S. market have dramaiically increased interest in this new automotive product. Automobile mnufacturets, regulators and environmentalists see HEVs as a potentially major avenue to increasing vehicle energy efficiency and reducing the emissions of air pollutants. Enam and environmental policy leaders view HEVs especialiy those that derive a significant portion of the propulsion energy from electricity as a stratqy to replace imported oil with domestic energy resources and reduce the emissions of carbon dioxide. Final@, electric utilities consider grid-connected hybrid electric vehicles a possible business opportunity. Batteries are an essential component of the hybrid electric vehicle types currently under development. In the functionally simplest KEV concept, the battery is employed to store energy captured in regenerative braking plus make-up energy provided by the engine, to keep the battery at approximately 50% state-ofcharge (SoC). The stored energy is used as needed to start the vehicle’s combustion engine and assist the engine during acceleration. The Toyota PRIUS and Honda’s recently announced INSIGHT HEV are exampIes of this “Power AssisuRegmration” (PAIR) -hybrid electric vehicle. Most of the hybrid vehicles currently under development at the three major U.S. carmakers also fall in this cntegoty. This type of HEV requires only a rather small battery that, however, must have very high specific power capability, as discussed below. Hybrid batteries and their controI systems can be designed to supplement not only the power but also the energy delivered by the combustion engine. In that case, the battery is charged by an off-board power source, and it gives the vehicle a limited range over which it can be driven with battery power only, with the associated advantages of zero local emissions and yet lower consumption of oil-derived fuels. Prototypes of hybrid electric vehicles with usable electric range termed “grid-connected”, “charge-depletion” or, in this paper, electric-range (ER) HEVs have been demonstrated. ER hybrids must meet different requirements, as discussed below. Yet another type of hybrid electric vehicle is created if a very smdl combustion engine is added to an electric vehicle to provide emergency driving rangeFrom a battery standpoint, there is little if any difference between the type of battery required for this “range extender” HEV and those for “pure” battery-powered electric vehicles. Ba#ery Requirements and Performance Targets Power assist/regeneration must meet demanding requirements. The most important of these, and the implications for the needed battery performance, are swnmarized in the first data