Electric Drivetrain Technologies

Introduction

The appeal of using electricity to power automobiles is that it would eliminate vehicular air pollution (although there would still be pollution at the power source), and that electricity can be reversibly translated to shaft power with precise control and high efficiency. The main problem with this use is that electricity cannot be easily stored on a vehicle. California’s mandate for the introduction of zero emission vehicles in 1998 has resulted in a major research effort to overcome this storage problem. The only commercially available systems for storage today, however, are the lead acid and nickel-cadmium battery, and both have limited capabilities. The lead acid battery’s limited storage capacity and substantial weight are ill-suited to a vehicle’s needs, although advanced versions of this battery reduce some of these limitations; the nickel-cadmium battery is very expensive and requires careful maintenance.

Electricity can also be produced onboard a vehicle by using an engine and generator. Simply feeding the generated electricity directly into a drive motor to power the wheels, however, would probably be less efficient than a mechanical transmission, because the combined generator and motor losses may outweigh transmission losses. The total system can be made more efficient, however, if the engine is operated at near constant output close to its most efficient point, and any excess electricity is stored in a buffer, which is used to satisfy the variable electrical demands of the motor and other vehicle power demands. Vehicles with powertrains combining a device to store electrical energy and another to produce it are called hybrids. The storage or buffer device can be an ultracapacitor, flywheel, or battery, depending on system design; the electricity producer can be an internal combustion engine or, perhaps, a fuel cell, which would be both highly efficient and almost non-polluting.

The sections that follow discuss new technology under development for batteries for electrical energy storage, fuel cells for energy production, capacitors/flywheels for peak power storage, and motors for conversion of electrical power to shaft power. The discussions focus on a selected set of technologies likely to be competitive in the future marketplace (at least according to current wisdom), and their efficiency and cost characteristics. The data and descriptions presented in this section can become out-of-date very quickly, especially if there are breakthroughs in the design or manufacturability of the technologies. Hence, the projections in this section represent an extrapolation of technology performance into the future based on information mailable as of mid-1994. New technology competitors may emerge very quickly and new findings may render existing “competitive” technologies poor prospects for the future.

Battery Technology

Requirements

A battery is a device that stores electricity in a chemical form that is released when an external circuit is completed between the battery’s opposing terminals. The battery, which provides both energy and power storage, is the critical technology for electric vehicles. Unfortunately, the weak link of batteries has been their low energy storage capacity--on a weight basis, lower than gasoline by a factor of 100 to 400. Power capacity may also be a problem, especially for some of the higher temperature and higher energy batteries. In fact, power capacity is the more crucial factor for hybrid vehicles, where the battery’s major function is to be a load leveller for the engine, not to store energy. Aside from increasing energy and power storage, other key goals of battery R&D are increasing longevity and efficiency and reducing costs.

Traditionally, the storage characteristics of conventional lead-acid batteries have been so poor that electric vehicles (EVs) have been extremely heavy, with poor acceleration performance and limited range. Battery technology research sponsored by the U.S. Advanced Battery Consortium (ABC) has sought to develop new batteries with improved storage and other characteristics. The performance characteristics of a battery relevant to use in vehicles can be defined by the following parameters, for which ABC has set goals.

The specific energy is a measure of the total quantity of energy stored per unit of battery weight. ABC has set a goal of 80 watt-hours/kilogram (with 100 Wh/kg desired) as a mid-term goal and 200 Wh/kg as a long term goal for this parameter. In contrast, conventional lead acid batteries have specific energy levels of 25 to 28 Wh/kg.

Specific power is a measure of how much power per unit weight the battery can deliver per second to handle peak requirements for acceleration and grade climbing. ABC’s mid- and longterm goals are 150 W/kg (200 W/kg desired) and 400 W/kg respectively for a 30-second pulse of power. Conventional lead acid batteries can provide as much as 100 W/kg when fully charged, but their peak power capability declines rapidly as they are discharged, and is about 60 W/kg at 80 percent depth-of-discharge (DoD). To some degree, specific power is a function of battery design, and especially trades off with specific energy. Hence, batteries designed for high power may differ from those designed for high energy.

The sustainability of peak power levels is an important issue for hybrid vehicles. The peak power values quoted in this section are based on a 30-second pulse. Batteries may not be able to sustain even half this peak level, if the duration is in the order of two to four minutes. However, the capability of the battery to deliver high power is a function of its design as well as the battery cooling system installed to prevent thermal degradation. At this point, it is unclear whether all of the battery types described below can provide half the rated peak power for several minutes, as is required for a hill climb.

Life can be based on both calendar years and charge/discharge cycles. USABC has set mid- and long-term goals of 5 and 10 years and 600 and 1,000 cycles respectively. Conventional lead acid batteries in electric car use have a life of only about two to three years and 300 to 400 cycles. For some batteries, calendar life and cycle life may present different limiting constraints, and the life itself is affected by how deeply a battery is discharged.

There are several other parameters that are of major concern, such as the power density and energy density, which are measures of battery power and energy storage capabilities on a volumetric basis (to avoid very large batteries), power and energy degradation over the useful life, fast recharge time, range of ambient operating conditions, maintenance requirements, and durability. USABC goals for some of these parameters are shown in table 3-10. In addition, there are special concerns with each battery type that include behaviour at low charge, special charging characteristics, and recyclability. This review of batteries is not meant to be comprehensive nor intended to cover all of the above factors. Rather, the intention of the review is to describe auto manufacturer concerns and battery manufacturer inputs on the current status of battery development, while the conclusions reflect only OTA’s opinion on battery prospects.

Credible specification of battery parameters is critical to judging EV capabilities, but in fact such specification is difficult to come by. Measuring battery parameters raises many issues, as the results are sensitive to the test procedure and ambient conditions employed. For example, most batteries display reduced energy densities at higher power levels, as well as during cyclically varying power draws (as will be the case in an electric vehicle). Yet, specific energy values generally are quoted at a constant discharge rate that would drain the battery in three hours (c/3). As noted, many batteries also display significant reductions in power density at low state-ofcharge, and at reduced ambient temperatures, while available data may be for filly charged batteries at 20oC. Finally, battery characteristics are often different among single cells, modules, and collections of modules required for a high-voltage battery. In many battery types, the failure of a single cell, or variations (owing to production tolerances) between cells often has significant impact on battery performance.

Auto manufacturers interviewed by OTA universally agreed that many battery manufacturer claims about battery performance and longevity are unlikely to be reproduced in a vehicle environment. European manufacturers have devised new testing procedures through their joint consortium, EUCAR, that appear to be more stringent and comprehensive than those performed previously by USABC or by DOE affiliated laboratories;68 similarly, USABC in 1994 also revised its testing procedures, which are now reported to be very stringent. Auto manufacturers stressed the need to test an entire high-voltage battery system with the thermal and electrical management systems included as part of the overall system to obtain a good picture of real-world performance.