Nickel Metal Hydride Batteries Expanding GM’s EV Portfolio

December 3, 1998 - General Motors is advancing to the next generation of batteries in its electric vehicles - the GM EV1 and the Chevrolet S-10 Electric Pickup truck. A move toward advanced technology nickel-metal-hydride (NiMH) batteries is aimed at improving three key aspects of performance -- driving range between charges, cold-temperature operation, and the service life of the battery pack.

Nickel-metal-hydride is an advanced battery technology for automotive applications. The batteries are manufactured by GM Ovonic, with research support from the U.S. Advanced Battery Consortium. GM Ovonic is a joint venture between General Motors and the Ovonic Battery Company, a subsidiary of Energy Conversion Devices, Inc. of Troy, Michigan.

With the availability of NiMH batteries in the EV1, it delivers the greatest range (75-140 "real world" miles), performance (in published tests, 0-60 miles per hour in under eight seconds) and power (137 hp.) of any production electric vehicle. The NiMH S-10 Electric Pickup truck delivers 65-80 "real world" miles on a single charge, and the greatest performance (EV America tests, 0-50 mph in ten seconds) and power (114 hp.) of any production electric vehicle for commercial use.

During the past decade, this type of battery has proven itself very desirable in high-volume commercial applications such as portable computers, cellular telephones, and video cameras. More than a dozen manufacturers currently produce 300-million NiMH cells per year.

A NiMH battery has little in common with the valve-regulated lead-acid (VRLA) batteries that powered the first-generation EV1 and S-10 Electric. Invented more than a century ago, the lead-acid cell uses a lead anode (negative electrode) and a lead-dioxide cathode (positive electrode) surrounded by a sulfuric acid electrolyte contained within a polypropylene case. Typically, six such cells are connected in series to form a 12 volt battery module. The NiMH battery was patented in 1986 by Stanford Ovshinsky, currently the president and CEO of Energy Conversion Devices and the CEO of GM Ovonic L.L.C.

With NiMH, the cathode is a nickel compound and the anode consists of vanadium, titanium, zirconium, nickel, and chromium alloyed to create a metal hydride (microscopic lattice structure containing hydrogen). The electrolyte is a strong alkaline solution of potassium hydroxide (KOH) and water. Both electrodes and the electrolyte are sealed within a stainless-steel case. Each NiMH cell produces a nominal 1.2 volts. Eleven cells are connected in series to form a 13.2 volt battery module.

NiMH batteries work by storing and releasing hydrogen ions during the charge and discharge processes. Just as with lead-acid, NiMH batteries are sealed, require no maintenance, and can be safely recycled at the end of their useful life.

To compare battery technologies, engineers study physical and electrical characteristics such as mass and stored energy. Two other ratings are specific power and specific energy. The energy performance per unit mass and volume is where NiMH batteries demonstrate a major advantage over any lead-acid design.

Specific power is generally quoted in watts per kilogram (W/kg)--the power available per unit mass. The NiMH batteries have equivalent specific power to the Generation I lead-acid batteries. In over-the-road performance, specific power correlates most directly to acceleration performance. With the optional NiMH batteries, the EV1 will continue to have spirited responsiveness. (In published Motor Trend tests, the GM EV1 accelerated from rest to sixty mph in 7.7 seconds, outgunning a BMW Z3).

Specific energy is quoted in watt-hours per kilogram (Wh/kg). Here the NiMH battery doubles lead-acid ratings with 70 versus 35 Wh/kg. The specific energy of a battery pack is directly proportional to an electric car's over-the-road range (all other factors such as weight, aerodynamic drag, and propulsion efficiency being equal). In other words, an EV1 with NiMH batteries can expect nearly twice the range of their lead-acid predecessors between recharges depending on conditions such as terrain, temperature, accessory use and driving style.

NiMH batteries bring other noteworthy advantages. A cycle life that's significantly better than the first generation lead-acid batteries and NiMH batteries accept high-rate charging very well so 15 or 20 minutes at 50 kW may be enough to boost the state of charge to the 80 percent level under optimum conditions.

NiMH batteries have only just begun to reach the commercialization stage for automobiles because they are more expensive to produce than lead-acid batteries. GM Ovonic L.L.C., supplier of the NiMH batteries, has been awarded a contract by the United States Advanced Battery Consortium (USABC) to focus on reducing the cost of this technology. NiMH batteries for the GM EV1 and the Chevrolet S-10 Electric vehicles permit GM to maintain the leadership role in the electric vehicle market.