Manufacturing EV1 and S-10 Electric NiMH Batteries

December 3, 1998 - GM’s EV1 and S-10 Electric Pickup truck will soon take a major step forward with the adoption of nickel-metal-hydride (NiMH) batteries. The key benefits are longer driving range than the first generation lead-acid batteries currently used in the EV1 and S-10 Electric, longer battery life and improved cold temperature operation.

In 1994, GM established a joint venture with Ovonic Battery Co. (OBC, subsidiary of Energy Conversion Devices) to develop and manufacture NiMH batteries for use in electric vehicles (EV) and hybrid electric vehicles (HEV). GM Ovonic President John Adams notes, "One of the reasons GM chose NiMH batteries over competing energy-storage devices is because many of the manufacturing processes necessary to make these new batteries are similar to the process we’ve perfected for lead-acid batteries. So the challenge was not to invent new processes but rather to improve existing methods from the standpoint of reliability and quality."

While GM was able to tap more than seven decades of lead-acid battery manufacturing expertise for NiMH production, there were still problems to solve and challenges to overcome to produce NiMH batteries with the required quality levels. One was to set our sights on error-proof manufacturing operations. Adams explains why mistakes are intolerable, "If you have a one-percent defect rate with starting-lighting-ignition lead-acid batteries, most of the cars relying on that energy source to crank their engines, will start every morning. But an EV battery pack is wired in series like a string of Christmas tree lights. Only one weak cell can limit the pack’s ability to work.

"The EV’s stringent reliability requirements push our whole production process toward that error proof goal. The control plans that spell out each job instruction have been written to prevent errors that could result in a product defect. In addition, we have electronic controls built into our processing to minimize the chance of manufacturing our product incorrectly. For example, we weigh all electrodes, and if weight is not within narrow specifications, they will not be used.

"A major change we made over conventional battery manufacturing is the use of continuous versus batch processing. Continuous processing requires extra engineering resources and greater investment in equipment but the payoff is reduced variation and less waste." Manufacturing manager Jim Greiwe, explains "we want production to flow like water -- without stopping. So we balance the flow to match our customers demand. Instead of gearing up to suit a machine’s capacity or cycle time, we adjust that machine to match the flow rate we want."

While the manufacturing process behind the NiMH and lead-acid batteries is similar, the raw materials are completely different. The nickel based materials account for 70% percent of the battery cost while lead accounts for only 40% in lead-acid batteries. This makes it essential to minimize waste during manufacturing. At GMO, efficient use of raw materials is a top priority.

GMO manufactures the positive electrode using nickel foam that is coated with a nickel hydroxide slurry. The negative electrode is composed of an alloy containing various metals and is purchased from our joint venture partner OBC. Both electrodes are separately cut to size and have a nickel tab welded to the top. After weighing and balancing, the electrodes are encapsulated in a separator material that provides the electronic insulation necessary to prevent contact between adjoining electrodes and holds the electrolyte necessary for ionic conduction through the battery. The electrodes are welded to terminals and the electrode assembly is placed in a stainless steel container.

During electrical formation, a process that consists of a programmed series of charge and discharge cycles, each battery is tested to assure it delivers its full rated power and capacity.

To manufacture a NiMH battery, GM Ovonic goes through some five dozen carefully controlled processing steps. The costs and effort are considerably greater than building a lead-acid battery but there is no doubt that doubling the energy density of the EV1 and S-10 Electric battery pack is well worth the extra toil.