The auto industry has focused so strongly on the lithium-ion (Li-ion) battery because of its light weight and good power density that it might seem the nickel metal-hydride (NiMH) battery soon will be displaced as the technology of choice for vehicle electrification. However, an argument for continued dominance of NiMH is being made by the world leader in hybrid-electric vehicles (HEV):Toyota.
The company isn't ignoring Li-ion. It is testing a plug-in-hybrid (PHEV) Prius fleet and is promising to deliver plug-in hybrids and all-electrics with this battery. However, Toyota also believes that the lower cost and proved durability and reliability of NiMH will keep it ahead of Li-ion until there is at least a 50% reduction in the latter's cost—and it sees that as possibly 10 years down the road.
Toyota sees the HEV as the mass market, with PHEVs and all-electrics very limited in sales. Some U.S. market studies show the alternatives may gain only a small market share (3% or lower in the near term) because of cost and limits to Li-ion life. Toyota and battery partner Panasonic have reduced the cost of the NiMH by almost 75% since the original 1997 Prius, according to Koei Saga, the Toyota executive who heads battery development.
Saga, who is Managing Officer, Advanced Technology Vehicles and Battery Development, spoke at a technical session during the North American International Auto Show in Detroit. He said that although current NiMH power technology is "mature," there is opportunity for further cost reduction. Toyota is believed to have industry-leading cost advantages for its full-hybrid, close to the more limited assist-only Honda system. And with its engineering improvements in more efficient motors and battery management, Toyota HEV costs are reportedly very close to those of a small diesel.
The NiMH that Toyota and other HEV makers use are the small-cell types, rated by Toyota at an energy density in the low 50s kW·h/kg vs. automotive Li-ion, which is in the 110 kW·h/kg and up range. The small-cell types were unaffected by long-term development disputes involving Cobasys, once a joint venture between General Motors Co. and ECD Ovonic, an NiMH technology pioneer. Control of Cobasys, which holds key patents in large cells with greater energy density, was purchased from GM by Chevron, which reportedly sought large production contracts that no carmaker would provide. Cobasys control was purchased last year from Chevron by a Bosch-Samsung joint venture. Under the new ownership, there might be more access to and use of large-cell NiMH technology.
Toyota RAV4 electric vehicles were made from 1995-2001, and since 1997 nearly 1500 were equipped with an early-development large-cell NiMH pack (12-V, 24 batteries, 288 V total, rated at 95 A·h), replacing a lead-acid array. However, a patent dispute between Toyota/Panasonic and Cobasys over the design ended the RAV4 EV project. Some 800 remain in service, all with the original NiMH pack and many with over 170,000 mi (275,000 km), a Toyota spokesman said. Only a few cells ever were replaced.
Ford built some 1500 Ranger pickup electrics from 1998-2002, initially with lead-acid batteries, later with a Panasonic large-cell NiMH pack similar to the RAV4's. The pickups were leased (most to the U.S. Post Office) until the project ended in 2004. Most were destroyed, but a handful ended up in private ownership. Although the Ranger and RAV4's batteries were 1990s technology, the large-cell design delivered power density in the mid-to-upper 50 W·h/kg range, and ECD Ovonic has shown advanced cells with densities of 80 W·h/kg.
Li-ion is lighter than NiMH and charges and discharges faster, so a smaller battery pack can deliver the bursts of power used by a hybrid. However, NiMH has been improved, and early issues with hot-weather charging and self-discharge are largely solved. Although nickel prices have been increasing, Li-ion also is affected as nickel-based Li-ion is Toyota's choice, Saga said, because it is a more robust design for cold-weather operation.
Saga is comfortable with the Prius's 10-year, 150,000-mi (240,000-km) battery warranty. Most estimates of Li-ion life are in the five-year range. GM reportedly hopes to provide the needed 10-year life for the forthcoming Chevrolet Volt PHEV by "de-rating" the Li-ion pack; that is, installing a pack that could go considerably further than the 40-mi (64-km) all-electric distance now promised. That pack, therefore, is somewhat larger, heavier, and more costly than the Li-ion packs other PHEV makers may choose. Might an advanced NiMH battery for a plug-in, even if still heavier than the Volt's Li-ion battery, come close in marketable quality at lower cost? Obviously such large companies as Bosch and Samsung see a future for NiMH.
Carmakers are moving cautiously with Li-ion. They must determine how far economies of scale can be pushed and how robust next-generation Li-ion chemistries are. High volume would be risky, as no carmaker can afford the potentially huge expense of a battery that fails to live up to projections on which pricing is based. So for the foreseeable future, low volume will be used to prove the technology. Risk will be managed by de-rating and by using market plans based on shorter life and leasing the packs.
The bottom line is that NiMH has the proved durability and the pricing advantage to dominate the market when it comes to high volume. Toyota is at 50 mpg with the midsize Prius and has made it clear that HEV improvements are far from over.