While lithium-based chemistries generally are considered to be the future battery technology for electrified vehicles, the auto industry’s pre-eminent battery expert and its leading producer of hybrid vehicles agree on this caveat: Don’t count nickel metal-hydride chemistry out yet.
“Nickel metal-hydride has been extremely reliable in use, and its cost per kilowatt-hour continues to come down,” said Dr. Menahem Anderman, President of Advanced Automotive Batteries, an industry consultancy. “Its performance has been proven for more than 10 years.”
Speaking at the 2010 SAE Hybrid Vehicle Technologies Symposium in San Diego, Anderman argued that for many hybrid applications, NiMH batteries can offer much of the performance of Li-ion packs, for a fraction of the total systems cost.
He acknowledged the power-density benefits of Li-ion and its potential for the upcoming generation of plug-in hybrid and EV batteries. Rising volumes and greater production efficiencies will help reduce Li-ion’s cost in the next decade, he said.
But Anderman also challenged the conference’s 340 attendees regarding lithium’s unproven history in the extremely tough automotive environment.
“Can the [battery] industry design a cell that will not catch fire under any circumstances?” Anderman asked the audience of engineers. “Does battery pack design guarantee that if individual [Li-ion] cells catch fire, the fire does not propagate?"
Anderman noted that automotive duty cycles and product life cycles are “very different than in the consumer electronics sector,” which Li-ion has dominated. Proper validation of Li-ion packs, including ensuring critical crush performance and overcharge protection, requires actual 10-year and 150,000-mile (241,000-km) durability testing.
Despite Li-ion being widely anointed as the next vehicle battery chemistry, Toyota is keeping its battery-chemistry options open.
While it plans to deliver plug-in hybrids and EVs with lithium technology and is testing a fleet of Prius PHEVs with Li-ion packs, the automaker also recognizes that the high durability, good reliability, and lower cost of NiMH will keep it in automotive battery use until there is at least a 50% reduction in Li-ion’s cost. This could take another decade to achieve.
Toyota engineers consider conventional gasoline-electric HEVs to be the “mass market” hybrids in the short- to midterm. Sales of PHEVs and EVs, the engineers expect, will be more limited.
Toyota and battery partner Panasonic EV have reduced the cost of automotive NiMH packs by nearly 75% since the original Prius debuted in 1997, noted Shunsuke Fushiki, a Toyota hybrid-vehicle engineer who also presented at the SAE Hybrid meeting.
The NiMH pack Toyota uses has an energy density of about 50 kW·h/kg. This compares with Li-ion, which in typical automotive applications is in the >110 kW·h/kg range.
Li-ion charges and discharges faster than NiMH, and it's inherently lighter. This enables a smaller battery pack that can provide the bursts of power required by a hybrid. NiMH, by comparison, offers nearly “bulletproof” reliability due to refinement of cell architecture and improvements in pack thermal management and power control, according to engineers.
Toyota sees opportunity in further NiMH cost reduction, mainly in manufacturing due to the company's long experience in HEV production. The chemistry also is considered mature and unlikely to change, unlike some of the still-evolving lithium-based chemistries. Some component-technology analysts believe Toyota’s systems costs on the current Prius are virtually on par with those of Honda’s much more basic assist-only IMA system.