A May 24 letter suggests that "no one has addressed what happens when tons of heavy-metal batteries wear out in a few years."
As the inventor of the Ovonic Nickel Metal Hydride Battery, Energy Conversion Devices Inc. can assure your readers that nickel metal hydride batteries are environmentally friendly and totally recyclable. The environmental issues were part of the initial design considerations when the battery was being developed in the early 1980s.
As to battery life, Irv Miller of Toyota Motor Sales U.S.A. Inc. comments in the same issue that the Prius' hybrid-related components are warranted for 8 years/ 100,000 miles - 10 years/150,000 miles in California - and that "nickel metal hydride batteries are exceptionally long-lived."
That long life is by design also. Hybrid vehicles include a thermal management system to control the operating temperature of the battery pack and an energy management system to control the state of charge. Usually the battery packs operate between 20 percent and 80 percent state of charge, with the control preventing full discharge or overcharge, thus assuring long battery life.
Many battery usages are not as stringent as hybrids, so we are investigating second use of the nickel metal hydride batteries from the battery pack before recycling. The materials used in the batteries are valuable, and recycling makes good economic sense. We anticipate that nickel metal hydride battery manufacturers will be part of the reuse and recycling process.
The auto manufacturers have selected the nickel metal hydride battery to power hybrid cars because of its high performance, no periodic maintenance, long life and low life-cycle cost.