Auto Electronics

HEV Battery Technology Advances Surely but Slowly

The ultimate goal of the Department of Energy's FreedomCAR& Vehicle Technologies (FCVT) program is the development of a commercially feasible fuel-cell vehicle that runs on hydrogen. Though steps are being taken toward that goal, hydrogen-powered fuel-cell vehicles are still several years from entering mass production. Much closer to showroom reality are hybrid electric vehicles (HEVs), which the FreedomCAR program views as an intermediate technology step to a pure hydrogen economy and hydrogen vehicles. But even though HEVs are already experiencing some commercial success, more work needs to be done to make these cars as popular as internal combustion engine (ICE) vehicles.

At present, the cost premium associated with HEVs still represents a barrier to their proliferation and much of that added cost can be attributed to the battery. In response to this problem, the office of the FCVT program is working with the United States Advanced Battery Consortium to advance the state of the art in batteries for various types of HEVs, as well as batteries for 42 V vehicles, pure electric vehicles and fuel-cell vehicles. The performance and cost goals for batteries in these applications are spelled out in a DOE document titled “Progress Report for Energy Storage Research and Development,” which was published in January of this year. This 122-page report spells out current research activities and the status of battery research efforts.

In a section discussing the Vehicle High-power Energy Storage program, the report observes that lithium-based batteries (Li-ion and Li-polymer are two familiar chemistries) represent the most promising high-power battery chemistries. Unfortunately, the cost of these batteries is prohibitively high on a kilowatt or kilowatt-hour basis. Therefore, current battery research is examining issues such as the cost of raw materials, materials processing, cell packaging, and module packaging.

R&D is also addressing lithium-based battery performance limitations, which include the reduction in discharge pulse power at low temperatures and the loss of power over time. Lithium-based batteries also need to be made more tolerant of abuse conditions like short circuits, overcharge, overdischarge, shock, and exposure to fire, etc.

The Vehicle High-power Energy Storage program also sets some specific battery objectives. One is to develop by 2010, “an electric drive train energy storage device with a 15-year life at 300 Wh with a discharge power of 25 kW for 18 seconds and $20/kW cost.” Another goal is to “reduce the production cost of a high-power 25 kW battery (light vehicle) from $3000 to $750 in 2006 and to $500 in 2010.”

In the long run, lithium-based batteries may provide the best path to meeting these goals. Their high energy and power density will provide more options for hybrid applications including all-wheel-drive capability and plug-in hybrids with 20 miles or more of all-electric range. But in the short term, the familiar NiMH batteries (already being used to build commercial HEVs) may have an edge.

Tien Q. Duong, manager of energy storage R&D for the FCVT program, commented that NiMH currently meets the performance targets established by the FreedomCAR program for HEVs. But NiMH batteries fall short in meeting goals for cost and calendar life. Existing NiMH batteries are more likely to have a 10-year lifetime rather than 15 years. (Some say a 15-year life is achievable with Li-ion.) On the other hand, 10 years might be sufficient to satisfy consumers' expectations.

Regardless of which chemistry is superior for HEVs, there is something of a catch-22 relating to battery cost. High-volume production requires low-cost materials and low-cost processing, but advanced batteries cannot achieve low cost until they are put into high-volume production. Consequently, advances in HEV battery technology will take time to seep into the marketplace as battery developers find niche applications in which they can prove out new technologies, while slowly ramping up production year by year.

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