A123 Systems, a company based in Watertown, MA, that will supply battery cells to Continental, has taken a different tack, turning to an iron-based cathode that is even more thermally stable than manganese oxide. Better still, iron is cheap and abundant. (See “More Powerful Hybrid Batteries.”) The electrodes are not oxide materials but phosphates, a chemistry that more closely binds oxygen, preventing it from being freed from the material, which would allow the battery’s flammable electrolyte to catch fire. Such materials do not allow for fast charging or delivery of big bursts of power, so researchers modified them, in part by doping the material and by forming the material as nanoparticles. The A123 batteries were developed for use in power tools but have since been modified to store more energy, making them better suited for use in electric vehicles such as the Volt.
The battery packs for the Volt must include complex electronics for ensuring that each cell is charged and discharged properly. If individual cells are overcharged, for example, the pack can fail. Unlike measuring the gas in a tank, it can be tricky to monitor the exact amount of charge in a cell. So battery makers often include more cells to provide a margin of safety, as a hedge against both running out of power and overcharging the batteries. The pack makers are developing better electronic equipment and algorithms for measuring charge, which could allow them to use closer to the bare-minimum number of cells.
Even as the new battery packs are being tested, GM is developing the rest of the vehicle, especially making sure that it meets targets for weight. Ultimately, Gray says, there could be tradeoffs between vehicle weight and battery size, depending on how the tests go. There’s even a chance that expectations for the battery pack’s lifetime could be lowered if necessary, although she emphasizes that the goal now is to have battery packs with 10-year lifetimes.
In June 2008, after analyzing the data from a year of testing, GM will evaluate if the technology is where it needs to be and pick a production supplier, Gray says.