An Afterlife for the Electric Car Advocates of electric cars and renewable energy have talked for years about repackaging the battery packs built for cars as home energy storage devices once they can no longer hold enough charge to run a vehicle. On Wednesday, ABB and General Motors announced that they are trying out just that idea with the battery packs of five Chevy Volts. When it is new, the Volt battery pack holds 16 kilowatt-hours. The prototype announced by the two companies promises a capacity of about 10 kilowatt-hours per pack, with five packs lashed together in an array that is supposed to provide two hours of electricity for three to five average houses. For the demonstration, the unit was providing lighting and audiovisual equipment in a structure in San Francisco where the experiment was announced. The batteries were hardly challenged. In the Volt, each pack is supposed to provide up to 111 kilowatts of power, the watt being a measure of how fast the electricity is delivered. In this case, the five batteries delivered only 2.5 kilowatts, or less than half their original capacity. Using the batteries at low power could extend their lives. The idea behind the prototype is two-fold: to provide a market for past-their-prime batteries, giving them a resale value that will lower their cost of ownership, and providing distributed storage that could be used to shore up weak spots on the grid or to absorb energy from intermittent sources like solar panels and wind machines and deliver it in a steady stream suitable for the power grid. ABB believes the installation could also be useful in a neighborhood with a lot of electric cars. The electricity needed to charge those vehicles could come into the area in a steady stream, and be stored by the battery pack and tapped at high speed whenever needed. In that function, the battery pack would resemble the tank on a toilet, which fills slowly but is available for quick discharge. The ABB/GM package also includes an inverter, a device that converts direct current from the battery into alternating current, the kind that comes out of an electric socket on the wall. In a statement, Pablo Valencia, G.M.'s senior manager for battery life cycle management, said that at the point that a battery pack was no longer suitable for a car, "only 30 percent or less of its life has been used." "This leaves a tremendous amount of life that can be applied to other applications," he said. Battery packs can also be used for frequency regulation, which means keeping the alternating current as close to 60 cycles per second as possible. That is now being done in some large-scale projects like a giant battery array attached to a West Virginia wind farm but not yet in small ones. The idea faces some challenges, even if the prototype works well. One is the low sales totals for the Volt itself; only about 24,000 have been sold in the last two years.