General Motors and grid equipment supplier ABB this week showed an early version of an energy storage system using battery packs from the Chevy Volt electric car. The two companies are partnering to build a prototype which they say can be used either for large-scale grid storage or back-up power for consumers. They estimate that 33 repurposed Volt battery packs could supply 50 U.S. homes for four hours during an outage or brownout. Batteries lose their storage capacity over time, but used car batteries are still viable for grid storage. GM estimates that once used car batteries have 70 percent of their initial charge, they can still be suitable for grid storage. Finding a viable method to repurpose used EV batteries after seven to ten years of driving isn't just a question of consumer convenience and driving range. Because batteries are the most expensive component of electric cars, car companies and battery makers are trying to ways to draw more money from that asset with grid storage.