Battery in a drum rolled out

That's enough to power a computer workstation for 2 or 3 days, or to recharge a cell phone almost indefinitely, according to Younicos, the German company that invented the pack.

The mobile energy storage unit, called Yill, is a couple of feet in diameter, about the size of a small bass drum, with a handle and wheels. It recharges in about four hours in a 220-volt electric outlet.