http://en.wikipedia.org/wiki/Toyota_Prius

A Sealed 38 module nickel metal hydride (NiMH) battery pack providing 273.6 volts, 6.5Ah capacity and weighing 53.3 kg (118 lb) [12] is supplied by Japan's Panasonic. They are normally charged to 40-60% of maximum capacity to prolong battery life as well as provide a reserve for regenerative braking;