Unlike full hybrids that can run on batteries alone, both power sources—engine and electric motor—operate full time. This let Subaru fit a much smaller and more compact motor, adding just 200 lbs (100 kg) of weight including the battery pack.
A crucial component of the B5-TPH is its manganese lithium-ion batteries, co-developed by Fuji Heavy Industries (Subaru’s parent) and NEC Corp. With 50 percent greater power density than the Nickel-Metal-Hydride batteries in hybrids today, Li-ion batteries also offer much faster recharge. The weak spots of Li-ion batteries have traditionally been their life cycle, which varied with use cycles rather than time, and their heat generation. Subaru and NEC claim to have solved these problems, though the company has released very few details. Experimental numbers of the TPH powertrain will appear first during 2007 in versions of the Legacy sedan, in Japan only.