They are NiMh batteries, just like the rechargables you can buy at Wal Mart internally. The ones in my Prius are round, and about the size of a "D" cell battery. I don't remember how many cells it has total, but the battery pack makes for a nice little shelf in the back section of the trunk by the seat. If I didn't point it out to you, you wouldn't notice it's presence. The new battery pack in the '04 and newer Prius, the cells look like cassette cartridges. If the car is involved in a major collision, the system triggers relays that totally isolates the battery pack from the vehicle.