I don't personally own a hybrid or electric vehicle, but I've been interested in them for a long time and have watched quite a few programs about them. One thing that has always bothered me is the fact that most (if not all) electric vehicles or hybrids use hundreds of battery cells in series instead of just having one large battery designed for the vehicle. So why is this? Why do expensive electric cars like the Tesla Roadster have what appears to be large banks of standard laptop batteries rather than one big battery cell? In some cases they even look like just a bunch of standard rechargeable AA batteries you'd find at the store.

With many prototype vehicles, I know that the production numbers are low and they may need to rely on off the shelf parts rather than custom designing and fabricating their own batteries; but what about cars like the Prius that are mass produced? Surely there's a different reason they use hundreds of small batteries instead of a single large-capacity battery.

Isn't it more efficient to have a single large battery rather than a bunch of small ones? At the very least, the amount of material/mass used in housing and shielding the hundreds of small batteries would be eliminated, as would be the wiring needed to connect them all together. So is the use of battery packs to allow malfunctioning/dead battery cells to be swapped out rather than having to replace the entire "battery"?