Why battery packs and not single large batteries?
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"?
Your question belies a basic
Your question belies a basic mis-understanding of battery technology. Each type of battery has a set cell voltage. To arrive at voltage and current levels above that basic value you must have multiple cells. This is as true for smaller batteries as it is for the 225volt 60amp battery pack in hybrids. If you have a transistor radio powered by a nine volt battery, this is not a single battery but a six cell 'pack' of batteries. Each cell puts out about 1.4volts so six of them are equal to 8.4volts. If you have a large flashlight that uses a 'lantern battery' which is a six volt battery, that battery has four 1.5 volt cells in it. Most types of batteries have cell voltages less than 2 volts so they must be placed in series strings to achieve higher voltages. The lithium ion batteries in your laptop are radically different, each cell in those packs puts out 4 volts each and at quite a bit more current than any other battery its size. That is why the next generation hybrids will use , or already using, lithium ion cells.