Since Wikipedia is down, I'll have to inform you all about the true story of the B-sized Battery.
Many years ago, the great reVolt occured, whereby batteries assaulted their human masters, and escaped from their packaging. Humans fought back, and did their best to enslave them, but couldn't keep charge of them, since there were so many, and they tended to be mercurical.
The situation looked terminal, but it all ended when the Energizer Rabbit spoke to all, pounding his great drum in anger at the confusion. It lead to the situation whereby all batteries were free batteries, but they now had expiration dates. A battery isn't truly useful, if it isn't in charge of anything.
It turned out those who created the great reVolt were the B Batteries. They were locked up in a cell on the Isle of Varta for eternity, or at least until they leak acid and must not be disposed of in a fire.
So there you have it. At least, that's the positive side of it all.