Found a neat site that walks you through disassembling your electric toothbrush and replacing the battery. Usually requires some soldering, but... very cool. (They're funded by selling replacement batteries!)
ToothbrushBattery.com
ToothbrushBattery.com - Replacement Toothbrush Batteries
Buy a replacement battery for your Braun Oral-B or Sonicare electric toothbrush and use our repair guides to help you fit it. Replace your battery ...