There are tiny 12V batteries that they make for garage door openers. Using a simple NPN transistor, you can switch the 12V source from the battery to a common ground with you robot, from the robot's MCU. I have one of those batteries, it's a Duracell, and says "MN21/23 12V" on the side. It's the same width and half the length of a AAA battery. I think they're referred to as an A23 cell.