A few months ago, a friend had a MacBook Pro battery that started swelling up. He agreed to let me have it, and now it has wheels:
The battery is 3S 2P (3 series, 2 parallel = 6 cells), so it peaks at 12.6 V. This is pretty much the highest voltage the motors can take before starting to take damage, but they spin fast!
The choice to mount it upright was a bad one, but the maze solving competition limits all robots to an 8×8 inch square, so the robot has a shark fin now, which makes it awesome.
The power port was conveniently shaped for me to etch a PCB and get access to the pins. After that, mounting the battery to the robot was a piece of cake.
Ye shall be called Jaws. Actually, I named it Tweedledee.