[BrianG]

I meant modded shopvac for the storage capacity. But yeah, I'd use the main guts from an iRobot to base the guidance and drive/control systems on. Just need to "amplify" the drive motor power so it can drive something heavier around. A simple high current H-bridge on the iRobots outputs to beef up the current would work - maybe even boost the voltage at the same time...

And since we are thinking a bit outside the box here, how about a non-contact charging system? When nearing the end of the charge, have the vacuum drive to the base, which uses electromagnetic coupling to charge the cells? Basically, half a transformer at the base, and half on the vac. They'd have to be fairly close together though.