I'm getting pretty interested in building an electric motorcycle, and I ran across this little hack to charge multiple batteries with one charger. It uses a 4020 counter that's pulsed by my dear friend the 555 to activate a series of relay pairs to switch a single charger sequentially between battery cells. A more advanced version could use a microcontroller to monitor the state of each cell to ensure even charging. If you're thinking of constructing an uber-ups, this could be useful.
http://www.hackaday.com/2006/10/15/sequential-battery-charging/