You will need to charge your batteries!
At a typical Maker Faire you might get 10 Amp of 120 volt power to your pit. Keep that in mind when choosing a charger.
The charger you choose will also have to work with the batteries you use. SLA are different than Lipo, etc.
Some teams (waffle Kart, LVL1 hackerspace) use the iCharger series of chargers, specifically the 3010 or the 306 to charge lithium batteries like the Leaf or Volt cells. these battery chemistries, and all chemistries besides Sealed Lead Acid (SLA) batteries need to be balanced while they are charging, that is, the charger must monitor the voltage level at each individual cell and not just the total voltage of the entire pack, to prevent over-discharge.
Check out this write-up on using the icharger from lvl1 hackerspace: http://wiki.lvl1.org/PRS_iCharger_Monstrosity
Someone should write a lot more about battery chargers here!
See Also: Battery Charging Tips