When you add more batteries, get the zipzaps se batteries. They are NiMH rated at 1.2V, 70maH and more importantly, they are much shorter than the standard bit battery (10mm dia x 10mm lg). Each zipzaps se battery contains two of these batteries soldered together (by a clip) and shrink wrapped.

Using these batteries is also the easiest way to dual-cell a bit. You simply cut the shrink wrapping down the center of the battery, so you can bend the soldering clip allowing the batteries to lie side by side. Just solder two wires to the batteries and they slip inside the chassis (between the steering system and the original battery location) with no problem. Then you simply solder the wires to the charging contacts.