if hey are NiCd batteries, check the wiring from the non working battery to the ESC, i had a buddy that put a mamba in his, and he had the wires actually melting the insulation cause of the high voltage... also if you have tamiya plugs, switch to the deans, i did that to my rusty and got longer run times too :P
HPI Hellfire SS
Losi Mini LST2
Losi Mini Raminator
There is no such thing as budget RC.