One thing to be aware of when using rechargeable batteries where the
kit may have been designed for dry cells is the probability/
possibility of batteries appearing flat prematurely. There isn't a
huge difference in the voltage generated by a flat dry cell and that
of a half charged NiCd or NiMH cell.

Many of the guys running RC planes and boats run the receivers on 5
rechargeable cells (at 1.2V each) instead of 4 dry cells (at 1.5V
each) to overcome this problem.