RE: Adding another receiver battery
Is your nicad worn out? Or just need longer flights, or? You don't necessarily have to go to the A123 technology which would require a new kind of charger. It might be simpler just to get a 6.0 volt Nimh (metal hydride) pack of higher capacity. It would be exactly like the one you have except Nimh cells instead of the old nicad type. Your existing charger might already be Nimh capable anyway.
Ernie