At Common Sense RC we have the perfect setup. We sell a pack designed specifically for replacement for the Rx pack. We also have a very nice voltage regulator that gives a constant 6 volts for the whole charge. This is very nice as your servos never loose any torque or speed.
We also have a replacement pack for transmitters. It includes a built in low voltage cutoff so you never have to worry if you accidentally leave your transmitter on.
Thanks,
Common Sense RC
www.CommonSenseRC.com