If you still use the old 2 channel 27Mhtz or the 40Mhtz 3 channel FM transmitters you may find that current rechargeable cells are a very tight fit and the back will not go back on. They seem to be about a millimeter greater in diameter.
Experimenting I found that a 10 cell flat pack of AAA cells at 1.1Ahr will fit in without cutting away any mouldings. Cut the + and - wires in the TX. and connect a female servo lead.
Solder a standard male servo lead to the AAA pack and plug in and play. The charging socket is still connected so easy to charge.
As these Tx's were originally suitable for 12 volts with dry cells no problem on voltage and I have found that switched on they last for a whole afternoon.
I asked The Component shop to make up 3 sets of batteries for me and have used them for over a year now.
Roy