what you need to do if the battery compartment is the one that takes individual batteries and is not removable is get youself a dremel and cut the battery tray out, taking care not to cut any of the clips that hold the back onto the transmitter,
next get yourself a futaba extension lead like this one
http://www.component-shop.co.uk/asse...300extn-03.jpg
and cut off the smallest end (male) and solder the red and black wires to the points that you remove the original battery compartment leads from,
put yourself a piece of stiff cardboard on top of electrics below the point you have cut out making sure to have the other end of lead you have soldered to board pertruding from behind cardboard,
screw back together and get one of these batteries making sure to choose futaba in the connector box,
http://www.component-shop.co.uk/asse...p2100aai-s.jpg
this will plug straight into the connector you have soldered to the board,
then get one of these leads
http://www.rcmodelcentre.co.uk/Futab...prod_1230.html
this will plug straight into your charger and into the battery, charge at 500mah if you do buy that battery, it will last longer
or you could just buy a couple of chargers that both come with 4 free batteries, like this one that does 15 minute recharge
http://media.rei.com/media/o/1415796.jpg
ps: i have done the above mod on one of my transmitters my son uses