G'day Crash,
As Chuck said, go with 6volt nicad or LOW internal impedence NiMh, or as I did, in my Katana, use 2, 4.8 volt nicads, 1, 1700Mah, the other is 1100Mah, just use 2 switches, & charge setups & plug the second one into a spare slot in the RX, it gives you redundency.
Works a treat, & you have 2 separate current paths, through 2 separate switches, the weight is worth it. If you look carefully you can see the 2 switches, 1 has a red lead, & the test /charge/ bind socket.
See here. http://www.rcbatteryclinic.com/