NiCd batteries are fine while they last, but the chargers which typically deliver a 300mA continuous trickle current eventually kill them. Although the Cd is now regarded as an environmental poison, they're still for sale, because Lithium batteries are so much more expensive. So here's the refit I did using LiFePO4 (Lithium Iron Phosphate) cells from Hobby King. This type of cell has much better intrinsic safety than other Lithium Ion cells, including LiPo, while the power density is not much less.

I ordered 4 of these cells and a Turnigy Accucel 6 charger for a total of $AU50 delivered. This is combined with an old Dell laptop charger which delivers 5 amps at 16v, which is as much power as the Accucel 6 can regulate. The photos which follow are pretty self-explanatory. You can see the bodgey'd up balance connector, so the charger can keep the cells in balance. In retrospect I should have bought balance cables and these connectors too.