It's hard to get to the correct voltage with NiCD or NimH - you end up with 3.6 or 4.8. If the charger is well designed it should be putting out 4.4, and constant voltage, where the NiCD or nimH want constant current.
When you eventually find the right replacement battery, you will likely find it cheaper to buy another flashlight than a replacement battery - at least that was my experience.
Comes of buying things on sale at CTC, I guess.