It is much easier and safer to charge NiMH batteries than the more recently developed and rather dangerous lithium based batteries. NiMH batteries are also much more tolerant of abuse than lithium based batteries.
With NiMH batteries you can employ constant current charging (up to a voltage limit), trickle charging, or float charging. Charge balancers are still necessary for NiMH battery packs, as attested by the design of the Prius battery packs.
You need to either do quite a bit more research, or, as suggested earlier, hire an professional to design the charger/balancer.