RE: Hitec 5625 -- 5645
The 56xx series of Hitec digitals, if run on 4.8v, don't like servo extensions, Nimh batteries, and everything that can cause a voltage drop. Nimh batteries have a high internal resistance so a high current draw from one or more digital servos can cause a voltage drop and the servo get crazy, lock and stop working for a short moment (as long as the voltage is dropped). Extensions and plugs have some resistance also so it is easy to get voltage drop here. On 6 volts, the voltage must drop very much before the servo stops working so it is an easy way to prevent that situation.
Personnaly, I do not run on 6 volts but I use 2 battery packs plugged directly on the receiver via their own switch. I use Nimh batteries and when the batteries get near discharged, I experience some problems with my rudder servo (a 5625). My 5925s are on the elevators and do not have this problem, even with the 2 24 po extensions. The rudder servo is plugged directly on the receiver on a pull pull setup so no voltage drop here.
All in all, a NiCd will handle 56xx series digitals better on 4.8v than any other battery on the market but 6v Nimh is also a good alternative since it would weight about the same but give more speed and torque to the servos.