[Spitball]

Kip51035 - I've E-Mailed the Futaba agents in the States twice for precise maximum voltages that my list of their receivers can work on, and their answer was very vague except don't operate them off A123s!! When pressed, they simply replied 6,0v max and said to use NiCds and not NiMHs as these can't supply the servo current, 3amps (sic)!!??
Obviously they don't know and couldn't be bothered to fully consider my question or research for the answer. So, I won't be operating any of my receivers directly off A123s - I had no trouble getting exact figures from C-C BERG and HiTec BTW. I would be hesitant to operate a HiTec receiver off A123s also; why not use a cheap little GWS 2-amp ESC and ignore the motor connections and use it just as a BEC with 5volts out?! Some of the smaller servos shouldn't be used above 6,0volts and some even above 5,0volts. (GWS Picos I believe, for one.)
Kip, I didn't reread your post - you weren't talking about A123s even, sorry. Did you e-mail HiTec? I didn't get a definative answer from them. Lot's of people use 5-cell packs and get away with it - I think when they say 6volt, they mean 5-cells just as when they say 4,8 they mean 4-cell packs. I can't really answer your point still but why not use a small and cheap GWS 2-amp ESC, ignore the motor leads and use it as a BEC; that's what I intend to do with my E-Flite Ascent on B&D VPX 2-cell A123 pack - they are available for ten or twelve dollars.
The use of a diode, while it would work, sort of, I couldn't recommend.
C-C BERG said their receivers are OK up to 7,4volts BTW!!