There's only about 8% loss in efficiency over 4.8v at 7v. 5v is absolutely more than fine. For my EF Exra I just wanted troublefree batteries that have a consistent voltage and can be charged fast. Answer = A123. I don't want a mix of NiMh and A123's in my planes. 1100mah A123 ignition is good for 8 flights, 2300mah Rx 5.9 flights. I rarely do more than 5 and if it happens I can balance charge them through the charge receptacle at 4 amps. Especially on the Rx, NiMH can vary in output voltage. A123's stay at 3.3v up to 98% discharge. The new gen Hitec digital servos can deal w/ up to 7.4 volts.