A123 Voltage nominal rating is 6.6 volts for a 2S battery. Full charge puts the voltage upwards of 7.4 volts. Compare to a LiPo or Li Ion whose nominal voltage is 7.4 and full charge voltage is 8.4 for the same 2S battery

A 5S NiCd or NiMH has a nominal voltage rating of 6.0 volts and a full charge voltage of6.8 -6.9 volts. It doesn't sound like alot of difference but that's significantly lower than that of an A123 2S battery

Some servos simply can't handle the extra 1/2 volt, especially with freshly charged batteries. JR's servos almost universally can handle it, but that's just JR. FWIW 2 U

One thing that competition pilots always consider is servo output consistency, first flight of a contest to fifth, at the end of the day. That's the reason many of us use LiPo's regulated down to 6 volts usually. It doesn't matter if the LiPo is at full charge of 8.4 or nominal charge of 7.4. The servos see 6 volts throughout the day and perform identically (speed and torque) from start to finish. Airplane response is identical start to finish for the day.