Again, when Futaba refers to servos working on 6 V, they're referring to a 5-cell NiCd pack, which will be rated at 6V, which is the nominal voltage of a 5-cell pack. Servos that are rated for 6V will work properly when a 5-cell NiCd pack is fully-charged. Fully-charged a 5-cell NiCd pack can reach 7.5 V, but it will quickly drop to about 6.0-6.2 V and hold that for a considerable time before it drops off. At 5.5 V, the pack would be considered 'dead'.