You would not operate with more than a 5-cell NiCd or NiMH battery pack. Please be aware that some Futaba servos are limited to 4-cell NiCd or NiMH packs. There is no specification for other types of battery packs. Normally, you'd regulate at just under 6.0 VDC. If you have a system that can regulate at that voltage and provide enough current to the servos, you can use any battery pack that will provide the voltage and current. A 2-cell Lithium pack without regulator is definitely not recommended.