Hi WLB,

Futaba does recommend only using 4.8V packs with any digital servos that are rated for 4.8V. But you should be okay to use 6 volts on a 4 cell pack, with them as long as are you are using a 6 Volt regulator.

You should not use a 5 Cell pack though as these packs when charged can carry a much larger voltage and this will run the risk of damaging the PC board in the servo.

I hope you find this information helpful. You can also reach our Futaba support technical team at 217-398-8970, or via fax at 217-398-7721.

Sincerely,
Krysta
Lead Futaba Customer Service and Programming Technician
Also you may want to check our Website at (http://www.futaba-rc.com)