That depends what you have connected to it.

The EZ-B V4 can operate from as low as 4v. The input voltage is passed through to the Vcc of each port. If you are running 7.2v servos from it then it would make sense to use a 7.2v supply and regulate any 5v sensors. If you are running standard servos on 5v then it makes sense to run on 5v, and so on.

Capacity wise, again it depends on what's connected and it's use. The higher capacity (mAh) the better as it will last longer.

Thanks ritch

The datasheet hasn't been finished yet - but the center pin is positive on the plug. :)

For power, use at least 7 volts. I recommend 6 AA rechargeable batteries

Thanks Dj :)