What do you mean by the maximum battery voltage? Do you mean the voltage the batteries supply to the NXT, or the voltage the NXT supplies to sensors and motors? If you put fresh batteries in the NXT, you can get a reading of abouts 10 volts, although it is nominally 9 volts. I use the LEGO battery pack. When that is fully charged it delivers about 8.2 volts. I think the maximum voltage supplied to sensors and Motors is 5V, but I would have to check the hardware specs to be sure.