It looks like Ralph Cunninghams ignition and last I knew he did not recommend using over 6.0 volts . A freshly charged a123 will have over 7.2 volts and a nominal voltage of 6.6 . A Nimh 6.0 pack will peak slightly higher when freshly charged but the nominal voltage will be a lower 6.0 v. I would recommend using a dropping resistor or a 5.0v voltage regulator if using more than a 4.8 pack .