From: Alternator trouble

[Quentin Jarrell 671]

if you have accessories(radio, lights, ect.|) in the vehicle on while just the key is turned to accessory your voltage might drop to 11.5-12 volts. But if nothing in the vehicle is on and you turn the key to the on position your voltage should be close to 12.6 volts.

Also try a DMM at the wires going to your stinger volt gauge and see if they are the same it could be a possibiltiy the gauge is off a little.