Chevrolet Repair: alternator voltage too high?, 2008 chevy trailblazer, chevy trailblazer

Question
2008 Chevy Trailblazer. With the truck running the voltage measured across the battery is 15.5 volts.My issue is I have a DC to AC inverter in the truck that will only take a maximum input of 15 volts DC and keeps shutting down.Chevy claims the alternator is "within GM spec".If that is true,is there a way to get the voltage below 15v like installing a resistor in line? I tried turning everything on in the truck (high beams,AC,radio)but the voltage will not budge.

Answer
Hey,
  If 15.5 volts is what they say is within GM spec it must be the case but I have not seen one charge that high before.  I have to admit I haven't seen any 2008 trailblazers with charging system problems so I just haven't checked any.  For the last 30 years or so target voltage of all vehicles has been roughly 13.2 - 14.8 volts.  If I saw a vehicle charging at 15.5 I would assume something was wrong with the alternator,wiring or PCM.  If you called them and they said 15.5 is spec well.....  If you install a resistor in line the alternator will just compensate and produce a higher field dutycycle.  Its not going to be that simple.
  If 15.5 is normal what you will have to do is make an external 12v regulated circuit.  It's not a complicated circuit to build but you will have to have a pretty good comprehension of DC circuits and how to make one.  Sorry I can't be more helpful I just havent run into that before.

wayne