Inverter always shuts down with high voltage alarm with APU Running.

15 volts sounds like alternator problem or voltage regulator. If you have a tester see whats coming out of the alternator.

 

I don't think they're low voltage anymore, not like they used to be where 13.4 was the norm. They all seem to be running higher, mine runs 14.1 steady and if I'm not mistaken can run up to 14.8.

 

There are a couple of ways to handle what you're asking. There are automatic voltage stabilizers for aviation but even a cheap one isn't really cheap. A rheostat would allow you to do it manually but for safety you'd need one with at least an 80 amp capacity to match the max loading of your inverter. Rheostats of that capacity tend to be sold bare, ie: without a housing, because they're normally installed on an equipment cabinet. I know a way to do it automatically with a simple transistor circuit but unless you have the skillset it's sorta pointless to explain.

 

Your voltage shouldn't be spiking to 15 volts. You either have Alternator/voltage regulator problems which I doubt because you would of had it prior.

What I think is you have a poor ground which will cause a voltage surge. Double check the main ground at each end and that they have good connections. Most inverters have a second chassis ground which is a smaller wire that goes from the outside chassis of the inverter to a truck chassis ground like the cab of the truck (not to the battery).

The main cables from inverter to battery should be less than 6 feet, preferably under 4 feet.

 

Check ur ground like condo cruiser said but mainly check that ur wires are clean. I was having that same issue, and after I went thru two inverters cuz the guys in the shop said it was a bad inverter, they cleaned the connections and I didnt have a problem after that.