I bought a Black & Decker power inverter and installed it in my service truck to keep my cordless tool batteries charged.  I wired it from the battery, through a fuse to a relay that is energized only when the engine is running. When one charger failed, I chalked it up to old age. Then when another one stopped working I tested the voltage to and from the inverter. I was getting 14v DC in and 108v AC out. I bypassed the relay and connected directly to the battery without the engine running and had 12v = 120v. I called B&D and their “rep” wasn’t much help. After much complaining he agreed to send me another one. It arrived in a timely manner and I installed it with the same results. I returned them and went to the local Interstate Batteries store. After telling my story to the counter person he let me try one of their inverters. The results, 12v DC = 120v AC and 14v DC = 120v AC. So I bought and installed it.