I had the same issue. Cracked the unit open looking for a coin type battery that I could replace and found the battery is soldered to the board! After some extensive research I found a good replacement part (Panasonic ML-621S/DN), ordered 2 from mouser electronics. De-soldered the old one, and soldered in the new one, and I'm up and running again. It wasn't terribly easy though, definitely need some steady hands to fix. Product is unfortunately designed to fail. These rechargeable batteries are good for a few years then you'll need to replace again. At least I'm not out another $150 and rewiring in my truck to install a new one.
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