Mine became useless also in a few months-Walmart of course refused to take it back-lost receipt. Historically these units were junk and should have been recalled---I was shocked at the amount bad reviews online.
Had one from Harbor Freight for 5 years-worked great. I tried to install new charger-no luck, so I opened it and attached a trickle charger and that got it working again. Had to extend the bolt only on the positive terminal so I would have a way to connect the charger to it once the case was back together-negative side is connected directly to the battery. I made a small hole in the cover on the positive side to the longer bolt would extend through so I could connect the trickle charger to it easily.
From now on I will be very careful about saving my Walmart receipts. I always expected the Harbor Freight products to fail...but they have surprised me most of the time.