They were discussed on this forum in 2012: The HF float chargers can put out 500 mA but don't unless the battery needs it. I posted the schematic on ARF some time ago, found here. I made a simple low voltage alarm to solve that problem.

I've used these cheap Harbor Freight float chargers for years knowing they can drain the battery if they fail. It or the other one, I can't remember which one, malfuntioned in a way that melted the wiring, which could have caused a fire. One failed and output too much current (no longer float charge). One for the RV house battery and the other for the RV truck battery. A few years ago, I bought a more expensive pair from Walmart. I understand a little more every time I do a teardown, so hopefully I will be able to decipher more if I return to this device in the future.Don't trust anything cheap. The DC output - which I guess would be above 100V DC - is buffered by a big capacitor, and I don’t understand very much beyond that. (White wire.) It first goes through a nonreplaceable fuse (better than no fuse I guess) and then immediately into the rectifier. But I realized I was wrong when I followed the copper trace for the line voltage. My first impression (and assumption) was that the big coil in the middle was the transformer, and I had thought it was used to step the voltage down from household 110V AC to something lower, then passed into a rectifier to obtain DC voltage somewhere near the 14.4V maximum for lead-acid battery. It also meant I could easily follow circuit board traces to see how much I understand. It’s almost refreshingly retro to see a circuit board with only through-hole components. I also note that this label (and all the warnings) would not be visible when the device is mounted.ĭisassembly was straightforward with only four screws to remove. Harbor Freight no longer lists item number 99857 on their site. When we want to charge/maintain the battery, we plug an extension cord into its stubby power cord. It was designed to be permanently mounted under the hood of a car. I got it from a discard pile just to take apart and look inside. I don’t know why this particular battery charger/maintainer was discarded, but I wasn’t going to hook it up to a real battery to find out.

Cen tech battery charger wrong