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. I got it from a discard pile just to take apart and look inside. It was designed to be permanently mounted under the hood of a car. When we want to charge/maintain the battery, we plug an extension cord into its stubby power cord.
![](https://newscrewdriver.com/wp-content/uploads/2022/04/cen-tech-12v-battery-charger-99857-bottom.jpg?w=1024)
Harbor Freight no longer lists item number 99857 on their site. I also note that this label (and all the warnings) would not be visible when the device is mounted.
![](https://newscrewdriver.com/wp-content/uploads/2022/04/cen-tech-12v-battery-charger-99857-disassembled.jpg?w=1024)
Disassembly was straightforward with only four screws to remove.
![](https://newscrewdriver.com/wp-content/uploads/2022/04/cen-tech-12v-battery-charger-99857-pcb.jpg?w=1024)
It’s almost refreshingly retro to see a circuit board with only through-hole components. It also meant I could easily follow circuit board traces to see how much I understand. 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. But I realized I was wrong when I followed the copper trace for the line voltage. (White wire.) It first goes through a nonreplaceable fuse (better than no fuse I guess) and then immediately into the rectifier. 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. 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.