Given the less-than-steller reviews and HF's reputation for electronics, I should not have bought this thing, a 12 Volt 6 or 2 amp (switch selectable) charger for car batteries. The problem is that instead of providing the tapering charge and switch to trickle it is supposedly designed for, this "charger" discharges and kills batteries instead. (In contrast, I have a 4 amp Schauer charger that is about 40 years old which still works just fine.)
Instead of throwing out the HF unit, I was thinking that it has a perfectly usable case, transformer, ammeter, and wires with alligator clips. Just the crapulent Chinese electronics are bad so I'd like to trash that stuff and turn this into an "old school" type charger like my Schauer.
The transformer secondary is center tapped and reads about 28 volts AC across the ends, 14 volts on each leg from the center. The original electronics are on a small board with 7 transistors, 2 SCRs, a couple of diodes, and numerous resistors. (That's a lot of stuff just to kill batteries!) Searching online I'm finding a bewildering array of home-brew battery charger circuits, everything from simply using a bridge rectifier on the transformer secondary to more complex circuits to taper the charge and either switch off or go to trickle when the battery is fully charged.
The behaviour of my old Schauer is to gradually reduces current until the battery is fully charged, where it will stay at a low level and not hurt the battery if left on overnight or even for a few days. I have not opened it up yet to see what's inside, but being an early 1970s unit I'd be surprised if there is much aside from a transformer and rectifier.
Anyone have a simple circuit handy that works well for this? I've seen a few plans online that just say to use a bridge rectifier of suitable capacity, but I want to be reasonably sure that this thing won't kill any more batteries.