Roger Blake wrote:
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.)

nice.

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.

It might be easier to fix the thing, unless you want a project.

I can tell from here that it's probably using some sort of half-bridge
setup. Apparently two extra diodes cost more than a smaller more efficient
transformer. Heck, it's unlikely to have the correct sized transformer to
start with, they just removed two diodes to save whatever it costs to
keep the school kids/prisoners fed.

It might be good to mod the thing to crowbar if it overshoots 13.8 or 14.1
or whatever you want your batteries charged to. There should be some sort
of rectifier to prevent the battery from discharging back into the
charger, check on that.

I can't say they're good or bad, but I've made NiMh chargers that were
just constant current sources that maxed out at the float voltage. The
charging current tapers off as the thing starts to choke itself out, but
it does work. The only reason I even had the constant current section was
to prevent the transformer from burning out if you threw a heavily
discharged cell into it. Old NiMh batteries really didnt like fast
charging either.

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.

How large are the batteries you're charging? A brute force charger may be
ok for smaller batteries, but if you're trying to charge some 2 ton off
the grid battery locker, you're going to have to smell that transformer
burn up.

What parts are on that board you have now? Can you post a photo?