Pretty good post on that Peter.

There are a few more things though. When the battery is bad it is usually a near open circuit. To restore it you need a much higher voltage and you have to babysit it unless you build a cool circuit for that. When it does not pull any charging current how can it charge ? And then sometimes you get a shorted cell.

Another thing is if it a not a gel cell you need to agitate it. Proof ? Charging a battery I kick it around a little and the ammeter on the charger goes up. That means I knocked the bubbles off the plates.

And then if you really can't get it to take a charge, (pull current) put epsom salt in it. That will restore almost any of them BUT, for one you can only do it once. If you do it again you risk a shorted cell and actually the thing might blow up. That is rare but it happens.

Another thing is the desulfater. You take 120 AC, put a light bulb in series like a DBT and hook that up for a while. A 100 watt bulb on 120 only lets a little more than one amp through, so it is safe.

The AC washes the plates, gets the nonconducting coating off them so they can work. It has to conduct or you get nothing. They actually sell desulfaters but you can do it with a light bulb. Some of the ones they sell use a higher frequency and I don't even know who to ask if that is better.

Anyway, there is no minimum charging current, it is all in how long it will take. Just to do not exceed the maximum. Charge it at 50 amps and the plates get all bent out of shape. And the thing could blow up, you know even if it can supply 50 amps it is not meant to do it all day. It can't. By the same token you cannot charge it all day at that rate.

Know what, just go to the boneyard and get an alternator from a 1970 luxury car. Get a motor and a belt. Those things look at the circuit and try to understand why the made them that way. They knew how to charge a battery.