If you have a charger, even a smart one that is a TRANSFORMER charger and not a "switching" (inverter) supply, you might be able to make a modification to "unsmart" the thing. If you can't figure that out, you could just add either two diodes (center tap) or 4 (in a bridge) and make a simple rectifier output that is not "smart."
Even "dumb" chargers will sometimes not activate a really dead battery. I sometimes series two "dumb" chargers (for 24V) to "boost start" a really dead one. Watch the meters, and when they come up after a couple/ few minutes, revert back to one charger
You can also do that with an extra good battery and a charger. Series the good battery and the "dumb" charger for 24V and again, watch meter for output to rise
When I was a kid, and I still have what is left of it, we had an old "tungar bulb" charger. These are always set up to charge batteries in series, because the tungar bulb has quite a bit of operating resistance, so rather than trying to get more current out of the charger to charge more than one battery, you put the batteries in series and crank up the "tap switch" on the charger. Because they do this---increase voltage--they are also useful for boosting dead batteries
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Above example is a "20 battery"--that's 6V so if you are charging at 8V that's 160V !!!! out of that charger to charge all 20 6V batteries in series!!!
