Cool motor.
Stuff like that makes we wonder why the modren Tesla wasn't called Edison...except well the truth is obvious.
I love that old stuff they made up for lack of control electronics with sheer clunkin' copper bar that'll run short circuit the rest of it's life and come back asking for more.
Reminds me of these old follow spotlights we have in a venue I used to haunt.
Carbon-Arc
Effectively a welding rod in front of a lense.
Here's the traffo.
None of this noncey shunt business..full load through the ammeter!
You can use anything you want for a charger. This whole 3 stage business is a product of marketing saying they havenay got the poower to run constant voltage.
All chargers are designed to run max output at over-voltage with tail current feedback that tapers current at absorption fixed voltage, the float is a load and parasitic compensation state (0.3A usually) after the zero current feedback threshold is reached at absorption voltage. The level of refinement of the principle is reflected in the price.
If they're lead give them 2.47V per cell until the specific gravity stops improving or the tail current is zero.
Probably best not do them in series until after you do any recovery business. The high internal resistance of some will play havoc with the balancing.
I'd say an adjustable power supply is yer best bet.
Unbridled solar will get you there in a pinch. A charge controller would give you some control.
Fero-resonant chargers are handy too.
Anything modern and lightweight will reject an old battery because they're designed to be annoying that way.
This is a technique as good as the best around;
That's all I ever do with MorningStar except my way's automated with a more constrained voltage.
Just to add:
The current taper happens naturally as the battery is charged. The battery voltage increases and opposes the source voltage reducing the current as a result.
You don't have to worry about it a whole lot other than keep water over the plates and stop the charger after they stop improving specific gravity or current acceptance.
If they start smelling funny like hydrogen (bad eggs) retire them.