I'm looking for charger/maintainer -- either an off-the-shelf unit or a
schematic so I can build my own -- suitable for a flooded deep-cycle
battery, either a pair of 6V "golf cart" batteries in series or a Group 31.

I don't want an RFI generator: I already have one.

I already have a few 13.8V 25A power supplies, one of which I would be
quite willing to "crank up" to a higher voltage to compensate for the
voltage drop in the control circuitry.

Something like the West Mountain Radio "Super PwrGate" (PG40S) would
work, although the automatic switch-over feature is not essential. That
device is intended for Gel-Cell and AGM batteries and not recommended
for flooded batteries -- absorption-charge voltage too low and
float-charge voltage too high, their tech support people tell me.

"Perce"