I acquired a marine battery charger/conditioner (Promativ 40-3). I
just tried it on the battery from a lawnmower which at first showed
less than 7 volts (12V battery). My car battery 10 amp charger could
barely get any current into the battery (barely moved the meter).
Leaving the battery on the new charger overnight seems to have
restored its ability to hold 12V.

What I am wondering is, would it be reasonable to use this charger
(It's rated for 40 amps) to charge smaller batteries like the 5 amp
hour gel cells from a UPS? I would think that it is regulating for
voltage, and monitoring the battery voltage to determine when to
reduce the charging voltage. I have a number of these batteries which
I'd love to restore to semi usable condition. Is it reasonable to use
this charger on them?

Bob