I have a BatteryMinder, a BatteryMinder Plus and a Pulsetech Xtreme charger that I keep on various batteries from my lawnmower to the RV 24/7. The batteryminder will only do 1 battery but the plus version along with the Pulsetech will do multiple batteries. I think the pulsetech can be put on an almost dead battery where the batteryminders are float chargers only or at least need a certain good amount of power in the battery to start working.
I use the BatteryMinder Plus and am very satisfied. I got it when the battery on a remote stationary engine deteriorated down to 3.5 volts after field mice ate though the solar charger wires. It took a charge but would not even hold it overnight. Put the BatteryMinder on for a few weeks and it now holds a charge. The battery is certainly not back to 100% but I would say the BatteryMinder worked wonders on a maltreated 10+ year old battery.
Phil & Susan
2002 Hurricane 30Q, F-53 Chassis
'05 PT Cruiser Convertible with SMI brake & Blue Ox towbar