If you start to rotate the chargers, then the battery at lower voltage will start to see some higher voltage.
Perhaps the cheap chargers will keep the battery at 13.2 but is not able to put in enough amperage to get it above 12.6?
Right now, because the 12.5 volt battery is only about 50% full, and will need something on the order of 50 amp hours to fully charge it, they will not get that no matter how 'good' the maintenance charger is, they normally only put out 0.5 - 1 amp per hour, sometimes less.
I would get a larger amp capacity charger on the discharged batterys very soon!
Fred.
