OK- - - -now we're making progress. The charger will probably go to max output if one of the 3 cells is shorted, and that's a definite possibility. As cell plate material sloughs off a battery it falls into the bottom of the cell, which has a reservoir that's able to hold quite a bit of material. If lost cell group material gets deep enough to contact the bottom of the plates, it shorts the cell out. Then you have a "4 volt" battery instead of a 6 Volt, because the shorted cell is no longer in the circuit.
Ignore the comment about a reversed cell- - - -it can't happen on a single cell of an automotive battery. I've seen totally discharged batteries connected improperly and recharged backwards, but I doubt if that's what has happened to yours. A cell shorted out from debris buildup is much more likely.
Jerry
