'04 villager 4-Very weird battery problem

[golfmech1]

So I have a '04 villager 4 48v in my shop (I work at a golf course) that belongs to my bosses friend. He brought it in a little over a month ago complaining that it was dead and not charging. He had replaced the batteries about a month earlier (trojan t-875's). I checked the wiring and found no problems. I checked each battery individually and found one battery (one of the center ones) was at -2.1v. Thats right,-! The polarity was reversed. Thinking this was most likely a problem from the factory I replaced the battery with a new one. Cart charged fine. Sent it on it's way. Now, one month later it's back. Same problem. This time it's the battery with the main + battery cable on it. I removed the batter and put it on a 12v/15amp charger for a little bit to swing the polaity back. I put it back in the cart and charged over night and this morning, the battery was still dead and the polarity was reversed again. I have checked the charger receptical wires, and they seem to hooked up properly and not reversed. I used a different charger last night as well. Called my local club car dealer and he is stumped on this one too. Any help at all here would be very appriciated.