The Charger that I use when the batteries are out of thye MH is an Automatic one with the 15 and 2 Amp Settings, and a 6 Volt 15 Amp Manual charge setting. This is the one I used to try and put some power into the battery before letting it sit overnight. As to the question concerning the placement/location of the battery position in the MH, as I recall, this Battery was nearest to the Positive input. So, it would be nearest the Xantrex Inverter/Charger/Converter when being charged/or maintained when the MH is plugged into SP, and would have power passing through it to another battery. From the suggestions and comments, it appears that for what ever reason that middle cell is beginning to fail.
One other thought is....some of these Xantrex Inverters will try and top off batteries, I forget what this is called, but I suppose if this were to happen it may cause a battery to get hot and dump the electrolyte. Just a thought. Thanks for your comments. David