I was thinking of the disturbing battery issue I had on board last year. When all of a sudden one of my batteries refused to complete a full charge using my Dual Pro Quad 4 battery charger. It took a few times of repeated charging but, finally the battery came back to being comparable to the other batteries in the 48 volt propulsion string and has stayed there ever since.
Some thoughts on what happened:
1) The main cause seemed to be that I was hanging a 25 milli amp load on this particular battery to run the Paktrakr Battery Monitor. I had done this years before with no problem but,
2) I had also removed the 48 volt Marine Air X Wind Turbine over the winter to replace it's blades and have it repainted. So the only charging that was happening was from the 48 volt solar panels and an occasional charge from the Dual Pro when I check up on the boat. During the winter solar availability was limited to short daytime charging. So that 25 ma drain started to add up. Possibly sulfating the battery enough to prevent it from ever really getting a full charge over the winter months because of the constant 25 ma load.
Over this past winter I no longer had the Paktrakr hooked up to that battery and I had the 48 volt Marine Air X wind turbine also helping with the charging. The battery that was causing the trouble last year now charges with the rest of the batteries. The lesson learned is that to avoid taking any load off a single battery in a series string. Also when a battery suddenly refuses to charge all is not lost and one may not need to run out and get a replacement battery.