Actually, this Optima battery has done the same self-discharge over about a month or two several times the past two years. Happens whenever I go out of town. I bought a solar charger to keep it topped up, but it is apparently insufficient. When the van is in regular use I don't have a problem. The external draw is as mentioned 10-20mA, so quite low and not an explanation. I re-charged it with a smart-charger set to 8A bulk, and it went through the three stages without reporting errors. (bulk, absorbtion, de-sulfatation cycling). Now I have used the van for about a week since I came back and all seems well. As you suggest, one cell might be bad (as opposed to all). I use NCd and NiMH batteries lots at school, and individual cells can have radically different self discharge curves. The Diesel westy AFAIK came with a group 41. In the past other than the Optima I have also used regular and calcium/maintenance free wetcell group 78 by lengthening the terminal wires. Have to be a bit more careful not to short the grp 78 to the metal engine cover if using the top posts, and the standard battery hold down at the bottom of the tray no longer fits the notch on the battery. Martin --- On Mon, 6/23/08, Dennis Haynes <d23haynes57@HOTMAIL.COM> wrote A battery dropping to 10 volts without explanation is a sign of a bad cell. Either a cell is dried out and not producing power or shorted. Either way it needs to come out of service ASAP. The alternator is working extra hard trying to charge a 5 cell battery and localized heating can cuase the battery to burst. Yes it can happen to AGMs. It will be a dryer mess. At 6 years you have exceeded its typical life so just let go. I would suggest a normal strting battery but something larger than the group 41. I believe the Diesels were equipped with group 48 as is typical for the larger Audi's and Mercedes
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