Join Date: Aug 2006
Location: Arlington, VA
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Re: Dead AGM bank
Unfortunately, my friend, it sounds like ALL your batteries need to be replaced. The age of batteries is not a good indicator of their condition. As you may know, it's much more important HOW THE BATTERIES ARE TREATED.
AGMs need to be FULLY CHARGED frequently...at least once a month...in order to live a reasonably long life. They're great for some applications, but for most boaters they are more expensive and more troublesome than necessary. And, they won't last anywhere near as long as good flooded batteries or gelled batteries.
The voltages you're seeing are WAY TOO LOW. An AGM battery which is reasonably well charged and with the charger removed should measure 13+ volts and, after sitting overnite, should measure at least 12.7VDC. I think it's safe to assume that all your batteries are shot. Only way to tell for sure would be to do a true 20-hour load test or check the batteries with a sophisticated (and expensive) internal conductance meter like the Midtronics Series.
That's the bad news.
The good news is that in all likelihood you don't need to spend all that much money for AGMs. Flooded deep-cycle batteries will likely be fine, depending on how you plan to use the boat.
You need to check the output from the battery charger to be sure the voltage is high enough, and it should be a multi-stage "smart" charger, NOT a ferro-resonant automobile charger. Anything else will kill your batteries in a short time.
The best bang-for-the-buck in house batteries are golf-cart size....6-volt batteries...in pairs. Two of them in series will give you 12V, and they have thicker plates and will last longer and take more abuse than will 12V batteries.
If you can provide some more info about your cruising plans, battery charger, etc, maybe we can help you further.