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Discussion Starter · #1 ·
Came back to the boat after 5 weeks away and found the two AGM group 31 batteries (1 year old) completely discharged. Boat was plugged into show power, BUT the marina had some work done on the piers, cut the power to the slip and forgot to restore it a few weeks back.:mad:

What am I looking at? Are the batteries essentially toast and I need to head out now and get replacements? Or are they ok, just with shorter expected lives? Or something else?

Thanks,
Jim
 

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Jim-
Yes, you have shortened their life and possibly damaged them.

But there is no way to tell other than recharge them, and then test them. Every battery maker will be a little different and a "100% discharge" is the cruelest service for any battery. Part of the question is how long they were totally discharged, longer being worse.

For a car battery (SLI type) four to six total discharges can totally consume the battery. For deep cycles...I'd take a guess from hazy memory they are at least 10x more robust. So charge them, gently if possibly, and fully. Then see what you've got. They're probably still quite good enough for your normal use.

There is a gizmo called a "battery brain" that will also cut off batteries when the voltage drops below a set point. You can find them on Amazon, on eBay, all the usual suspects including some auto stores. One more thing that can break of course, but a good way to make sure you really really have to try hard to do this again.(G)
 

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Discussion Starter · #6 ·
at 11.5 they will be fine. according to my chart that leaves 20% life left. so it was just a really deep cycle. just be glad you didnt have a bilge pump get stuck on. what did wear the batteries down that low?
The only thing that was on was the bilge pump. Over those weeks we had some tremendous rains and water can come down the mast and into the bilge. The bilge pump appears to be operating normally. I spent the day flushing the water tanks into the bilge and the pump came on and off as expetected.
 

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I wouldn't think 11.5 was that big of a deal. Our inverter/charger has the cutoff mentioned above, but I don't think it kills the bank until 10.5v.

If they were damaged, the marina would seem to be some responsibility here. Clearly, they can't be responsible for every cut in power, however, I would shut systems down or get there to recharge otherwise, if I knew I was out of power for weeks, not days.
 

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The only thing that was on was the bilge pump. Over those weeks we had some tremendous rains and water can come down the mast and into the bilge. The bilge pump appears to be operating normally. I spent the day flushing the water tanks into the bilge and the pump came on and off as expetected.
Time for a new boot then. You don't have one of those auto-sensing rule pumps that spin every 2 min to detect water, do you? also make sure you use a multi-meter to find if you have any power draw. Many boats are left on a mooring ball for 6 months and come back to batteries at 90% left.
 

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I look at what else is going on with leaks ,electric or water :)

My boat lives on a mooring and has been through the same rain and there should not be enough water coming down the mast to run a battery down

Even my J24 which had no bilge pump never got more than 8 to 10 gallons in the bilge after a hurricane
 

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11.5 isnt catastrophic but its worse for agms than old flooded I have been told

I would do this:

charge them at a very slow rate for a long period of time...and then give it a nice blast on high charge after you have acheived normal voltage for your batteries

I know this goes against the normal equalize first(bulk charge) at high rate and then float charge but I have experienced decent results bringing back dead batteries like that

I just did it on my old truck battery that I have on the boat for projects and tools and stuff and that baby got down to 10...I charged it for 3 days on a trickle charger or 1 amp charger and then blasted it only after it got to a steady 12.5 after sitting a while...

that battery is still doing fine although I know its been damaged and its cycling abilities have decreased I still wouldnt through it in the trash

fwiw
 

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I just had the same thing happen to me. The batteries had 0% charge, but when I checked them they had 11.5 volts across the posts. The young man at Auto Zone said he would suspect the charging system as both batteries read the exact same. My boat is a 1986 Catalina 30 with and old charger in it. It is not self tending. My question is: How do I check the charger to see if it is functioning properly.
Thanks
Jeff
 

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With an old-fashioned analog or dumb charger, you could just connect a voltmeter to the output leads and see if it was putting out ~14.4 volts. With the new chargers, even the cheap $30 ones sold in auto stores, many of them are designed to prevent charging fires if they are connected to a dead battery and if they sense the battery voltage is below ~11 volts (or whatever set point is used by that maker) they will not put out any power.

So in order to test a charger and be sure it is working, you'd need to connect it to a "good" battery, one that is probably over 11.7 volts. (11.5 being defined as dead with no load on it.)

Since both batteries are connected together, it is possible that they equalized with each other, and that is the reason both are "the same". Not necessarily a bad charger.

I'd suggest buying an inexpensive* charger, even something like a 2A or 6A cheap dumb charger, and connecting it to one battery at home. See if it comes up, repeat on other battery. If both come up, take them out to the boat and see if the boat's charger keeps that voltage up--or doesn't.

*unless you have use for buying a better one, at least something that can be more useful for you. Or, borrowing one. Or dropping the batteries at a battery warranty center & distributor, where they can actually load test them for you. The auto chain stores often don't have highly educated help.
 

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This is another reason that solar--even a small system--is helpful. At my home marina it is all too common for the power to go out, often because of something a slip holder did, and without solar dead batteries are often the result.
 
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