REALLY Dead Battery ??? - SailNet Community
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post #1 of 20 Old 09-13-2016 Thread Starter
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REALLY Dead Battery ???

Our 28 footer has very simple electrics with a single group 27 deep cycle servicing house and our 15 HP outboard. It is on a mooring, usually with a 30 watt solar panel with PWM regulator, but since the battery voltage was 12.7 volts when we left the boat two weeks ago, i did not hook up the panel.

Going to the boat today, everything was dead. The voltmeter read 2.6 volts directly from the terminals. I know all breaker circuits were OFF at the panel. I have two devices that are not wired through the breaker panel: a bilge pump (maybe with a stuck switch??) and an incadescent bulkhead light ( that could have been left on).

So my question is: would a constant 2 week draw from a light or bilge pump reduce a group 27 deep cycle battery to 2.6 volts? Or could there be some other failure within the 9 year old battery itself causing this voltage reading?

In other words, does this battery just need a good slow recharge or is it toast?

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Re: REALLY Dead Battery ???

9 years.....you got your money's worth....move on and get a new one. Don't waste your time trying to revive it.
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Re: REALLY Dead Battery ???

Yes, either of those could do it. Even a 12W lamp is 1A, and will flatten your battery in less than a week.

Edit : missed the bit about the battery being 9 years old. There are two scenarios :

1) Your battery suffered an internal failure and went flat on its own. You need a new one.

2) You flattened the battery by leaving something on, and now you need a new one.

Good news, your 9 year old battery was so sulphated that it had lost most of its capacity, so you didn't lose a battery. 20% of one at the most.

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Last edited by MarkSF; 09-13-2016 at 08:06 PM.
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Re: REALLY Dead Battery ???

168 hr in a week... 1 amp bulb X 2 weeks = 336 ah = 3X to 6X the capacity of your battery.

You're lucky it had measurable voltage.

Normally, a good charge at 5 AMPS to full followed by an hr at TVG should have the battery back in service with just a few months of useful life lost.

But for a 9 yr old battery it probably didn't have a few months left.
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Re: REALLY Dead Battery ???

Ya did good getting 9 yrs. out of it

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It might be worth a little battery CPR. It's a bad discharge, but it is still just a discharge. Amazing what a deep cycle will survive. Many chargers will not even try to bring back a batt from that voltage. My best luck has come by starting with a really stupid shunt charger @ 1A. Smallest, dumbest charger there is. Leave it on until battery voltage reaches 12 or 13. Now hit it with a bigger charger -- 5 or even 10A. Drive it to 14.4v. Let it rest. Drive it again. If you have a smart charger or high-amp alternator, great. If not, just keep cycling it manually with a shunt charger. Then unhook it & let it rest. If it settles out around 12.2v, carry on as before. You've done some damage, but it's like hitting a '95 Civic with a shopping cart. Least of its problems.

A battery with internal damage will behave very erratically. You can't mistake collapsed plates.
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Re: REALLY Dead Battery ???

Sounds like you may have left a light on, but even if everything is shut off, there remains a parasitic load from many electronic devices that can slowly drain a battery. You wouldn't expect parasitic load to kill off a healthy battery, but being 9 years old could mean it had very little of it's original amp hr capacity left. The 12.7 volts you saw does not indicate capacity.

As some have mentioned, you could try to recharge it. There are two risks. First, she takes a charge, but really has very little capacity and you find yourself without a battery, while sailing. Second, damaged batteries can be an explosion danger, particularly when charging.

As others mentioned, nine years is a very good useful life. You're replacing soon one way or the other.


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Re: REALLY Dead Battery ???

You must have a horseshoe some where if that's the first time your battery has gone dead in 9 years. It happens to me a couple of times a year, usually because I leave the key on. I wouldn't worry about it, try to charge it back up. It might be worth checking the fluid levels too. If adding water and charging doesn't work, buy a new battery and hope you get another 9 years problem free.
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Re: REALLY Dead Battery ???

Time for a new battery. Being in the field I test batteries daily for actual Ah capacity. I have two AGM's running on 20 hour discharge tests as I type this. 10.5V, while under discharge, is considered 0% SOC.. The resting recovery voltage, after a 20 hour discharge to 10.5V, is between 11.1V and 11.4V depending upon how long you let it sit at 0%, which you simply don't want to do for more than a few hours. Taking a batteyr to 10.5V then immediately recharign to 100% SOC won't damage it as much as taking the same battery to 50% SOC and not getting back to 100% SOC for a week or more. It is teh time at the low SOC that really causes the damage to the battery. Anytime you discharge to below 10.5V or 11.4V rested, both are 0% SOC, serious capacity degredation going to happen.

Taking a battery down below 3V is grossly beyond 0% of capacity and permanent damage will occur. Yes there is more energy below 10.5V to be had, but not a lot, and it drops off rapidly below that point. Below 10.5V loaded, (unless short duration in-rush loads) permanent damage is occurring.. I have tested far too many batteries, known to be testing well before an over-discharge, to suggest not replacing them...

Most recently was a bank of very expensive batteries that had been testing at about 96% of rated capacity in February of 2016. The owners crappy Rule bilge switch stuck ON and murdered his bank. They sat at well below 6V for approx 3-4 days before he found the issue. I retested the batteries in late July and they delivered between 27% and 28% of rated capacity, and this was after going through a lengthy recovery process... In other words the batteries had become boat anchors.. You got 9 years, pony up the $80.00, move on and call it a day.
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Last edited by Maine Sail; 09-14-2016 at 09:33 AM.
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Re: REALLY Dead Battery ???

Thank you all, I do intend to get a new battery, i recognize that 9 years is a very good service life. I was really more interested in the battery science, whether the 2.6 volt reading was accurate and whether that value clearly pointed to either VERY deep discharge, or an internal failure.

Now I gotta find a good quality 12 volt group 27 deep cycle in the Boston North area. Maine Sail's informative article on how many deep cycle batteries really aren't has me looking for either a Trojan or US Battery unit.
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