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Old 14-10-2019, 21:07   #1
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Weird battery issue (12v to 6v)

Has anyone ever had a 12v battery suddenly start outputting 6v?



We have a single 4d battery (15 months old) which has been working fine, but today the lights wouldn't turn on. When I checked the battery monitor, it was only reading 6.35v on the battery; I verified this with a handheld multimeter as well.



Only thing I can think of is an internal fault with the battery, though I can't think what would have caused it.



Any other thoughts?
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Old 14-10-2019, 23:24   #2
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Re: Weird battery issue (12v to 6v)

I'm surprised the battery monitor is still on if it's only getting 6v.

are you sure you didn't drain it down to 6v? have you tried to charge it?
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Old 15-10-2019, 08:11   #3
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Re: Weird battery issue (12v to 6v)

Yea, the battery monitor was the only thing that would still power up.

The battery has been connected to the alternator (along with the start battery, which is still charged). So I don't think it's a lack of charge issue. Is it even possible to draw a 12v battery down to 6 volts?
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Old 15-10-2019, 08:25   #4
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Re: Weird battery issue (12v to 6v)

Would it be possible for a 12V battery to actually be 2 x 6v in series internally? If one of the internal cells died, the remaining cell would be reduced to 6v, theoretically.
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Old 15-10-2019, 11:41   #5
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Re: Weird battery issue (12v to 6v)

Sounds like shorted cells but three at one time? Yank it out and see what happens charging it shore side.
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Old 15-10-2019, 11:42   #6
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Re: Weird battery issue (12v to 6v)

That was my thought.. maybe an internal connection break that somehow disabled half the cells?


I called the manufacturer and they suggested getting a charger that will charge a 12v battery that has been drained to 6v and see if it comes back up; just in case it somehow got inadvertently discharged that deeply. So I guess we'll start with that.


I'd love to hear if anyone else has any other ideas.
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Old 15-10-2019, 16:14   #7
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Re: Weird battery issue (12v to 6v)

Yes, I’ve seen 12 volt batteries that only put out 6 volts. It happens all the time when batteries are bad or deeply discharged.

So. Do you have a battery charger other than the alternators mentioned in your post? A battery charger to keep a battery topped up and supply house loads like lights and refrigeration is a necessity. Such a charger can be solar or from Ac mains.

At 6 volts a 12 volt battery is deeply discharged, and a deeply discharged battery is a damaged one ( due to sulfatization of the plates),

You need to get this battery on a mains powered charger to see if you can bring it back. First bring it to a full charge (if possible), and then equalize it to remove sulfate from the plates.

There’sa good chance that the battery is bad and that no amount of charging will restore it. You should be there whilst the battery is charging. A battery with a bad cell can get hot enough to catch fire or explode. If the battery gets hot while charging, turn off the charger and buy a new battery.

Oh. If a new battery is required, ditch the 4d — they’re horrible Deep cycle Batteries — and replace it with a couple of 6 volt golf cart batteries wired in series.

If all of the above is Greek to you, hire a marine electrician.
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Old 15-10-2019, 16:52   #8
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Re: Weird battery issue (12v to 6v)

See if your alternator is hot--if it is even warm when the engine has not been used, your little cut-off relay is either not working or has been bypassed by someone wiring the voltage regulator incorrectly.
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Old 15-10-2019, 16:56   #9
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Re: Weird battery issue (12v to 6v)

I've had 2 separate instances where 8D batteries got shorted inside. They wouldn't hold a charge and got quite hot when charging.
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Old 16-10-2019, 07:47   #10
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Re: Weird battery issue (12v to 6v)

I suppose I should supply a bit more backstory for this..


The battery isn't in our boat, it's actually in a camper we built and sold and the new owner is having troubles. Part of what I posted in the first post was my own observations (6v readings) and part is what I was told by the new owner (lights were working fine one day, then stopped working suddenly).


The battery doesn't have any other charger except for the truck alternator (connected by a run of 4 gage cable w/ 100amp breakers on either end), which will charge the battery anytime you drive the truck. I suppose it also has solar, but the panels are briefcase style so they're not charging the battery unless you set them up. The only loads in the camper are a Dometic cooler (which shuts itself off if the voltage drops below 11.4v), LED strip lights, battery monitor and a solar charge controller display.


I suppose this is where I'm struggling with this issue.. My wife and I pretty religiously pay attention to our power usage and battery SOC, so I just can't fathom how someone could discharge a battery so severely that it is only outputting half it's rated voltage.


I know the ACR won't combine the batteries if the voltage is below 10.8v on either of the batteries, so I suppose it's possible that the new owner wasn't paying attention and drained the battery below that point with the lights/etc; then continued to not pay attention to the battery until the voltage dropped so low that even the lights wouldn't turn on.


I guess I will tell the new owner to get an external charger and see if the battery will come back up to the correct voltage (obviously paying attention for hot spots on the battery as it charges), otherwise they'll have to get a new battery.


Thanks for the replies/discussion everyone.
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Old 16-10-2019, 13:25   #11
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Re: Weird battery issue (12v to 6v)

No.

The failure is due to the unit continuing to be used for a very long time past the point

70-80% SoH

where it should have been proactively replaced.

Doing so is dangerous, lucky it didn't explode.

Good for scrap only, replace it ASAP, making sure with a proper deep cycling type, which those automotive sized 12V units rarely are.
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