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Old 03-01-2024, 14:52   #1
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Two AGM in Parallel. One gets warm.

I have two GPL-8DL batteries in parallel.

They are of uncertain age, but I have used them for 4 years here in Mexico. They seem to have reasonable capacity still but I have not tried to measure.

The boat sits at a dock with the 110V charger on float, and a solar system on float as well. I believe the charger settings for both are correct.The Heart monitor shows 1.8 Amps charge at the moment. The positive lead goes off one and the negative off the other so that should not be a cause of imbalance.

My concern is that today I noticed one battery is noticably warm to the touch at 36ºC. The other is at 29ºC, and the inside of the hull where they sit is at 24ºC.

This implies to me that one battery is discharging through the other in a loop and that one battery is defective.

Am I correct in assuming it is time for new batteries?

If so, the easiest thing physically is to just replace them with similar new batteries, but I am wondering if I should do that, get golf cart batteries, or consider one of the newer sorts -- carbon foam or lithium?
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Old 03-01-2024, 15:10   #2
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Re: Two AGM in Parallel. One gets warm.

The warm one probably has a shorted cell.
Disconnect it and I'll bet the voltage drops within a short time to around ten Plus.


I have had LiFePo4 cells in my boat for over ten years, all the toys on the boat and the cells work just like new.
Same thing in my home. LiFePo4 backup with grid intertie. Love them.
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Old 03-01-2024, 16:31   #3
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Re: Two AGM in Parallel. One gets warm.

Thanks. Can I just drop in a lithium battery and make a few adjustments to my existing charger and Victron solar or am I going to have to change the charger?

I'm looking at this battery for a start. https://www.amazon.com.mx/HiXiMi-Blu...dp/B0CD7NX88D/
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Old 04-01-2024, 01:58   #4
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Re: Two AGM in Parallel. One gets warm.

Swap them over and see if it makes any difference.
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Old 04-01-2024, 02:26   #5
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Re: Two AGM in Parallel. One gets warm.

Quote:
Originally Posted by senormechanico View Post
The warm one probably has a shorted cell.
Disconnect it and I'll bet the voltage drops within a short time to around ten Plus.
Had the same problem and this is exactly how I verified it.
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Old 04-01-2024, 04:16   #6
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Re: Two AGM in Parallel. One gets warm.

Come on guys, measure the current through each battery positive cable before deciding what to buy or even take apart!

You must diagnose and the clamp meter that we all carry (right?) is the tool.
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Old 04-01-2024, 04:50   #7
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Re: Two AGM in Parallel. One gets warm.

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Come on guys, measure the current through each battery positive cable before deciding what to buy or even take apart!

You must diagnose and the clamp meter that we all carry (right?) is the tool.
Disconnecting the charger and letting the battery voltage stabilize—as everybody has suggested—IS 100% diagnostic of a shorted cell. It’s a different test than you describe, but just as effective.

Given the physical symptoms, I’d bet my morning coffee on a shorted cell.

But I do agree that telling the OP what to buy to replace is damn silly when nothing at all is know of his system or use pattern.
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Old 04-01-2024, 05:14   #8
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Re: Two AGM in Parallel. One gets warm.

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Disconnecting the charger and letting the battery voltage stabilize—as everybody has suggested—IS 100% diagnostic of a shorted cell. It’s a different test than you describe, but just as effective.

Given the physical symptoms, I’d bet my morning coffee on a shorted cell.

But I do agree that telling the OP what to buy to replace is damn silly when nothing at all is know of his system or use pattern.
For the meter you don’t need to disconnect anything… you have the measurements in seconds.
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Old 04-01-2024, 10:29   #9
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Re: Two AGM in Parallel. One gets warm.

For those wondering what kind of meter to get for this, make sure it’s clamp supports DC current as well as AC. Here is a good entry level meter:

https://www.amazon.com/ANENG-Multime.../dp/B0BKVVFP21
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Old 04-01-2024, 11:38   #10
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Re: Two AGM in Parallel. One gets warm.

Shorted cell.
Fully charge, disconnect both completely, text voltage next day. IF shorted, the bad one will be low voltage.
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Old 04-01-2024, 12:06   #11
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Re: Two AGM in Parallel. One gets warm.

Quote:
Originally Posted by allendick View Post
Thanks. Can I just drop in a lithium battery and make a few adjustments to my existing charger and Victron solar or am I going to have to change the charger?

I'm looking at this battery for a start. https://www.amazon.com.mx/HiXiMi-Blu...dp/B0CD7NX88D/
Not quite there are lots of different ways to wire up LFP batteries. However, its not complex.

Do you also have a separate engine start battery and what sort of age is that in?

There is a thread running about converting to 24v which you might find interesting and on a larger yacht has some merit.

https://www.cruisersforum.com/forums...ts-282406.html

Alternatively, there are lots of threads about adding LFP on CF.

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Old 04-01-2024, 12:54   #12
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Re: Two AGM in Parallel. One gets warm.

OK. All good advice, Thanks.

I turned off the solar and shore chargers, and the house battery switch, then confirmed there was no draw from the pair,. I then measured a half amp flowing between the two at rest when disconnected from charging or load.

I would have expected it would take more than a half amp to generate that temperature difference, but the current should be zero, I assume. I assume also that it might change with time. Should I check over an hour or so?

As for disconnecting the batteries to see how they settle overnight, I live on this boat and have a DC fridge and lights. I suppose I could disconnect them one at a time, but would it be harmful to disconnect both and run the house on only the charger float? Seems chancey.

The other option is to disconnect them one at a time.
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Old 04-01-2024, 12:57   #13
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Re: Two AGM in Parallel. One gets warm.

I think if you disconnect one at a time you might find the bad one shows up real quick, but it will solve the mystery. Have a torch handy.

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Old 04-01-2024, 12:59   #14
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Re: Two AGM in Parallel. One gets warm.

The start battery is a just a group 24 car battery. The house and start are connected by a Blue Sea SI-ACR. The alternator is managed by a Balmar ARS-5 regulator. Solar is a late model Victron BT. The boat is a an Oceanis 351.
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Old 04-01-2024, 12:59   #15
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Re: Two AGM in Parallel. One gets warm.

A torch?
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