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Old 04-01-2024, 13:02   #16
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Re: Two AGM in Parallel. One gets warm.

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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.
A half amp is nothing, not enough to worry about so yes the next step is to disconnect one and keep a close eye on the voltage of the one still connected. After a couple of hours measure the voltage of the disconnected one.

Are you sure both batteries are the same age? Often there is a date somewhere on the battery.
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Old 04-01-2024, 13:04   #17
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Re: Two AGM in Parallel. One gets warm.

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A torch?
You might find the lights go out real quick, so one of these Linky
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Old 04-01-2024, 13:05   #18
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Re: Two AGM in Parallel. One gets warm.

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Originally Posted by allendick View Post
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.
You need to disconnect them both, and let them rest. It isn't the current flow directly causing the heat. It is that with a shorted cell, one of the batteries is now a 10V battery, that is being overcharged as a 12V battery.

You could do that one at a time. Disconnect one, let it rest 12 hours, measure the voltage, then swap and do the other.
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Old 04-01-2024, 13:14   #19
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Re: Two AGM in Parallel. One gets warm.

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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.
Oh that makes life simple. Other than some heavy duty fuses you are pretty much set up for LFP. However, not impressed with the one you linked to. I think you might choose a better named brand with a reputation for supply better quality batteries locally. You really only want to fit them once and not have to deal with problems once at sea. You should have bluetooth and check how many can be linked together in parallel in case you want to expand the bank later on. LFP can be addictive.

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Old 04-01-2024, 13:25   #20
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Re: Two AGM in Parallel. One gets warm.

Thanks. I just linked that one because it can be had here easily but then I noticed that it would arrive late Feb. The price is a bit steeper than some suggest.

There are some local dealers, so I have to do some research.

I can limp by for a while on just one oof the 8Ds, or even both. I just worry if I leave the the one that is heating connected, things could get worse and maybe dangerous, plus it would be a drain when not at dock.
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Old 04-01-2024, 13:37   #21
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Re: Two AGM in Parallel. One gets warm.

Well, I think I have the warm one disconnected from the other and the main switch off. The warm one reads 12.82V. The cool one reads 12.65V. After ten minutes it is up to 12.72V.

Go figure.

Could it be that the warm one is the one that is working and the other has high internal resitance?
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Old 04-01-2024, 15:02   #22
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Re: Two AGM in Parallel. One gets warm.

An hour later: 12.82 and 12.73.

Very curious. Maybe load tests are called for?

Oh, I see my profile says Sidney BC. Yes, I have a boiat there but am living on my boat in La Pa, Mexico. Getting batteries here is a bit different.
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Old 04-01-2024, 15:31   #23
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Re: Two AGM in Parallel. One gets warm.

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Originally Posted by allendick View Post
An hour later: 12.82 and 12.73.

Very curious. Maybe load tests are called for?

Oh, I see my profile says Sidney BC. Yes, I have a boiat there but am living on my boat in La Pa, Mexico. Getting batteries here is a bit different.
The slow voltage increase is the good battery recovering from the constant discharge it was connected to before you separated it from the bad battery.
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Old 04-01-2024, 15:43   #24
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Re: Two AGM in Parallel. One gets warm.

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The slow voltage increase is the good battery recovering from the constant discharge it was connected to before you separated it from the bad battery.
So you think a battery that stays at 12.82 steady has a shorted cell?

Edit: OP hasn’t answered question on age of batteries, date code on them…
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Old 04-01-2024, 16:02   #25
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Re: Two AGM in Parallel. One gets warm.

I looked. I cannot see anything but the batteries are enclosed on three sides. On top, each has an address that begins with 2009. I doubt that is a date.
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Old 04-01-2024, 16:10   #26
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Re: Two AGM in Parallel. One gets warm.

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A torch?
A cylindrical metal device for storing dead batteries… has a light on one end…

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Old 04-01-2024, 16:14   #27
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Re: Two AGM in Parallel. One gets warm.

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I looked. I cannot see anything but the batteries are enclosed on three sides. On top, each has an address that begins with 2009. I doubt that is a date.
So you think they are equal age? Do they appear newish or old?
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Old 04-01-2024, 16:17   #28
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Re: Two AGM in Parallel. One gets warm.

They appear to be identical and my friend who installed them assured me thye were good and they have been. Now I am wondering if they need to be equalised.

I had though that AGMs should not be but am reading this article: https://lifelinebatteries.com/blog/c...agm-batteries/

These are Lifeline brand GPL-8DLs
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Old 04-01-2024, 16:22   #29
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Re: Two AGM in Parallel. One gets warm.

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Originally Posted by allendick View Post
They appear to be identical and my friend who installed them assured me thye were good and they have been. Now I am wondering if they need to be equalised.

I had though that AGMs should not be but am reading this article: https://lifelinebatteries.com/blog/c...agm-batteries/

These are Lifeline brand GPL-8DLs
Can your friend remember how old they are?

Lifeline is a very good brand. They invented AGM batteries.
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Old 04-01-2024, 16:26   #30
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Re: Two AGM in Parallel. One gets warm.

I doubt he knows. I have a Heart monitor and a Balmar Smartgauge and they appeared to have the roughly full rated capacity until recently according to both. They did get discharged to 30 % at one point and in the last while the monitors have not agreed. Coincidence?
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