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Old 19-03-2018, 07:40   #1
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Different current from house bank batteries

Hi,
I have 3 x 12V batteries in parallel. When turning on a few consumers I have measured how much current is leaving each battery.
1. 4.0 A
2. 1.2 A
3. 4.4 A
If I understand correctly this indicates that the internal resistance of the 2nd battery is higher then of the 1st and 3rd so it is more aged. (The batteries are 5 years old). I would like to delay replacing the batteries till next season. What should I do? I suspect that having all 3 connected together is worst then only having 1 + 3. Should I consider the 2nd battery dead and disconnect it? (and leave with the less battery capacity)?
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Old 19-03-2018, 08:44   #2
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Re: Different current from house bank batteries

A proper load test on each would be a more accurate measure of SoH.

Were they all purchased at the same time? I assume the same model.

A bank made up of paralleled strings have inherent balancing issues, but 3 is often given as a safe maximum.

If the one is really that much worse than the others, I would probably replace all three sooner than later.

Worst case, go to a 2-batt bank and put in a reserve one, possibly using the "bad" one if it's not too bad. But don't just buy one new one now planning to mix it with others purchased later.

Obviously keeping everything fully charged as often as possible becomes more critical.
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Old 19-03-2018, 09:03   #3
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Re: Different current from house bank batteries

It could also be that the wiring is not balanced for the cells. If your feeding plus and minus from the same side that can cause different resistances in the wires which effect which battery works harder.

You can swap two battery's and re read the current to check that. Ideally Plus is taken off in one direction/end and Minus is taken off at the far end of the bank, which equal length interconnects.
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Old 19-03-2018, 09:11   #4
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Re: Different current from house bank batteries

Yes, the incorrect pattern is very often found online.

This is correct:

https://marinehowto.com/wp-content/u...rectHookUp.jpg

https://marinehowto.com/installing-a...ttery-charger/
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Old 19-03-2018, 09:16   #5
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Re: Different current from house bank batteries

Quote:
Originally Posted by john61ct View Post
Yes, the incorrect pattern is very often found online.

This is correct:

Updated link to make it an image.
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Old 19-03-2018, 17:49   #6
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Re: Different current from house bank batteries

The center battery is reading low? have you cleaned all your connections? has one battery used more or less water than the others? are they all the same resting voltage?
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Old 19-03-2018, 19:04   #7
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Re: Different current from house bank batteries

I have all the + and - line connected to the same point with heavy same length cables.
Will do some more testing
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Old 20-03-2018, 08:18   #8
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Re: Different current from house bank batteries

Disconnect one battery at a time, and check for a set of 2

Be sure that contacts are similar,and ok.

Also V at rest will tell you if a bad battery draws from the other two.

V is about the state of battery, A draw may depend on some few more factors (oxidized wires you can't see...)
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Old 20-03-2018, 08:31   #9
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Re: Different current from house bank batteries

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Originally Posted by rotorman View Post
I have all the + and - line connected to the same point with heavy same length cables.
Will do some more testing
I hope not literally the same point.

Do you mean to posts on the same battery?

Or are you wired as per the diagram just above?
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Old 20-03-2018, 08:36   #10
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Re: Different current from house bank batteries

Quote:
Originally Posted by rotorman View Post
Hi,
I have 3 x 12V batteries in parallel. When turning on a few consumers I have measured how much current is leaving each battery.
1. 4.0 A
2. 1.2 A
3. 4.4 A
If I understand correctly this indicates that the internal resistance of the 2nd battery is higher then of the 1st and 3rd so it is more aged. (The batteries are 5 years old). I would like to delay replacing the batteries till next season. What should I do? I suspect that having all 3 connected together is worst then only having 1 + 3. Should I consider the 2nd battery dead and disconnect it? (and leave with the less battery capacity)?
Your suspicions are correct! I would bet you can get to next season eliminating the bad one. You may have to watch your usage.
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Old 20-03-2018, 08:44   #11
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Re: Different current from house bank batteries

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Originally Posted by sailorchic34 View Post
It could also be that the wiring is not balanced for the cells.

thats my thinking too, and its very common
out of a bank of three batteries, power ought be distributed from the middle one
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Old 20-03-2018, 09:26   #12
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Re: Different current from house bank batteries

I don't by the wiring. Paralleled batteries are electrically the same no matter where the taps are. As long as the wire ga. is sufficient connecting them.
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Old 20-03-2018, 09:34   #13
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Re: Different current from house bank batteries

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I don't by the wiring. Paralleled batteries are electrically the same no matter where the taps are. As long as the wire ga. is sufficient connecting them.
I totally agree in that too. Electrons motion follows simple rules.
I would make sure also there is no hidden resistor in the connection such as corrosion.
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Old 20-03-2018, 09:59   #14
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Re: Different current from house bank batteries

I suggest cleaning ALL of the connections to the underperforming battery and testing again.
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Old 20-03-2018, 10:01   #15
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Re: Different current from house bank batteries

So much wrong in the last few posts.

Look at the source of the diagram I posted, read the whole site, lots of good learning there from Maine Sail, to say a very experienced professional is understatement.

Obviously some bits may be expensive overkill for small boats or those on a budget, but they're very solid on the fundamentals

which **are** both real, and important.
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