question

oleksandr-p avatar image
oleksandr-p asked

Unbalanced parallel battery bank

I installed a UPS setup which consists of Victron MultiPlus 12/1200/50 and four parallel connected batteries 12 V Deep Cycle GEL 90 Ah each (360 Ah total). Batteries connected via busbars (5x20 mm - general and 5x20 each battery connection). Busbars are connected to the MultiPlus via 35 mm2 wires (positive 1.8 m, negative 2.2 m). During charging/discharging battery A consumes/supplies 10% of the total current, while the other ones consume/supply 30% each. What's wrong with battery A? As I know current should be equal to all the batteries.

Before I got the busbars the batteries were connected halfway with wires (35 mm2). The situation was the same - battery A- 10%, other ones 30% each.

battery
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5 Answers
kevgermany avatar image
kevgermany answered ·

Sounds as if battery A is faulty. You can test by removing the other batteries, applying load to A and measure voltage drop over time. Then replace A with another battery. Do the same test and compare.

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JohnC avatar image
JohnC answered ·

Hi @Oleksandr_P

Even from new, batteries can arrive with differing resistance. So then you apply the recommended charge V, the ones with less resistance accept more current, get hotter, develop less resistance, then take more current again. This is usually noticed in practice by their Temp, and a touch-test can often help. Ease back on their Charge V and don't push them so hard under charge.


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oleksandr-p avatar image
oleksandr-p answered ·

Hello To All.

It seems @JohnC was right.

I discharged all batteries together at 20% (SOC80%), then I charged every battery individually till storage mode and voltage V=13.25. The average consumption of the system is about 130W (max 200W), so the current is 130W/12V=10.8A, 10.8A/4 batteries=2.7 A per battery. But in reality, the discharge current from the batteries is distributed as follows:

Total – 10.8 A (100%)

Battery A – 0.7 A (6.5%)

Battery B – 3.9 A (36%)

Battery C – 3.4 A (32%)

Battery D – 2.8 A (25.5%).


After several hours of discharge (about 6 hours) I noticed that the current have been changed as follows:

Total – 10.8 A (100%)

Battery A – 2.7 A (25%)

Battery B – 2.8 A (26%)

Battery C – 2.7 A (25%)

Battery D – 2.6 A (24%).


As we can see the current is almost equal for each battery.

So I decided to investigate this situation for a few days:

I increased the loads up to 300 W and was discharging batteries for about 6 hours:

screenshot-2023-01-12-075723.jpg


As we can see current was changing every hour and in the end, it becomes almost equal for each battery. Then I charge every battery individually. The next days I was discharging batteries with a load of 130-150 W. I noticed the same trend: at the start, the current is different for each battery but with the next hours it heading to equal for each battery.

Then I decided to charge batteries together. Right after the 6th hour of discharge the grid has been recovered and the batteries start charging (charge current in Victron Connect setup 37A) but again current is distributed unequally:

Total – 37.5 A (100%)

Battery A – 7.9 A (21%)

Battery B – 11.6 A (31%)

Battery C – 9.4 A (25%)

Battery D – 8.6 A (23%).


I am afraid that during discharge at power about 150-200W batteries B and C will fail faster than the other ones (in accordance with the datasheet the rated capacity is 20h, which means the discharge current should be not more than 90Ah/20h=4.5 A, but in my case, during the beginning hours discharge current of batteries B and C more than 4.5 A (6A and 5.3 A respectively for battery B and C)).

Maybe I'm worrying too much and this situation is normal?


aaa.jpg (114.2 KiB)
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JohnC avatar image JohnC ♦ commented ·
@Oleksandr_P

Interesting research. The results not unexpected though, and I'd consider them 'normal'. Don't get excited about the C20 rating, that's not a limit, and it's probably fine to design to an average of 10% A/Ah under charge/discharge. And even then that's not a limit.

What gets up my nose is batt makers speccing a highish Absorb V range (say 14.4 - 14.8V), not mentioning that the 14.8V is for low batt temps. Then the user picks the highest setting, applies Temp Comp, and gets more again. Then of course those figures came out of a lab with a single battery and make no mention of what can happen when you parallel them.

Easy does it. Charge them to an Abs V as low as you can get away with in the time available. By all means monitor the string differences if it's convenient. But you may find a remote IR temp scanner used occasionally will give a good feel for any critical imbalance.

Lithium batt users may laugh at this, but Li's suffer it badly. Tis why they need a BMS.

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Alexandra avatar image
Alexandra answered ·

@Oleksandr_P

Ideally you would want a much shorter bus bar and the exact same length cables to each battery connected to the bus bar.

And from the inverter to the bus bar the exact same length path.

How you have done it some will always be cycled deeper than others. It is quite typical of battery stacks to behave like that. You see that behaviour even in Pylontec stacks. I also dont like it

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oleksandr-p avatar image
oleksandr-p answered ·

Thank you for the answer @Alexandra. Previously batteries were connected halfway via 35mm2 wires with almost the same length (see screenshot) but the problem was the same. I also switched batteries, but battery A always gave/consume less current. I do not think the shorter busbars will solve the problem. screenshot-2023-01-12-095915.jpg


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Alexandra avatar image Alexandra ♦ commented ·
So then it is solved. It is a battery problem.
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