Parallel batteries charging at different rates

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igvan asked Jun 3, '22

Parallel batteries charging at different rates

I have two 12v 300ah LiFePo4 batteries, purchased at the same time by the same manufacturer. They are wired in parallel, standard + to +, - to -, main + coming off one battery, main - coming off the other. I have been charging them with a Victron Smart Charger up until this point, but today finally got my MP 3000 up and running. When I plugged in to charge, the batteries were receiving wildly different currents, something like 30 amps into one and 55 into the other. I dialed the MP input current way down and was still getting a huge difference, 17 into one and 33 into the other. After removing the batteries from charge, the one which received a higher charging rate predictably starting dumping load into the other until they were balanced.

I have had some issues with one of the batteries having cells that appear to not be too well matched. The cell voltages stay close during much of the charge/discharge curve, but two of them run away toward the top of charging, and the other two never reach 3.4v. This is the battery that was receiving the much higher input current today. The manufacturer keeps insisting batteries are totally fine, which I am finding increasingly unacceptable.

Anyone have input into this? Thanks much

battery charging Lithium Battery multiplus ve.bus

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Answer 1 · 2022-06-03T05:00:13Z

snoobler answered · Jun 03, 2022 at 05:00 AM

Check that ALL connections are torqued to the same value.

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Answer 2 · 2022-06-03T07:01:29Z

kevgermany answered · Jun 03, 2022 at 07:01 AM

It does appear to be a battery fault. Problem is how to prove it and get something done about it.

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Answer 3 · 2022-06-04T12:22:16Z

igvan answered · Jun 04, 2022 at 12:22 PM

I retorqued all my connections, everything was good. After the batteries sat for a night, they began charging quite evenly when I put them back on the charger in the morning. They performed mostly evenly throughout the day of charging and discharging. Today, put them back on the charger, again, super different currents going into each battery. Turned the charger off, fired up my 12v air conditioner, discharge rates were super different. Charger back on, charging rates still very different.

I am far from an expert on this, but it seems as though the batteries just have extremely different internal resistances. Does that sound like a fair assessment?

If I continue operating like this, which the battery manufacturer wants me to do and tells me is perfectly normal (lol), what do you all think it will do to the health and overall lifespan of the batteries? Is it safe to operate like this repeatedly? Any input is appreciated.

Thanks much

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Stefanie (Victron Energy Staff) ♦♦ commented · Jun 04, 2022 at 02:20 PM

@IgVan
A parallel connection of two different (even if they are the same brand and manufacturer) batteries and also different cell internal resistances. In my opinion, this is an absolutely expected result if the battery consists of unmatched cells.

Even if you cannot compare this directly, I tried this at home with a new 330Ah Victron Smart Lithium and an old and very used 100Ah Victron Smart Lithium. The latter has a few cells which are more or less end of life.

The charging behavior is similar to what you describe, very different charging currents and not only because of the different battery capacity (the symptoms are the same). Hardly predictable actually. Only when both batteries are balanced does it return to normal and the charge or discharge current flows into or out of the batteries at about a 3:1 ratio. With decreasing SoC, the ratio changes rapidly again as a result of changing internal resistance and everything starts from the beginning.

I don't think it will do any harm to health or lifespan of the batteries as long as they are operated within the specifications. The BMS should take care of ie. low or high cell voltage.

A possible way fora warranty claim could be a capacity test at C20 for each individual battery and see if the results obtained match those of the manufacturer's specifications.

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Answer 4 · 2022-06-04T13:45:42Z

snoobler answered · Jun 04, 2022 at 01:45 PM

Is it safe to say that current is evenly shared below say 90% SoC, but above that SoC, current is shared unevenly?

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