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diy solar

One 3.2v lifepo4 180ah cell having large voltage swing compared to other

warly

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Joined
Apr 11, 2021
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I have a battery system with 3 sub-batteries:
- bat 1: 16 x 135ah 3.2v
- bat 2: 16 x 150ah 3.2v
- bat 3: 16 x 180ah 3.2v

They are prismatic cells bought on aliexpress.

I do not have a BMS, I have several type of active and passive balancing per battery:
- Each battery has a 2A ejpower+ active supercapacitor balancer (also used to monitor cell voltage and resistance)
1618161190189.png
- bat 1: cheap 1.2A passive balancer

1618160966346.png
- bat 2: passive 5W resistance triggered at 3.7v for each cell
- bat 3: no other balancing system

I do not use a BMS, I have triggers based on individual cell voltage that would turn on heaters or big resistance to dissipate energy and reduce cell voltage. The solar installation has about 8KW peak capacity, but being in Ireland I never saw more than 6.5 KW peak. The charger are 2 victron multiplus 3000/32 working in parallel. Half of the solar power comes from DC, half come from micro-inverter through he Victron. The system is 100 % off-grid.

My question is regarding the behavior of some of the cells in the installation. Most of cells are just being kept very close in voltage most of the time, and I can only see some discrepancy when the solar power is at max and the battery near full.

1618161492633.png

This behavior looks normal to me, when the battery starts to be full, some of the cells with slightly more or less internal resistance and capacity start to be full before other and see their voltage increase, then the dump load triggers or sun goes away and all come down to be balanced.

However, some cells have a much wider swing in voltage, as shown in the following picture:

1618161462271.png

I cannot really explain what is happening to the blue cell in that case. That cell seems like it has a problem, has its voltage increasing or decreasing much more than the other cells, even when we are not near battery full or empty. I had two cells like that, I removed one and check it, the internal resistance in on-par with the other cells, and the capacity is also the same. I suspect those cells where deficient to some degree but I could not detect it with my initial balancing and capacity test. However quite rapidly I noticed their voltage changing much more, but I do not know what is the physical reason of that problem.

I suspect they may not have been brand new and got some cycles already from a previous life, but they have the same size, looks the same and match their capacity. Maybe they got overcharged before, but internal resistance would be different. If anyone has a suggestion at what is going on or what I can do to know, that would be amazing!
 
Can you get a plot of cell voltage vs current? One thing that happens a lot is that a bad/week connection of a bus bar creates voltage swings that are interpreted as a bad cell. I suspect that might be happening in this case.

Just last week I was helping a guy that had such a bad swing in voltage of one of the cells that the BMS would cut off. He was afraid he had a bad cell. I had him take the bus bars off, clean them and re-install everything.....and the problem went away.
 
Can you get a plot of cell voltage vs current? One thing that happens a lot is that a bad/week connection of a bus bar creates voltage swings that are interpreted as a bad cell. I suspect that might be happening in this case.

Just last week I was helping a guy that had such a bad swing in voltage of one of the cells that the BMS would cut off. He was afraid he had a bad cell. I had him take the bus bars off, clean them and re-install everything.....and the problem went away.
sand them with 600 grit sandpaper, clean with isopropyl, add some noalox or oxguard, reassemble. Hopefully that will keep the problem from returning.
 
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