Need to quickly Integrate replacement cell in battery bank. Looking for advice.

TrailAgent

New Member
I built a 24v battery using eight 3.2v lifepo4 cells for use in a camper van. I top balanced them a few months ago and have been using them under light loads (under 10a) and charging them at a max of 25a. A few days ago I installed a 24v air conditioner in my camper van and tested it out on the batteries. At full power the AC pulls about 28a. All worked well except one cell discharged very rapidly when using the AC and went out of balance. The other cells are at nearly identical voltages. I replaced the cell that went out of balance thinking it may bad. But the replacement cell is at a slightly higher state of charge than the others. What is the best way to either discharge the new cell or charge the other seven to bring them closer together so I can do a quick top balance? Unfortunately, I leave next week with the camper and will need my battery system to be in shape by then. Below is a screenshot of the voltages. The new cell is no. 60EDFF7C7-18E2-4797-B5CB-BA2BF229CC84.jpeg
 

TrailAgent

New Member
I should add that I have a bench top charger so I could charge each of the 7 cells to match the new cell. Would that be better than discharging the new cell to match the other seven?
 

RCinFLA

Photon Sorcerer
If voltages are accurate the one you replaced is about 80% other are a bit below 50%.

If you are leaving and really need to be sure you get it right, redo a top balance.

You will never get it matched well by just discharging the one or bringing the others up to match the highest voltage cell..
 

jwelter99

Solar Addict
I built a 24v battery using eight 3.2v lifepo4 cells for use in a camper van. I top balanced them a few months ago and have been using them under light loads (under 10a) and charging them at a max of 25a. A few days ago I installed a 24v air conditioner in my camper van and tested it out on the batteries. At full power the AC pulls about 28a. All worked well except one cell discharged very rapidly when using the AC and went out of balance. The other cells are at nearly identical voltages. I replaced the cell that went out of balance thinking it may bad. But the replacement cell is at a slightly higher state of charge than the others. What is the best way to either discharge the new cell or charge the other seven to bring them closer together so I can do a quick top balance? Unfortunately, I leave next week with the camper and will need my battery system to be in shape by then. Below is a screenshot of the voltages. The new cell is no. 6View attachment 59794

You can try to discharge that one cell using a 12V lightbulb or better is a power resistor. But doing that may not solve the issue as the challenge is these cells have such a flat voltage output from 10% to 90% SOC you likely still are not balanced.

I personally would discharge that cell with a resistor to get it close, parallel all the cells overnight, then top balance them back up. That's the only way to get them all to the same level reliably.

Another quick solution would be a cheap active balancer and hook it up and let it do its thing. If you have quick access to one that might be the fastest solution, and in theory can be run while the battery is in use - so while you are out camping.
 
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TrailAgent

New Member
You can try to discharge that one cell using a 12V lightbulb or better is a power resistor. But doing that may not solve the issue as the challenge is these cells have such a flat voltage output from 10% to 90% SOC you likely still are not balanced.

I personally would discharge that cell with a resistor to get it close, parallel all the cells overnight, then top balance them back up. That's the only way to get them all to the same level reliably.

Another quick solution would be a cheap active balancer and hook it up and let it do its thing. If you have quick access to one that might be the fastest solution, and in theory can be run while the battery is in use - so while you are out camping.
Any advice on the best type of resistor to use? Would simply connecting a lightbulb work?
 

jwelter99

Solar Addict
Any advice on the best type of resistor to use? Would simply connecting a lightbulb work?

Lightbulb will work, but be slow.

I use a pair of 0.5 ohm 100 watt resistors in parallel to get 0.25 ohm. This gets you about a 13A discharge rate.

 

TrailAgent

New Member
So I got my batteries top balanced and ran the ac. All was good. Each cell discharged at a consistent rate. However, when I charged the batteries back up they stayed in sync until hitting about 85% capacity and then the last cell spiked and the BMS turn off due to over voltage. Also, the cell in 6th position (and 7th) failed to keep up with the other batteries. I am using 4 cells from one batch and 4 cells from another batch. Should I mix them up or put them in any particular order?
 

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TrailAgent

New Member
Quick update. I ran the ac for about 5 minutes and the batteries leveled out almost immediately.
 

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TrailAgent

New Member
Quick update. I ran the ac for about 5 minutes and the batteries leveled out almost immediately.
Batteries still are not charging in synch even after I top balanced them. Cell 8 jumps to over voltage and cells 6 and 7 never seem to go past 3.35. Could it be the BMS? I’ve changed the cells and copper bars.
 

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jtvt

Solar Enthusiast
You could try a voltmeter and see if the bms readings are correct. You could also swap cell 8 with cell 6 or 7 and see if the voltages follow the cells.
 
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