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Different voltage cells in DIY battery

ChrisSnow

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May 20, 2021
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Hello all.

I've built my first solar and DC-DC charging system for my camper van with a DIY lifepo4 12v battery.

It uses 4 used CATL 150ah 3.2v cells in series. and this JBD BMS (the 150A one): https://www.aliexpress.com/item/1005001564046325.html Attached are images of the parms. I use the stock Lifepo4 'normal' pre-programmed values. I took the image of the main screen of the app after I'd run the battery down until it cut out.

I have a new Renogy 30A DC-DC hooked up to it: https://www.amazon.co.uk/Renogy-DC-DC-board-Battery-Charger/dp/B0891SJTD5

The system works pretty well in that it can run a heat gun or a microwave oven for a good long time so I'm fairly happy, but cell 4 is always high in voltage compared with the others and cell 1 always low, so maybe I'm not getting best performance out of it.

I'm pretty sure that in one of Will's very many videos, he'd said if you have a BMS, don't worry about balancing, the BMS will take care of it after a few cycles. However I've only seen cells 3 and 4 balancing in the Android app. It's version 3.1.1026. It is the admin version.

Any help much appreciated!

Chris. Screenshot_20210910-212158.pngScreenshot_20210910-212319.pngScreenshot_20210910-212409.pngScreenshot_20210910-212438.pngScreenshot_20210910-212456.pngScreenshot_20210910-212512.png
 
Did you perform top balancing before putting the pack together?
Did you verify all the connection and also move the suspect bad cell to another location to see if the problem follow it?
 
Cell 1 is way out of balance at a much lower state of charge than the other 3. Either charge up that cell with a suitable charger, or top balance the cells with a 3.36 volt charger, or charge all the cells individually to 3.65 volts, or discharge the high cells with a suitable load.
The objective is to get the cells at their ideal 'full' volts similar to each other.
Check also the connections to cell buss bars and bms cables, compare cell bms volts with a meter direct on the cells.
The app reading of 100%soc does not match the cell volts.
Check that cell capacity is set in the app.

Mike
 
Thanks for the replies all. Busy with other jobs on the van today so it has sat on charge from the PV as it's a rare sunny day here in UK. This is how it's looking now. Obviously the cells are now a lot closer in voltage. Is this masking a problem or problems, or is the balancing functionality of the bms taking care of things for me? If I can avoid having to take the whole thing to bits to do stuff to cells individually then I'm happy about it; not least because I have no sperate charger or test equipment bar a cheap DVM.Screenshot_20210911-191538.png
 
Since taking more care examining the screen grabs, a number of points.

The displayed state of charge does not agree with the cell or battery voltage, via the app go to 'function setting page' and select 'reset capacity'.
The big difference in the first picture between the voltage of cell 1 and the others is due to cell imbalance and the fact you were actually at low state of charge (not 100% as shown on the display ). The second picture once the cells had charged a bit showed more in balance. The display showed 100%, still in error, and for some resion discharge path was open, what stopped discharge?
The bms will not start balance until the cells are over 3.4 volts.
Although the cells are closer in volts, cell 1 is at a lower state of charge than the others , very aproxametly cell 1 is 40%, cell 2,3,4 are 50%.
As you get near full charge the voltage difference will increase and eventually one of the cells will hit the 3.65 limit and the bms will stop charging.
The balance built into the bms will attempt to reduce the charge and thus volts above 3.4 volts, but may not make too much difference due to the low 'bleed off' current.
If you do nothing to get the cells in balance,( the bms will take forever as the inbalance is large) the battery is still usable but effectively your 150 ah battery will be less, perhaps only 120 ah, as the top cell limit is stopping charge before all are charged to the same voltage, and on discharge the cell with the lowest charge is stopping discharge early.

The cells will all have different storage capacity. There is nothing you can do to change that and your pack is limited to the capacity of the lowest capacity cell times 4 ( not considering active balancing).

The best you can do is to ensure the cells are balanced in voltage when they are full or empty. Most systems adopt the balance when full approach.
Since your battery is built and in use and you have no AC charger and no other test equipment this is what I suggest .

Continue with solar charge of the battery, try to reduce loading so it gets to near full ( reset capacity as suggested, the app will display a more correct capacity reading)

Eventually one of the cells will get to 3.65 and the bms will stop charging.

Obtain a suitable dc load, I used a 12v 50 watt headlamp bulb. Attach suitable leads with 'probes'. The objective is to connect temporary for several seconds at a time, to the cell with the highest voltage, when the cell volts are over 3.4 volts and the battery is charging. After some experimenting it's possible to relate the connection time to the voltage reduction. Since all the cell volts are displayed via the app its easy to see the effect.
If you are using the default setting in the Renogy unit it charges to 14.4 volts for lithium, thus the aim is to get all the cells to as near as possible to 3.6 volts when the charger gets to 14.6.
This may prove difficult and using 'user settings' in the charger to lower the voltage to a lower value, say 14.0 volts, 3.5 volts per cell will be more realistic.

Mike
 
]The display showed 100%, still in error, and for some resion discharge path was open, what stopped discharge?
I'm afraid I don't know what stopped the discharge.
The bms will not start balance until the cells are over 3.4 volts.
Thanks for the detailed and useful reply I hope to be able to get around to tackling the individual cells at some point. I'll see if the app allows me to reset the capacity. Which setting determines that balancing won't begin after 3.4v please? The settings screen grabs above have the 'balancing turn on voltage' at 3.2v and I don't see any other setting that jumps out at me as affecting the same.
 
screen grabs above have the 'balancing turn on voltage' at 3.2v
Missed, that I was working with a phone. The usual value is 3.4 volts and the precision 0.015 volts.
Ideally you only want balancing to take place when under charge and where cell divergence starts to become prominent. 3.2 volts a cell is within the normal working range where cell imbalance will not show up.

Mike
 
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