What’s happening to my bms

[Plopez210]

New to building batteries, I’m getting single cell over voltage alert when charged. Seems to be balanced when under load.

Not sure if it’s a settings or cell problem

I did not too balance when assembled

Eve280k cells. All came from docan at the same time
16s
Jbd 200 bms

 

[FilterGuy]

The voltage on a LiFePO cell remains prety close to the same from about 15% to 80% SOC. Even between 10% and 90% SOC there is not much variation in the voltage of the cell, Consequently, unless the battery is nearly full or nearly empty, the cells will appear to be 'balanced'. It is not till the battery gets full or empty that you start to see any significant differences in the voltage.

This sound exactly like what you are seeing.

 

[HRTKD]

Do a Top Balance on your cells. This will help eliminate the problem you're seeing.

The other option is to lower the charge voltage on your charger. This will give the BMS more time to balance the cells. Unfortunately, it can take many charge/discharge cycles to achieve a true balance.

 

[Plopez210]

The voltage on a LiFePO cell remains prety close to the same from about 15% to 80% SOC. Even between 10% and 90% SOC there is not much variation in the voltage of the cell, Consequently, unless the battery is nearly full or nearly empty, the cells will appear to be 'balanced'. It is not till the battery gets full or empty that you start to see any significant differences in the voltage.

This sound exactly like what you are seeing.

What device do you recommend for top balancing?

 

[Plopez210]

Do a Top Balance on your cells. This will help eliminate the problem you're seeing.

The other option is to lower the charge voltage on your charger. This will give the BMS more time to balance the cells. Unfortunately, it can take many charge/discharge cycles to achieve a true balance.

Thanks for the response, is there a good way to top balance effectively on large cells like these?

 

[HRTKD]

Here is the Top Balancing guide from the Resource section of the forum.

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...

diysolarforum.com

 

[WindWizard]

If you want to top balance without tearing apart your pack then just hook a light across the highest cell after they are greater than 3.4 volts.

https://www.amazon.com/gp/product/B076MKZS35/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

Connect them to the highest Cell with jumpers and then wait for Cell voltage to go down. It takes a while but it does work. My pack is 16s 8p so I needed to have a bit more than the light to balance them so I used these resistors which work well. I mounted them in a heatsink because they do generate a lot of heat.

https://www.amazon.com/gp/product/B07KJYTHZK/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

https://www.amazon.com/gp/product/B01MT1XI7K/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

I never top balanced my cells before putting them into service. It takes a bit of time once they start to run to get them balanced. It is a juggling act but I have never regretted it. I figure that to connect all 128 cells in parallel would take a long time to top balance them including all of the time wasted connecting them and disconnecting them.