JBD BMS settings

[c3auto]

Hi all
4S 200ah lifepo4 cells (sinopoly) with JBD100A BMS.
Charged with solar (MPPT allegedly)

Im after some advice regarding the tweaking of the BMS (overkill app, but JBD BMS) settings.

I've attached:
1) current settings
2) cell undervoltage alarm from last week (delta 0.078v)

general questions:
Q1) total battery capacity seems to have increased for some reason (i set it originally at 200,000mah)
Q2) turning off discharge and charge, my voltage dropped from 13.40v to 13.39v but the capacity changed from 99% to 78%?
Q3) can the delta 0.078v be improved? I did top balance them initially.

I think my main issue is that ive set the config settings wrong (possibly too conservatively).
My main priority is cell longevity.
Thanks

 

[SeaGal]

I'd say your cell undervoltage trigger of 3075mV (with release at 3100mV) is too conservative. Better to set your inverter to stop discharge at a given SOC (say 15 or 20%) and use the BMS to do its job and protect the cells if something goes awry.

The BMS should be the fall back, not the primary trigger for stopping discharge.

Voltage is not a good measure of SOC, and if you are drawing a big load, the voltages will drop quite a bit. So personally I'd set the Under voltage protection on the BMS lower, like 2500 - 2600mV and rely on the BMS to stop discharge when the SOC gets low.

Delta of 78mV is OK at the very top of bottom of the range, but seems large for resting cells around the 3.1 to 3.15V range to me. I'd do another top balance and check all connections are good to cell #1.

Edited to add: 100% cell voltage setting of 3400mV will mean the BMS will report 100% to your inverter when a cell reaches that value, which will stop it charging. This gives no time for the BMS to do any balancing, as balancing is set (correctly) to start at 3400mV. Hence I'd increase that value to 3450 or 3500mV. This could be a factor why the cell balance has gone adrift.

 

[c3auto]

I'd say your cell undervoltage trigger of 3075mV (with release at 3100mV) is too conservative. Better to set your inverter to stop discharge at a given SOC (say 15 or 20%) and use the BMS to do its job and protect the cells if something goes awry.

The BMS should be the fall back, not the primary trigger for stopping discharge.

Voltage is not a good measure of SOC, and if you are drawing a big load, the voltages will drop quite a bit. So personally I'd set the Under voltage protection on the BMS lower, like 2500 - 2600mV and rely on the BMS to stop discharge when the SOC gets low.

Delta of 78mV is OK at the very top of bottom of the range, but seems large for resting cells around the 3.1 to 3.15V range to me. I'd do another top balance and check all connections are good to cell #1.

Edited to add: 100% cell voltage setting of 3400mV will mean the BMS will report 100% to your inverter when a cell reaches that value, which will stop it charging. This gives no time for the BMS to do any balancing, as balancing is set (correctly) to start at 3400mV. Hence I'd increase that value to 3450 or 3500mV. This could be a factor why the cell balance has gone adrift.

Thanks for the long post Seagal, I have a very basic inverter and it doesn't have the cut off function but Il do my best to update the settings as suggested

 

[SeaGal]

Thanks for the long post Seagal, I have a very basic inverter and it doesn't have the cut off function but Il do my best to update the settings as suggested

In that case, maybe a cut off of cell reaching about 2900mV would be ideal? It may take some tweaking and testing to see what the voltages dip to when your typical load is applied, compared to what their resting voltage is, i.e. with no load. At the end of the day, you don't want it cutting prematurely if you've just turned the kettle on! Maybe others on here using simpler systems with 12V LiFEPO4 packs can chip in with more experience than me.

Regarding the capacity change... from my experience (I have 16s Overkill/JBD BMS), the BMS will recalculate its estimated % SOC every time you write new data to the BMS (e.g. if you tweak a value such as cell voltage or a cut-off limit). It then bases the SOC by interpolating the cell voltages that you've entered for each 10% SOC. After that, it will just measure current in and current out the calculate the SOC, but if the max or min cell voltage is hit, then it will 'jump' to 100% or 0% accordingly.

My experience is that the BMS's estimate will drift over time, so if you never get to 100% (or 0%) it will end up being 'out'.

I am guessing that your BMS had either hit the max cell voltage recently or it had calculated that the input charge it had been receiving got up to 99%. Then presumably you updated the BMS settings (even if you didn't change any values) and it re-calculated the SOC.

Your 2nd image shows voltages of 3345, 3349, 3351 and 3351mV. That averages 3349mV. You have "defined" 3350mV as being 80%, so the BMS is correctly saying 3349mV average (or maybe it used the 3345mV min voltage, not sure ?‍) is 78%.

Hope that helps.