JK BMS settings with possible wrong Battery Capacity - what could goes wrong?

[vapi]

Hi, I bought 16 lifepo cells of 320Ah. A review on the site says they are not realy 320Ah but maybe 300Ah (from his tests).
I have connected these cells in series to a JK B2A24S20P BMS with BT support and in in application I have to setup Battery Capacity.
I have set 320 Ah as the seller says but I wonder what could happen if the cells are not 320Ah. What if I set 300Ah in the app but the cells are 320Ah?
At this moment I set Battery Capacity as 310Ah, and the BMS app displays 56.17V, Average cell Voltage: 3.51V and Remain Battery: 100%.
The highest voltage on a cell is 3.53V.
I suspect the cell voltage should be 3.6V when remain Battery is 100%.
Can someone explain how to understand above info? BTW, at this moment I don't have the possibility to test the cells capacity. Thanks

 

[BentleyJ]

3.6 to 3.7V per cell corresponds to Li NMC sometimes referred to Li-ion. LFP cells are rated at 3.2V nominal and fully charged, resting voltage will be 3.35V.

At 3.53V AND (tail current has dropped to 2%) during a charge cycle the cells are full and the BMS is accurately indicating 100% remaining SoC. Unfortunately the accuracy of most BMS are rather poor compared to a separate monitoring device such as a Victron Smart Shunt.

You should probably set the cell capacity to 300Ah on the BMS. Its better to be conservative and use slightly lower values so the BMS does not give you false Remaining SoC values that are higher than what is actually available.

 

[mikefitz]

What if I set 300Ah in the app but the cells are 320Ah?

The SOC reading will not be exact. If there is some concern about the actual cell capacity, it would be logical to assume the capacity to be 300 Ah, ( if the actual capacity is higher, the only effect is more 'margin ' at a low SOC).
The BMS will sync to 100%SOC when cell volts exceeds the preset value in the BMS , typically in the region of 3.40 volts per cell.
A full charged resting cell will read 3.35 to 3.45 volts .
3.65 volts per cell is the safe limit for charging a LFP cell, in practice a low stress charge is 3.55 volts per cell or slightly lower, 56.8 for a 16 cell battery.
..

 

[vapi]

How does this Battery Capacity value influence the charging process? I would say the most important are the cell's voltages to determine the SOC and the start of balance.
I suppose that as long as a cell doesn't get to the full charge voltage value it can 'ingest' more energy.

 

[rogerheflin]

If the battery capacity is wrong and/or the SOC is way wrong then SOC may be say 25% and the battery may really be <5% and once the JK bms sees the lower voltage that indicates <5% it will reset from the wrong SOC to 0%(I have seen it do this). On the charging side it may say 100% early and stop charging (depending on the inverter), or if SOC is under actual the bms may see the voltages in the battery hit true full/100% with the SOC not 100%. The bms will reset to 100% when that happens. The more troublesome is if the bms registers 100% when it is not 100%, I have had to go into the bms and change a parameter--like the capacity by 1ah (and it resets SOC based on some unknown set of rules but typically that one be 100% and it will start charging again until the voltage shows it is full.

 

[mikefitz]

does this Battery Capacity value influence the charging process?

It has no effect on the charge process, that's down to settings in the charger.
The BMS can only stop current flow into/ out of the battery if protection levels are exceeded, these would be cell or battery overvolts/ undervolts or overcurrent or over/ under temperature.
There is no protection associated with SOC.