Should I trust the BMS or the charge controller for SOC data?

[WillAK]

I've just received some LifePO4 cells (280ah REPT) and put them into a 12v battery box with a JK 200a BMS. All came supplied just connect and go.

The batteries were all balanced when they arrived at 3.29v so I connected them all up and started using it.

I installed the app and set the perimeters 12V 4S 280ah and I used the batteries for a little while but each SOC indicator is showing something different.


The BMS app is showing 11.78v with 61%. but the charge controller and the SOC indicator are both showing 11.3v and less than 10%.

Which one is correct? And any advice on how I can keep an accurate account of the SOC with this?

Thanks
Will.

 

[MisterSandals]

Neither is going to give an accurate (coulomb counted) SoC until they are cycled a couple times. If either gives a voltage based SoC, then that won't improve over time. Based on voltage, the 11.3V = 5% SoC looks about right.

 

[Rocketman]

I don’t use those brands - so these are just more generic answers.

Most bms don’t count low amps of discharge and charge. So they get off over time.

Is your battery monitor a shunt based one?

They get off too, and are most accurate after a full charge, the accuracy goes down over time. My Victron BMV712 says(if I remember correctly) to have a full charge at least every two weeks. That eliminates the errors.

Good Luck

 

[upnorthandpersonal]

At 2.9V per cell, you're closer to 10% than 60% (more than likely even well below 10%) - I don't even tend to use my batteries below 3.0V. You can calibrate the BMS, but neither would be super accurate. If you need more accuracy, get a shunt like a Victron SmartShunt.

 

[12vDC]

My Victron components only agree on voltage when charging is taking place. Otherwise at rest, in Victron Connect they report differently from 0.15-0.20V from the shunt to the battery. Not sure if the difference in resistance value is acceptable, but its close enough for me. The bank does what Im asking of it.

 

[sunshine_eggo]

I didn't see it explicitly stated, but that charge controller value is worthless.

 

[timselectric]

Get a shunt.
Everything else is just guessing.

 

[time2roll]

I would trust the BMS over the CC. Battery voltage will also be a good indicator to verify the BMS number. Takes a bit of working practice to get a grasp of the changes due to load or charging. However if near the top or getting depleted, when it really matters, the voltage becomes very clear indicator. Give it a few months and it will be a breeze.

And if it seems to be mission critical with big fluctuations.... the battery itself might be on the small side for the usage.