Frequent lifepo4 system shutdown

[koenws]

Hi guys,

After a couple of months of good use, I’m now experiencing frequent shutdowns of my lifepo4 setup.

Setup:

• 12v system with 4x LifePo4 cell connected in series with HEYO BMS
• 3x 200w solar panels in series with EPEVER 50A MPPT Charge controller
• CNSWIPOWER 2500watt pure sinewave converter

The inverter sometimes resets itself when battery is full and the sun is shining (overcharge protection), but the rest of the system doesn’t shut down. However, these shutdowns incur for the entire system when too much power is used through the inverter. When switching the main power switch on and off, I can restart the system again.

System can shut down already when using my laptop, which runs at 96 watts on 220v, before it was able to handle a lot more...

I’ve tried draining the battery a bit, so the system can balance itself out, but the shutdowns keep incurring. Looking at smart bms, it seems like cell 4 is sometimes over or undercharging especially. The system does balance things out eventually, but always cell 4 which displays under-voltage first.

I included some pictures of the system and of the bms battery and settings.

Anything I can change in my setup to stop the problem?

Thanks a lot!

 

[Supervstech]

Your balance begin voltage is set at 3.0V? That isn’t going to do anything. Best to set around 3.4V or the imbalance has too much mass to be effective.
If #4 is always lowest and highest, it isn’t balancing correctly, and most likely has failed. If it hits 3.55 disconnect first, AND hits the 2.65low disconnect, it has a lower capacity than the rest and should be replaced.

 

[koenws]

Thanks for your quick reply. The system is only balancing when charging, if the balancing starts at 3.4v per cell, wouldn't that make the differences too great?

You think increasing the diff volt protection could be a solution?

Hope I don't have to replace cell 4, the system is quite new.

 

[SparkyJJO]

The charge curve is very flat on LFP, so "balancing" at a lower voltage could easy knock things way out of balance.

 

[Supervstech]

3.4V is past the bulk of the cell capacity, and the balancing requires less mv so more effective.

No idea if it will help or not, but it is worth a try.

 

[TomC4306]

You need to top balance the cells.
You have .504 volt differential between the lowest and the highest cell.
When you are charging, one cell is way ahead of the others and hits high voltage disconnect. When you're discharging one cell is way ahead of the others and you hit low voltage disconnect.
The problem cannot yet be identified as a low capacity cell. First, you must top balance this pack.

 

[koenws]

Alright, I changed the balance open start volt to 3.4, let's see how the system reacts.

I did top balance the cells before use, if the problem persists will check if can do again.

Cheers

 

[TomC4306]

Cell reference voltage in settings is also incorrect. For LiFePo4 it should be 3.2.

The cell delta is SO high that the BMS will never be able to balance this pack.