Sudden drop in SOC

[Jimmyburnworld]

We had a sudden and quite concerning drop off in our charge state yesterday whilst I was at work. I got a notification through Home Assistant and looked at the limited data I have configured and it looked like we'd dropped from 52% to 17% in nearly no time. Obviously a bit concerned about shedding over 3KWh in a short space of time I checked in with my wife who confirmed no fires!

The pack is over a year old now, built with Eve Grade B LiFePo cells from Fogstar.co.uk. It has a Seplos clone BMS.

I've pulled the logs from the BMS and I can see that cell 9 possibly looks weak. I'm going to balance the cells later this week as a precautionary however I would like to know why the BMS suddenly cut the SOC value? Was this some sort of re-calibration in response to Cell 9 going below a certain voltage?


Thank you in advance!

 

[42OhmsPA]

I would bet it's a reaction to the single cell dropping, as you suspect.
Top balancing will most likely take care of it and takes care of checking all connections.
Do you have a shunt on the system to verify capacity?

 

[Steve_S]

Your voltages are not quite right... There is a fair bit of deviation between cells. Also over driving the cells beyond the Working Voltage Range (3.000-3.400) is not doing you any favours at all, it is likey causing more issues than you realize, the chemistry is not designed to be above 3.450 and hence why the cells always settle down when charged. Seplos BMS also only does passive balancing which is for all intents & purposes useless on large capacity cells or unmatched & unbatched cells. An Active Balancer moving 2A can generally keep cells in synch nicely.

Clearly Cell 9 is your "dawg" and it actually rules the entire pack. It appears to be "The Runner" so it will reach High Volt & Low Volt Disconnect before the others. @ 2.89V and the BMS stating 7% SOC remaining, tells me your cutoff voltage is 2.800 Volts per cell. Just look at the differing cell voltages when at 100% and when below 7%, pretty much shows this to be happening.

You clearly state these are B Cells. That means they NEVER passed the full test suite and therefore cannot deliver the stated Amp Hours from the Working Voltage Range. Grade-A cells will deliver the actual 280AH from a 280AH battery between the 3.0-3.4 working range.

Corrections to do.
1) Check all connections to make sure everything is nice & tight.
2) Change the charging settings as follows:
Bulk/Absorb: 55.2V for 60 minutes (3.45vpc) (some call this boost) this rarely actually runs the full 60 mins because of EndAmps
Equalize: OFF
Float 55.0V (3.437vpc)
MIn Volts: 42.4 (2.650vpc)
Max Volts: 57.2 (3.575vpc)
Rebulk Voltage: 51.2 (3.200vpc)
End Amps: 14.0A (*1)
(*1): End Amps is calculated 280AH X 0.05 = 14A.
Coulumbic Efficiency / Battery Status Meter Efficiency for LFP = 99%.
3) Install a Smart Active Balancer
Set to start Balance @ 3.200, when cell delta is over 0.010V.
NB: Active MOVES power from Hi to Lo cells and does not waste it. When not running it uses nothing.

From my resources, may be helpful to have handy.

 

[Kenny_]

Thank you @Steve_S for all your help in all these threads. I'm trying so hard to learn....

 

[Steve_S]

Thank you @Steve_S for all your help in all these threads. I'm trying so hard to learn....

I try to help when I can, but years of doing so has added to my burnout on it and so I do less now on forums and focus on other things.
It's terribly tedious and exhausting constantly fighting mis & dis Information along with wives tales.

 

[42OhmsPA]

I try to help when I can, but years of doing so has added to my burnout on it and so I do less now on forums and focus on other things.
It's terribly tedious and exhausting constantly fighting mis & dis Information along with wives tales.

Please keep me in line if I share any bad information, I'd appreciate it.

 

[Steve_S]

Please keep me in line if I share any bad information, I'd appreciate it.

Hahaha, your pretty good !
Top Balancing won't fix a weak cell though, been through that merry go round a couple of times. Well with V1 & V2 cells not so much with V3 cells though...

 

[Kenny_]

I try to help when I can, but years of doing so has added to my burnout on it and so I do less now on forums and focus on other things.
It's terribly tedious and exhausting constantly fighting mis & dis Information along with wives tales.

I know exactly! I've been there a few times on other things, in other forums....

 

[Kenny_]

Please keep me in line if I share any bad information, I'd appreciate it.

Somebody certainly needs to keep you in line!!! J/K!!