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Balancing a troublesome CALB cell
#1
I have been using a 8S1P new CALB 100Ah LiFePO4 27V array with Batrium wmon4 BMS for about a year now to power my home water pumps etc. Charging is via the PWM charger in the PIP2424 inverter, set to 27.9 V charge.

I noticed that cell 6 has always been a little reluctant to stay in balance with the other seven cells, usually less than .01V difference.

However lately cell 6 is getting into wider swings as seen in the 11am charge pic below:




I realise that this is cooking the mon. Eventually, in a couple of hours all the cells will balance and then cell 6 may even finish up .01v lower than all the others.

Looking for suggestions oh how to rebalnce cell 6 to the others, (high or low) or is it a potentially bad cell?
Mike. DIY Offgrid in South East Queensland, Australia. 

17x250W Trina panels, 2P12S 48KWh / 24 x 2V 1000Ah SLA, 8S 2.4KWh / CALB CA100 LiFePo w/ Battrium WM4 BMS, 
1 x PCM60X + 1 x PIP24 inverter for house, 1 x PIP24 inverter/charger for utilities using LiFePo for trial.

Belt: Autostart 100A 24V petrol alternator/charger, Braces: 3KW genset.
Hydrotherm programmable heat pump 800W/275l storage hot water from solar/battery. ~0.8KWh/day

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#2
You may need to pull the pack out of service and do a capacity test on it specifically, or check the fuses on the cells (if you have these). It's possible you have a blown fuse, or 2, or 3.

Considering that pack 6 is 'higher', I would lean more towards blown fuses in that pack. And maybe, you are getting more and more popping as the loads are are balanced across the whole pack anymore.
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#3
Thanks for the prompt reply.

However, these are 8 x CALB 3.2V 100Ah prismatic cells in series, not 18650 packs, so there are no fuses involved.

I have also just noticed that the high level on this cell only occurs when there is no load on the system , causing high solar charging current, about 20+ amps.

As soon as a 220VAC water pump or such is running, the charge current drops back and feeds the pump via the inverter, and the cell 6 voltage drops back to normal level. Pumps stops, cell 6 goes high again.

Also noticed that when there is a high load put on the system (eg, 900W heat pump hot water system) cell 6 drops significantly lower than the other cells, will try to get a pic when the HWS heat pump starts at 1pm.

Here is pic of system with Heat pump on, so small charging current. Cell 6 now sits lower.
Mike. DIY Offgrid in South East Queensland, Australia. 

17x250W Trina panels, 2P12S 48KWh / 24 x 2V 1000Ah SLA, 8S 2.4KWh / CALB CA100 LiFePo w/ Battrium WM4 BMS, 
1 x PCM60X + 1 x PIP24 inverter for house, 1 x PIP24 inverter/charger for utilities using LiFePo for trial.

Belt: Autostart 100A 24V petrol alternator/charger, Braces: 3KW genset.
Hydrotherm programmable heat pump 800W/275l storage hot water from solar/battery. ~0.8KWh/day

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#4
Cool the mons, or set a lower charge target when in bypass, and set a lower bypass current so you aren't cooking the mon - cell voltage will rise irrespective of what you set the bypass voltage to when the mon is in thermal bypass.

.... and check cell 6 connections.

........ and confirm, with an calibrated voltmeter, that the mon is reporting the cell voltage accurately.
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#5
And here is the final 4pm fully charged, basically no load pic. Cell 6 sits about .15V below the rest. It may improve over night as all cells discharge to about 50V with night intermittent pump loads. Will look again tomorrow morning.

I've checked connections and voltages, all is well.

I'd really like to fix this.

Mike. DIY Offgrid in South East Queensland, Australia. 

17x250W Trina panels, 2P12S 48KWh / 24 x 2V 1000Ah SLA, 8S 2.4KWh / CALB CA100 LiFePo w/ Battrium WM4 BMS, 
1 x PCM60X + 1 x PIP24 inverter for house, 1 x PIP24 inverter/charger for utilities using LiFePo for trial.

Belt: Autostart 100A 24V petrol alternator/charger, Braces: 3KW genset.
Hydrotherm programmable heat pump 800W/275l storage hot water from solar/battery. ~0.8KWh/day

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#6
Since it goes high during charge and low directly during load you have a weak cell. You cannot balance away that fact.

Retest all cells for capacity and even IR and you will see whats going on...

You can potentially top balance it and leave it as such but cell 6 will drop faster than all other no matter what you do and most likely due to high IR have issues during charge and during last % of charge go way high. To play with this you need to reduce the charge during the last & of SOC.... With that said i would recommend to swap that cell out.
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#7
(08-06-2019, 06:26 AM)mldee Wrote: I'd really like to fix this.


What method are you using to determine that each cell is properly fully charged ?
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#8
Yep. I tend to agree with you.

This is the status at 7pm, all looks great!



Little load, no charging current, everything's happy.

I'll contact the supplier and try for a replacement as the cell is only 15 months since purchase new.

Thanks for your insights
Mike. DIY Offgrid in South East Queensland, Australia. 

17x250W Trina panels, 2P12S 48KWh / 24 x 2V 1000Ah SLA, 8S 2.4KWh / CALB CA100 LiFePo w/ Battrium WM4 BMS, 
1 x PCM60X + 1 x PIP24 inverter for house, 1 x PIP24 inverter/charger for utilities using LiFePo for trial.

Belt: Autostart 100A 24V petrol alternator/charger, Braces: 3KW genset.
Hydrotherm programmable heat pump 800W/275l storage hot water from solar/battery. ~0.8KWh/day

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#9
+1 to Sean's suggestion ie check the connections for cell 6, eg clean the cells terminals & cell-cell straps, plug/unplug the cellmon a few times, etc.

You might also try backing of the charge CV voltage a bit eg from 27.9 (~3.48V/cell) down to say 27.6V (3.45V/cell). I run my LiFePo4 packs at CV = 3.41V/cell
If the issue persists, the cell may be on the way out, the more it's used in the pack, the weaker it will get, the more range of voltages you'll see on it.

Are there any signs of swelling?

Also do you have auto balance enabled in the WM from ~90% SoC?
Running off solar, DIY & electronics fan :-)
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#10
(08-06-2019, 09:23 AM)Sean Wrote:
(08-06-2019, 06:26 AM)mldee Wrote: I'd really like to fix this.


What method are you using to determine that each cell is properly fully charged ?

Since these are sealed LiFePo cells, not really much I can do to ascertain charge except depend on the Batrium BMS, which has been quite reliable and also shows the state of charge of the other cells plus the overall state of charge.
Mike. DIY Offgrid in South East Queensland, Australia. 

17x250W Trina panels, 2P12S 48KWh / 24 x 2V 1000Ah SLA, 8S 2.4KWh / CALB CA100 LiFePo w/ Battrium WM4 BMS, 
1 x PCM60X + 1 x PIP24 inverter for house, 1 x PIP24 inverter/charger for utilities using LiFePo for trial.

Belt: Autostart 100A 24V petrol alternator/charger, Braces: 3KW genset.
Hydrotherm programmable heat pump 800W/275l storage hot water from solar/battery. ~0.8KWh/day

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