Balance parallel SOK batteries?

[chateau_nomad]

I have two 100Ah SOK batteries (heaters/low temp cutoff) in parallel in my motorhome, and plan to add a third. I am charging from 600 watts of rooftop solar. The cells in each individual battery appear decently balanced, and have remained so since my April 2023 install. The issue I'm having is often one battery reaches 100% (according to ABC-BMS) and the other only reaches 45% - 75% or so. This occurred randomly all summer long with both batteries occasionally reaching 100% SOC.

Before installing, both batteries were 100% SOC.
The batteries are in parallel connected to a Victron Lynx Power In busbar, which I converted to use fuses. All battery cables (2/0 AWG) are exactly equal length and size.
Positive battery #1 to positive battery #2.
Negative battery #1 to negative battery #2.
Positive battery #1 to Lynx bus.
Negative battery #2 to Lynx bus.

Is there something in my wiring - or something else - that could cause this?

 

[Q-Dog]

Do you have a way to isolate the 2 batteries? What happens when you shut down the 100% battery?

 

[KITROBASKIN]

How long have you had these batteries?
I have a pair like you (but no heaters) and have a similar reading from the SOK app. Don't trust the app is what I remember being told.

I tried to get one of the batteries to have more balanced cells and wonder if I made adjustments that now cause odd readings.

What does the SOK app say is their CALC Capa...? one of mine is 62Ah, the other is 156Ah. The low one never reaches 100%. If I remember correctly, one of them was about 105Ah initially. The other one seemed to always be suspect.

Do you feel like the cells themselves are functioning properly as far as expected capacity?

 

[KITROBASKIN]

Do you have a way to isolate the 2 batteries? What happens when you shut down the 100% battery?

I have run each battery separately and could detect nothing awry, but I have not taken instrument measurements (though did confirm voltage accuracy of the SOK app).

Curious to see what chateau_nomad says.

 

[JeromeS13]

How long have you had these batteries?
I have a pair like you (but no heaters) and have a similar reading from the SOK app. Don't trust the app is what I remember being told.

I tried to get one of the batteries to have more balanced cells and wonder if I made adjustments that now cause odd readings.

What does the SOK app say is their CALC Capa...? one of mine is 62Ah, the other is 156Ah. The low one never reaches 100%. If I remember correctly, one of them was about 105Ah initially. The other one seemed to always be suspect.

Do you feel like the cells themselves are functioning properly as far as expected capacity?

This has been my experience, as well. The ABC app is very inaccurate, unless you fully cycle both batteries very frequently. What I usually look at is to make sure they're both charging and discharging at close to the same rates.

 

[chateau_nomad]

I'll check today and see what the app says... full sun today. The batteries are buried under the bed... accessible but not easy. Later today I'll get in there and take battery post readings and compare to the ABC-BMS app.

Any suggestions how to accurately measure ACTUAL amp hours in my setup? I have a clamp DC multimeter. Would turning on my 3k inverter and running a 750 watt heater - then averaging the clamp meter reading be accurate? I also have a kilowatt meter I could plug the heater into... although that wouldn't account for the inverter parasitic draw. Suggestions welcome!

 

[KITROBASKIN]

Victron Shunt?

 

[chateau_nomad]

I have a QWORK battery monitor... Amazon - basically a cheaper Victron shunt. It's currently reading 42.5% SOC. Attached are current screenshots of the batteries:

 

[JeromeS13]

As long as your shunt settings are correct for LiFePO4, I would trust that more than the app readings.

 

[chateau_nomad]

Every diagram I've seen says to chain the batteries as I have them. I'm wondering if the first battery has less resistance and getting more current?

 

[Q-Dog]

What is "CALC Capacity" and why is it so different between the 2 batteries?

 

[chateau_nomad]

That's something I'd like to know too... is it a BMS issue... or a battery cell issue? Who knows! My brother -in-law had $Battle $Born batteries and Victron "complements". No issues like this at all. And finding answers is VERY difficult.

 

[Q-Dog]

If I were guessing I would say it is "calculated capacity" which would suggest your batteries are not balanced. I would fully charge each one independently, then put them back together.

 

[chateau_nomad]

If I were guessing I would say it is "calculated capacity" which would suggest your batteries are not balanced. I would fully charge each one independently, then put them back together.

Yes, it's "calculated capacity" for each individual battery - calculated supposedly by the battery's dedicated BMS. According to the individual battery cell readings (bottom of ABC-BMS display, and "V Differ"), the cells are balanced about as close as possible.

Before installation in April of 2023, I completely discharged (to cutoff) both batteries... then fully charged them individually to cutoff. Both indicated 100% on the app, cells balanced and calculated capacity was something like 105Ah on both batteries.

This unbalanced (between batteries) issue started maybe a couple weeks after installation. At that time I removed them and repeated the discharge/charge cycle. I calibrated the battery monitor for 100% = 200Ah.

All was good for a few more weeks... both seemed to top off at 100% and the battery monitor (shunt) agreed. But now when one battery gets to 100% SOC, it seems to stop charging (as it should), but the other battery downstream in the parallel chain ALSO stops charging. ??? Which leads me to believe maybe there's another wiring scheme which may work better??

 

[Benjamin Yang]

It will automatically balance the two batteries if connected in parallel could it be a BMS issue?

 

[JeromeS13]

What is "CALC Capacity" and why is it so different between the 2 batteries?

In my (not so) professional opinion, there's an incorrect setting in their firmware settings. Unless you're fully cycling the batteries from 0-100-0 frequently, that number consistently gets smaller and smaller. I do not pay any attention to that number now. I just go by the SOC % from my shunt.