LFP Battery Capacity / SOC Question - Cell Voltage vs Shunt Percentage

[MrM1]

I am trying to learn my actual State of Charge of my battery bank.
I'm not totally new at all this, but still learning. I'm not sure what I am seeing in my Shunt Percentage vs Cell Voltage of my battery bank.

I have a 24v system with a Schneider SW 4024 Inverter, two Classic 150s SCCs, and a 5200 watt array
Using Busbars for combining from positive and negative and equal cable lengths from each of the 3 batteries I have
- A 2p8s battery of 304ah eve cells / Overkill Solar BMS with an entered Amp Hour of 608
- A 2p8s battery of 230ah eve cells / Overkill Solar BMS with an entered Amp Hour of 460
- A 8s battery of 271ah CATL cells / Overkill Solar BMS with an entered Amp Hour of 271

The Midnite Classic 150 SCC (with Shunt and Whizbang Jr for reading Amp Hours) is entered in as 1,339 Total Amp Hours for the Battery Bank.

Here's the Question:
- The MidNite Solar Shunt And all the BMS's pretty much agree. This morning the system is at 17% SOC.
- The Capacity / Net Charge according to the Midnite Solar shunt is -1011 Ah (which lines up with the total Amp Hours of 1,339Ah if at 17%)
- The SCC is reporting the Total Battery Bank voltage as 25.7v with a total load of 594 watts
- The Array is turned off

BUT ... even with a Load of 594 watts on these batteries,
- My Individual Cell voltage of each of the 40 cells is on average at 3.24v per cell even with a steady 594 watt load
- The Delta between all cells is no more than 0.005v

So ... Wouldn't the Cell Voltage indicate that the SOC is actually higher than what the Shunt is Reporting at 17%?
- Wouldn't the SOC Actually be closer to 35% according to cell voltage?

Is something Calibrated Wrong ?

Do I go with the Shunt or the Cell Voltage Chart?
I guess i could just run the whole thing down until the BMSs started turning off the batteries (LVC set to 2.7 in the BMS). Then I'd see how low the Shunt read. That'd be fun ?

 

[DIYrich]

Chart is a "guide". The discharge curve is too flat to be precise (not much voltage difference between 80% SOC and 20% SOC.

You need a shut that counts amps in and amps out. Charge to full, and discharge to whatever zero level you want. Then periodically charge up to 100% to keep it calibrated (it doesn't perfectly read amps in/out, so it will drift over time).

 

[MrM1]

Chart is a "guide". The discharge curve is too flat to be precise (not much voltage difference between 80% SOC and 20% SOC.

You need a shut that counts amps in and amps out. Charge to full, and discharge to whatever zero level you want. Then periodically charge up to 100% to keep it calibrated (it doesn't perfectly read amps in/out, so it will drift over time).

Thats probable what is needed . A full charge, full discharge and recharge

 

[Steve_S]

I use Midnite Systems including the WizBangJr.
I set the Wizbang to 99% Accuracy for LFP. Sadly the software & I believe the Jr itself is only capable of One Decimal Point accuracy which for LFP (any Lithium chemistry) is not really good enough. Consider that from say 26.5V to 26.6V is from 3.312Vpc to 3.325 volts per cell.

The above being said, with my Midnite Settings and BMS' being set accordingly (including voltage correction) the WizBang does report fairly accurately. I have however found that by the time the WizBang says 20% Remaining, it is off by a few percent (batteries are higher).

Now I run 24V and have 2x174AH, 3x280AH, 1x105AH in my bank, all in parallel with equal length wire, identical fusing etc.
The WizBang registers Load Out & Charge in from SCC &/OR Inverter/Charger without problem.
I Bulk/Absorb to 27.3V (3.412Vpc)
EndAmps set to 10A (because of my mix)
Float is set to 27.2V (3.400)

EndAmps for 304AH Packs = 15.2A
EndAmps for 230AH Packs = 11.5A
EndAmps for 271AH Packs = 13.5A
FOR Your Setup, I would set EndAmps to 12.5A I would not go lower than 11.5A, the packs will be quite good @ 12.5A

EDIT-PS: I should have mentioned that the WizBangJr uses the voltage params you've set to determine what is 0% or 100%. The part that fools folks often, is that when you charge & float LFP to a set voltage IE 27.2V while in float, they will settle a bit as soon as the incoming SCC voltage starts to drop and that will show the packs as 99%. That's OK and expected.

I just doodled this up to help people and I will post it into the resource section with my other charts.

 

[MrM1]

So even by your new graphic @Steve_S I am in the lower middle of optimal operating range according to cell level voltage at 3.24v, not the 17% implied by the Midnite Solar Shunt and Whizbang Jr?

 

[Steve_S]

Tweaking out the Midnite stuff isn't hard with the Windows Software (gosh like a time warp using that).
Right now my bank says 97% and charging (nice & sunny) on the Wizbang and my battery pack BMS' also indicate 97-98%.
I'll be in Absorb within the hour and that will run for 1 hour (setting) but usually ends in 40 minutes then flips to Float.
Once I hit Absorb Voltage the WizBang says 100% and stays there until there is battery draw.

My previous settings etc are all here: I did tweak the numbers a bit (Bulk/Absorb to 27.3V (3.412Vpc) Float is set to 27.2V (3.400)

Steve_S-Tech: My Final Config for Midnite Classic SCC & JKBMS' w/ Large Bank (works amazingly well) Includes Charge Profile & BMS Config

INTRO: I have debated posting this for a while until I got everything tweaked in and working as best as possible. The End Result is Gobsmackingly Good ! I cannot speak for other SCC and such as each is different & unique in their own ways BUT this may likely be a Good Reference. Civility &...

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