Different SOC % on two batteries connected in parallel w/ same cell voltage?

[roark]

ok, I think this is weird so I thought I'd post and see if someone can explain this to me.

I have two 4s 180ah calb 12v batteries setup w/ overkill 120a bms each. I just got done top balancing both batteries. I'm running a test on these batteries. After balancing I tied the two 12v batteries in parallel for a 360ah bank. I hooked up a 750w simple inverter with two big fans and am draining about 68W or 5a out of each battery according to the bluetooth app that comes w/ the overkill. Nice app by the way.

The strange thing is one battery shows 60% SOC at 13.23V and the other shows 83% at 13.22. The cells are all around 3.308-3.310 on the 60% battery, and 3.304-3.307 on the 83% battery. I guess I expected the SOC to be very close between the two battery setups.

Can Anybody explain this?

Thanks!

 

[#40Fan]

How is the inverter connected to the batteries? Are both cables from the inverter connected directly to one battery (ETA: or do you have cables coming from each battery connected to a busbar and then the inverter connected to it as well)?

After charging both batteries, did the SOC reset to 100% on both batteries?

 

[roark]

yes bus bar between the two. yes 100%. I thought the SOC was specifically set based on the cell voltage, and just an estimation. The bluetooth app shows that both batteries are discharging at the same rate. ie. the 10amps pulled by the inverter is reflected as 5a on each batteries bms. So, that's why I'm confused as to why the bms on one battery reflects so much of a difference in SOC.

The settings in the bms app show a cell volatge of 3300mv is 60% and 3350mv is 70%. 100% is at 3500mv.

So, why is the battery that shows 83% at a cell voltage of 3308mv to 3310? Below are the screen shots of the now 81% battery

 

[roark]

Solved! Capturing the screen shots I noticed that the only difference was this battery had 'balance only when charging' enabled. The previous posted battery had that disabled. I changed and it fixed the SOC. Kinda weird that is somehow affecting SOC.

Here is the other battery:

 

[#40Fan]

That'll be a good one to remember. Thanks for letting us know.

 

[Ellcon123]

Also remember that LFP has a flat voltage curve so voltage itself is a poor indicator of SOC. My 3 packs can see 10% variation. SOC gets reset when fully charged so it's not an issue.

 

[mikefitz]

I changed and it fixed the SOC. Kinda weird that is somehow affecting SOC.

When you make any changes to BMS perameters, the SOC defaults to best guess based on the capacity settings and battery voltage, (regardless of the SOC setting prior to the changes).
Happily the SOC matched the other battery.

Mike

 

[RCinFLA]

The strange thing is one battery shows 60% SOC at 13.23V and the other shows 83% at 13.22. The cells are all around 3.308-3.310 on the 60% battery, and 3.304-3.307 on the 83% battery. I guess I expected the SOC to be very close between the two battery setups.

Assuming the cells have no current loading, the 3.308v to 3.310v cells are slightly above 50% SoC. The Columb counter showing 83% is wrong and needs to be reset to 100% SoC with a full charge.

You should always keep 'only balance with charging' ON and start of balancing above 3.40vdc.

Balancing at mid SoC (3.30v) will likely be unbalanced when you get near full state of charge when charging. This will likely cause a charging BMS shutdown due to an overvoltage cell.