Varying SOC in Parallel Batteries

Basil48192

This is probably more of a question for LiTime, but thought I would throw this out there.
I just recently upgraded my batteries to two, LiTime 100AH batteries which I connected in parallel on my 2021 320S.
So far (in my driveway) everything seems to work fine.  However, I have been testing them with a load (fridge) for the past few days.  The BMS app on my phone shows quite a difference in the state of charge between the two batteries.  One shows 78% and the other shows 85%.  The app shows that both are 'balanced' properly so I'm a little confused.  

I have sent a question to LiTime, but I'm wondering if this is common or I'm being too picky!

I have wired them properly (I believe) with the positive load wires connected to one battery and the ground bus bar to the other.  

Any comments would be appreciated.  

pthomas745

There are lots of questions being asked about the LiTime bluetooth "monitor" in various solar forums, the Will Prowse group in particular.  The feeling is the monitor is more voltmeter than much else.  It is hard to tell if the 7 percent variance is due to a poorly performing voltmeter, or this is normal behavior for that version of their app.  

Grab a multimeter and see if you can compare those measurements.  Trying to measure a lithium battery using voltage only is a crap shoot, where even tiny differences in battery voltage can show large swings on battery capacity measurements.

Yoshi_TAB

Hi,

I just ordered a LiTime battery and this was sent to me.  You may already have it, but passing it along to go through in case you do not.  Maybe there are some nuiances  Ultimate Guide of LiFePO4 Lithium Batteries in Series & Parallel – LiTime-US

Horigan

LiFePO4 batteries have a very flat voltage curve as a function of SOC.  If your SOC meter is measuring voltage, you will get the variations you're seeing between the two parallel batteries.  I wouldn't worry about it.

Basil48192

Thank you all for your comments!  I heard back from LiTime, though I'm not sure I fully understand their comments.  I think I will leave things as they are.  The response from @Horigan makes good sense to me.  
For reference, below is the response I received from LiTime:

The reason why the batteries are discharged one after another is that the voltages of the terminals of the two batteries are different.

The battery with a larger voltage bears all the discharge current first.

When the voltage of the battery with a larger voltage drops due to discharge, and the voltage drops to the same voltage as the battery next to it, the two batteries can be discharged together.

 

This is the normal discharge phenomenon of batteries in series and parallel.

It is recommended that the total discharge current of the two batteries reaches 50A, so that the two batteries can be discharged together.

 

Solution: Temporarily turn off the discharge of the battery that is discharging, and then resume the discharge of the battery; the two batteries can resume synchronous discharge.

If it is found that the battery that has just resumed discharge will charge other batteries briefly. This is a normal phenomenon.

Horigan

Hmmm.  That response doesn't make much sense.  Batteries connected in parallel pretty quickly balance out to the same voltage.  If one is higher voltage it quickly charges the lower voltage battery until they reach equilibrium.  What you're seeing is the minor variations in the SOC (voltage) meters.  I bet if you swapped the meters between the batteries, the percentage SOC would follow the meter, not stay with each battery.

Basil48192

@Horigan.  I agree.  In one sentence they say it's a normal phenomenon...then they say to correct it.  Your description of the flat voltage curve put my mind at ease to simply ignore the minor, short-term descrepancies.  Thank you again!