Is this self discharge excessive? (EVE 105Ah cells in 8P configuration)

[sduser]

I just finished charging my 8 EVE 105Ah cells in parallel to 3.4V, then 3.5V then finally 3.6V per cell with a rest in between steps. All seemed to be good at first, when I disconnected the charger they dropped from 3.6V to about 3.57V which I expected. I ran out of time last night to finish the fixture and cut the busbars so I left them overnight all connected in parallel. This morning I measured the pack voltage and it was about 3.54V. I measured again today about 8 hours later and the pack was at 3.537V.

I know they are in the knee of the curve and voltage changes with very little capacity change, but this seems a bit high of a self discharge. I would expect after the initial settling out the voltage would stay fairly stable.

Is this normal?

Next step is break down the pack and monitor each cell. I am hesitant to do this as 1) I don't want to leave the cells at 100% SoC for very long and 2) every time I remove and reinstall the nut on the stud I risk stripping the cell terminal threads.

Appreciate any input.

 

[softdown]

Did the battery temperature drop overnight? Perhaps anything over ~3.5ish volts is a "surface charge" known to dissipate rapidly? LiFePo4 cells self discharge faster than Li-ion. Perhaps they need an absorb period at 3.6V to truly hold that charge?

Just some ideas...

 

[nebster]

Yes, it is normal. There's a meaningful gap between the voltage when charging and when discharging. As much as 300mV at the very top of the capacity curve, and roughly 100-175mV in the middle. (The actual delta depends on how much current you move.)

 

[Xerxes]

Yep,
Won't stay over 3.5V even if you repeatedly recharge it with a few hundred milliamp/minutes to go back up to 3.65V
It might be the "settling" of the battery concept/syndrome some folks use to describe this behavior or that the SOC curve is so steep that a few milliamp self discharge is in the tens of millivolt range drop per day when over 3.45-3.5V.

Watch it when it gets to 3.4V and then see how fast the voltage drops.
If at 3.25V and you are losing 10mV/day then maybe then consider you might have a high self discharge rate.

Did the seller give you a test paper or video of the batteries when purchased?
This is pretty common in the last six months if bought through Alibaba.
Sometimes you have to request this sometimes they just do that.
If so what was the voltage difference when you received them 30 days later?


The only way I know to test is to do a Discharge test 30 days after fully charging and then do another full charge/immediate discharge and compare. A balancing BMS-even at 30mA will skew the results so disconnect during the 30 day settling test.

For what you describe I would not be worried.

 

[Ampster]

Mine seem to settle at 3.335 after a few days and stay there for weeks.(for those spare cells that are not active.) I actually only charge to 3.4 and have refloat set to 3.3 just to give them that little boost during the day but my rebulk in down at 3.2 because I do not want a lot of recycling at bulk current.

 

[sduser]

Awesome thanks for the feedback that's good to hear. So are they now considered top balanced? I hope to cut up some copper busbar tomorrow and convert these into a 2P4S battery and put some load on it to get under 80% SoC.

 

[nebster]

Yes, that would be a top balance. Well done.

 

[Ampster]

Yes, that would be a top balance. Well done.

Yes and to drive home the point, since they settle at 3.335 whether they were charged to 3.4. or 3.65, that is why it is difficult to tell if they are balanced if you measure them at their settled voltage or any where along that very flat part of the curve.

 

[sduser]

Thank you.