Self Discharge limits?

MurphyGuy

Solar Addict
I'm trying to figure out if I have an "actual" bad cell, or just one cell with a higher self-discharge rate than the others.

These are brand new LG Chem N2.1 cells that have never been cycled.. were destined for the Chevy Bolt.

I have a 25 kWh (480aH) pack that is 4/8P 14s. (LG Chem modules are stacked two 60ah cells to each module)

The first two runs with this battery went perfectly, albeit short as the grid power came back on fast. But this last outage, I ran the battery for a longer time and when I activated the off-grid system, I noticed one cell was 64 millivolts below the rest. Most were at 3.620 to 3.630 (storage voltage), but this one cell (#12) was at 3.570.

Running the battery down to 25% capacity and then charging it back up to 90%, and that cell was still 0.050 below the rest.. I could tell the BMS was working to bleed off the other 13 cells.
The grid came back up so I shut the system down.. decided to charge that specific cell with an 18650 charger (LOL). 480aH of cell and I had a 1400mW charger hooked up to it with alligator clips. (BMS Disconnected) It took quite a few hours of charging to bring it up with the other cells, which I let rest at 3.915 to 3.921.

I brought that #12 cell up to 3.916 and then disconnected the charger and monitored its resting voltage over the next 10 days using a Fluke every 8 hours or so. It dropped 0.001 volt right away, and now it has settled to dropping about 0.001 every 24 to 36 hours or so. In 10 days, we've gone from 3.916 to 3.909.

The rest of the cells in the pack haven't dropped a single millivolt after the first day of rest.. they were disconnected during a charge cycle but only about 10 amps were going into the pack at the time.

Does this sound like I have 13 really good cells and 1 normal? Or 13 normal cells and one trouble maker?

Anyone ever heard of these kinds of issues fixing themselves as a new battery sort of "breaks in" ? This entire pack has only been cycled 4 times, and most of that was bouncing between 3.6 and 3.9.. This was the first time it was ever discharged below 3.6 volts.. I'm wondering if it just needs to stretch its legs..
 

MurphyGuy

Solar Addict
So what is 4/8P?

And if you have 4? or 8? cells in parallel, how do you measure the voltage of a single cell?

Happen to have a pic?
Chevy bolt cells come in modules, two cells to each module.

I use 4 modules stacked together.. so if we count modules, its 4p.. if we count the actual cells then its 8p.

The 4p/8p stack is treated as a single cell when all in parallel..
 

epoch_time

New Member
Hi Murphy. I'm using nissan leaf lipo's. 7 modules 2 cells per modual or 14s. At 51.0 volts discharge I have .064 imbalance (hi to low cell difference) thats when I have lv5048 programmed to switch to utility power and begin charging batteries (from utility). At 56.0 volts programmed to be fully charged. I have a cell imbalance of .007 volts. These imbalance of .064 and .007 have not changed in over a year of continous operation.
Each evening i manually change lv5048 inverter power setting to utility mode. and in the morning I manually set inverter power setting to sbu mode (solar, battery, utility). My battery charge discharge range is approximately 80% -- 40%. Basicly I use about 300 watts of battery per day.
My battery functions like a cloudy day buffer using about 1500 watts on 50% cloudy days. 1500 watts is my entire battery capacity when battery is charged to approximately 80% an dischargt to 40% (imbalance .067 volts). I think you have one slightly less capacity cell. and 13 better cells.
 

MurphyGuy

Solar Addict
Hi Murphy. I'm using nissan leaf lipo's. 7 modules 2 cells per modual or 14s. At 51.0 volts discharge I have .064 imbalance (hi to low cell difference) thats when I have lv5048 programmed to switch to utility power and begin charging batteries (from utility). At 56.0 volts programmed to be fully charged. I have a cell imbalance of .007 volts. These imbalance of .064 and .007 have not changed in over a year of continous operation.
Each evening i manually change lv5048 inverter power setting to utility mode. and in the morning I manually set inverter power setting to sbu mode (solar, battery, utility). My battery charge discharge range is approximately 80% -- 40%. Basicly I use about 300 watts of battery per day.
My battery functions like a cloudy day buffer using about 1500 watts on 50% cloudy days. 1500 watts is my entire battery capacity when battery is charged to approximately 80% an dischargt to 40% (imbalance .067 volts). I think you have one slightly less capacity cell. and 13 better cells.
I was wondering about that as well and watched it very carefully.. actually, I'm still watching it.

I isolated the battery bank and disconnected the BMS and then used a small 18650 charger to bring the #12 group of cells up to 3.916 volts (inline with the others). It took 3 days of charging with the little 18650 charger to get it there! LOL.

I then disconnected the charger and, as expected, it dropped a couple millivolts the first 24hrs due to charge distribution..

I then measured it every 4 to 6 hours for the next 10 days.. it seems to be dropping 0.001 volts every 36 hours or so. Its down 0.009 and still falling.

Observations:
1) After being dormant for 5 months, the system was started with the cells at 3.62x and #12 was 0.064 below the others.
2) The system was started in the evening when we lost power and was discharged to 3.5v before the morning sunshine. Cell #12 was still 0.064 below the others.
3) The system was charged to 4.002 volts over 7 hours the next day. The BMS was warm as it was trying to bleed energy from the other cells to keep the battery balanced with #12. It was eventually able to narrow the gap to about 40mv difference between the highest cell and #12.

I ordered one of those hobby balance chargers.. I have two extra BOLT EV modules I had originally ordered in case I found a bad one. I'm charging that one up now to be in line with the battery bank and then I will pull the #12 group of cells, separate them, and see which one keeps dropping.. and replace it.

I think I will then exercise that defective cell and see if it changes its behavior.. My battery bank has only been cycled a few times and this is the first I've seen that #12 cell drop.
I'm always keeping records of the BMS readings and about once a month, I open up the battery cabinet and use the meter to check the bat voltage and record the number..
 

Smokin

New Member
You may have one of those funky bad cells with the folded insulator. If you do then that would bee cool to disect and show proof as to a bad lg cell that's all the rage now with the battery fires...May want to put thoes cells outside and build a concrete structure over them so when the do go up in flames it won't burn down the house down eat the cat and mate with the dog.
 
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