New LF105 cells seem to have a sudden drop in capacity, bad cells? Something else?

[TacomaJoe]

Reading the thread, I see mention of erratic readings. Is this something that you see while watching it on the app? My voltage displayed bounces around a little. I just did a 0.4C discharge and my display was bouncing between 12.72V and 12.74V but I contribute it to rounding. Mine is a 12V system in my trailer, I'll report cell voltage to be consistent. I started at 3.2975V and applied my 0.4C load (110A pull from 277Ah cells). At 5 minutes the cells dropped to 3.1825V and were at that voltage after 10 minutes so I called it a good number. I have 3 batteries and got the same results with each of them.

My JK BMS voltage reading matches up with my MPPT and my shunt readings. JK current seems to read a little higher that the shunt ($40 AiLi) so I can't really say which is correct but we're looking at 4% difference, so I don't worry about it. My voltage readings will drift with higher currents but that is reflecting the voltage drop of the battery wires.

My JK reading for SOC tracks with the shunt pretty close as long as I just look at the app. If I change a setting or switch the FETs, the JK does a recalculate guestimate and will be off until I get them charged up.

My MPPT floats at 3.5V and I had one battery with a little cell imbalance that would trip on cell overvoltage. Not a big problem accept that the other two would continue taking on a charge and would no longer be in the same SOC. Top balanced the battery and its fine now. JK balance never fixed it for me.

You need a full charge before running a capacity check and that requires sitting at 3.5V until the current drops off.

 

[Chris30559]

I saw you mentioned you checked voltage at the cells. Were you testing on the buss bars by chance. Or the actual terminal. I had a similar issue with these same cells. Thought they were bad, had some that where way off in voltage under load, and resting was fine, and some even read high at times. Nothing made any sense.. ended up being the connections on the buss bars, I was sure that wasn't the issue but in the end it was. These batteries have such tiny screws on the terminals, you need to tighten them pretty tight. Much more than you think. And it doesn't hurt to make sure the terminals are parallel with each other since the surface area is so small.i even sanded mine some with a flat bar between each one. If you hold a flat edge between the terminals where the buss bars go you might even see that it barely makes contact on some. After I meticulously went through each connection within the pack all the problems went away completely, I regularly pull 0.5c sometimes more and they hold strong down to 15% or so.

Oh and I agree with the other statements about your charge voltage. 3.45 is not enough. It bet they were never fully charged, Needs to be 3.5 per cell and hold it there until the current tails off to around 2 amps on a pack that size. For however long it takes. Even tho your charger might be putting out 3.45 per cell the voltage at the battery will certainly be less. If you only want to charge to 3.45 you would absolutely need to hold it there until current goes to 0a or it will not even be close to full. Since these batteries have such flat charge curve. Even a slight voltage drop in any connection could mean the difference of 20% capacity.

 

[Aldricoak]

100% with Chris30559. DO NOT connect JK Bms test leads to the bus bars, you WILL get wacky readings. I had the same problem as Chris and once I connected the test leads of the BMS directly to the terminal screws all my "bad" and "great" cells went away. My active balance could keep up and everything discharged and charged more evenly. I have learned the JK Bms are highly sensitive and will require some "tweaking" in regards to wire resistence as well.