Li Ion cells not all same V in parallel

[Dan the E-Bike Man]

I'm new to this but researching this topic has gotten nowhere. I am using an Imax B6 set to 1s @ 5 amps. I am charging 6 18650 cells all in parallel until the voltage reads 4.1. When i pull them off the charger and check the voltages of the individual cells some are 3.8.

Im starting off with same discharged V of 2.5
All cells are identical 3.8V 1800mA
All cells were bought new from new laptop batteries (not recycled)

Any ideas why this is occurring? I was thinking the internal resistance could be different but im not sure how to verify yet.

I built a 11A 6S pack with 18650 only to find the balance charging was not working for some reason. I have disassembled my packs to test the individual cells and have come across this issue.

Thanks in advance for any suggestions!

 

[dogman dan]

Sounds like you need to weed out some shitty cells.

 

[John in CR]

You need to leave them connected in parallel for quite a while after charging is the parallel connections are thin wires. Charge that very low cell up by itself first to make sure it's essentially dead already.

 

[Dan the E-Bike Man]

Thanks for your suggestions.

I am charging them in parallel and after i unparalleled them is when I read the 3.8v right after being disconnected. I charged the cells up to 4.1 separately and im going to try and recreate the issue.

 

[dogman dan]

Ok, so they are not the same V when NOT in parallel. I suspected that.

You have some cells that are either no good, or never got quite fully charged. Try charging them one by one, tedious I know, but it will show if they hold a charge when fully charged.

 

[knighty]

try leaving them on the charger for longer ?

 

[Dan the E-Bike Man]

I'm using the Imax B6 and when it hit 4.10V it stops charging, and I have found If I wait an hour the cells will all balance out. When building my battery packs I will most likely single these out since i want the packs to balance well. This makes sense now when I built my first initial pack to test my hub motor (xmas present) the balance voltages were all over the place after a charge. I just read some old textbooks about internal resistance so I think I will incorporate that in my cell testing and segregate my cells. The downside of this is ripping apart all my packs and starting over, but hey im having fun and learning alot!