That single cell is having a higher voltage than the others - you have to charge them to match the single
@Maxid asking you because I know you’ve soldered packs before spot welding them 
My DIY-Spotwelder is nearly done. Now I’ve thought about desoldering my 10s4p just to spot weld it. What is troubling me is that the poles won’t be “new and untouched”.
I’d use desoldering wick to get rid of most of it but would you recommend me doing this ? I’m not sure how the nickel strips will behave due to the rough surface.
I never soldered a 18650 pack 
pardon me then 
thought you had a soldered one up for sale once. Must’ve been someone else haha
I tried to sell my spotwelded 18650 pack and I soldered copper bars on top of the nickel strips. But it always seemed to be a really bad idea to solder directly to the cells
People on es are into “resistance soldering” lately and supposedly better than plain soldering. But some people say soldering is ok if ur good and quick
yeah when adding fuse wires it is fine as the wire is supposed to be thin and easily solderable. With 10AWG wire it is a different story though. The cable itself will move so much heat away from the solderspot, thus causing the entire area to heat up significantly more than with a thin wire.
So I’ve redone the pack and replaced 2 cells (1 faulty for sure due to disconnecting from the pack). The other one was for safety reasons since the insulator ring was off.
I used the 1s bms for charging packs and a heater block to discharge if needed. When reconnecting all packs/cells had the same voltage. The issue is that since then one pack has a higher voltage then the others. I’ve manually discharged that pack to the same voltage and when plugging into the charger (41,2v) the pack is getting close to 4,20v and the others stay behind at 4,09v-ish.
My guess is that the pack has one faulty cell or one disconnected cell which makes it a 1s3p and thats why it is charging up faster and beyond ? I’ll make one run later to drain the whole pack to around 50% and will plug in into the charger again to see if the issue still remains.