18650/charger questions

So I'm starting to charge 18650 cells that I've scavenged and need some feedback. I purchased a Thunder 1220 charger to use, I set the cell voltage to 4.2 (cells are in the 3.7 range). I put 10 cells in parallel to speed things along and apply only .5 amps, which is what... .05c? However the voltage shoots up when charging to 4.1v, it reaches 4.2v in a few hours and stops but then the voltage immediately drops to 3.75 or 3.8v which I assume is normal since the cells haven't received much energy yet. When you set the voltage on the charger, it stops charging when it first reaches that voltage and not what the cells settle on when the charger is no longer applying voltage. How am I suppose to full charge the cells this way? I think the highest I can set the charger to is 4.3v, am I doing something wrong here? People talk about charging at .5c or slightly less on these cells, would the voltage be closer to 5v when set that high?

Starting to sound to me like the scavenged cells, or at least some of them, aren't so great.

I'd start doing some of the tedious one cell at a time testing. Get rid of the really high resistance ones.

Yup, definitely not new and many are likely not great. I don't have a sense of a what a good cell and a bad cell look like when charging, how do you tell? Will the voltage on a good cell increase only a small amount when charging? If so, what is "a small amount"? I'm trying to figure out how I profile cells that might have high resistance.

after you charge up the lipo cans they should hold the voltage of the charger. they do not usually drain voltage off at the terminals like old lifepo4 does. that should be a clue about using dead laptop cells.

when you have assembled enuff cells to test capacity then you should connect them in series and measure the cell voltage of each one as you discharge them into the dummy load through the wattmeter. at this point your voltage measurements will show which ones have high internal resistance and which are low internal resistance.

the cells with a high internal resistance will have a larger voltage drop across them than cells with low internal resistance.

I did a pack back in the day with laptop 18650 cells. It just isn't worth it on used laptop packs, if you can find a large lot of new laptop packs or maybe a lot of batteries for a laptop or device that is maybe a year old it is a win but old used ones are terrible from my experience. Mine was a 20 amp pack and it would charge normally but would sag from 29v to 23v and got maybe half range.