Charging question - not reading voltage correctly?

Just a little background, I'm using a Thunder 1220 charger and I'm working with used 18650 laptop cells (in this example, working with a single cell), I'm also using a Fluke 117 multimeter to monitor the cell.

When I first turn the charger on the voltage read by the charger is 3.71v and the fluke reads 3.745v. Once I begin charging at .2C the charger now shows 4.05v and the fluke shows 3.790v, when the charger goes through its 5 second "rest" period the charger reads 3.72v and the fluke 3.75v.

Why is there such a large difference in voltage reading while actively charging? I figured if the voltage reading was off I could adjust a pot like I do to calibrate my other cheap multimeter, but if I adjust it then the resting voltage will be off. This doesn't seem like normal behavior, am I missing something? My cells all end up at lower voltages than what the charger is told to charge them to.

It sounds like the charger is faulty. I'd trust the Fluke over the charger, especially based on the results you're posting, but if you have another meter, try to confirm it.

If it is the charger, you'll either have to crack it open and do some component level troubleshooting, or replace it.

Found the problem and all is well! I've been using some extra alligator clips that had some fairly thin wire to reach the ends of my batteries (the ones that came with the charger are so short they are almost worthless). When I took these extra wires out of the equation the voltages read by the fluke and charger matched. Time to find refit my clips with some beefier wire.