[BrianG]

What should happen is that the charger charges at a constant-current until the cells hit 4.2v/cell. At that point, the charger switches to constant-voltage mode where the voltage is held at 4.2v/cell and the current dwindles down to shutoff.

If you are stopping the charge when you see the voltage exceeds 4.2v/cell, you are missing the whole CV part of the charge and could be missing out on ~10% of the charge cycle.

If the cells are getting beyond 4.2v, it's either a charger malfunction, an error in the voltage display, the peak threshold is set to something above 4.2v, or you are in the wrong mode (NiXX instead of lipo). Either way, I would hook a voltmeter to the charge leads and watch that instead. You could even hook another meter in series to measure current at the same time. This would show you what is going on with the voltage and current simultaneously, provided the meters are accurate and/or been calibrated correctly.

I have an old Hyperion EOS7i where the voltage display shows ~0.1v higher than what it really is according to two different meters. On my Xtrema, I set the CC/CV threshold to 4.15v/cell just in case, plus it gives me a little wiggle room if the cells aren't perfectly balanced before the chargers gets to the CV point.