What everyone else says - leave them on your charger until they reache 4.2 - your charger should figure out that they're full and stop there. If it won't go to 4.2, or if it goes above 4.2, get new batteries and/or charger.
I never vape my batteries below 3.6ish on a mech, but they routinely run down to 3.2 or so on my Sig box - if you get into the 2s you're entering the danger zone and damaging the batteries.