Guessing you have 18650 batteries?
My guess would be that it depends on if there are any additional safety features in the charger (like overcharge protection). I don't have anything that uses those batteries, but I do know that's important on other charger types. I have one charger that just charges for a certain amount of time (regardless of how full the batteries already were when you put them on) and another that reads the charge level and only charges till they're full. I definitely recommend the latter for any rechargables that I have used (but as I said, I have no 18650 experience).