18650 lithium ion 18650 battery could be used while using. To avoid over discharge.
It should be charged when device indicate low battery, if the device turn off automatically that battery have been overdischarge. It will affect battery life time. To avoid overcharging, Charger is fully charged, it should disconnect the power.
Generally, now the charger will stop charging automatically, there will not a big problems do not remove the battery from the charger for a short time. But if we left the battery on the charger for a long time, in late night, the voltage will increases, the charger will continue charging, causing overcharge. Long time not using, Lithium batteries should be charged up to 60%, separately stored in a cool dry place, every 4, 5 months supplementary charge . Avoid short circuit for the battery, high current discharge, it is adverse to the battery life.