in metal detector(SE PRO) battery don't over-discharged. voltage drops to 3-3.3v and device turn off.
and better don't leave battery alone when charge. or use good charger with thermal sensor.
Hi! I have made holder for li-ion 18650 battery. It amazing thing, need only three battery. and capacity of 2700mah are almost enough for 5 days.
made for battery size 18mm X 65mm
also lithium batteries don't have a memory effect. you can charge it when you want. don't need to discharge battery...