Deep discharge is the complete discharge of a battery cell by current drain. The terminal voltage drops below the final discharge voltage. In the case of batteries with series-connected battery cells, the battery cells with the lowest charge capacity are reversed. In addition, the deep discharge increases the memory effect and reduces the chargeable battery capacity.
Since batteries can be destroyed by deep discharge, they are provided with deep discharge protection. In lithium-ion batteries, deep discharge is prevented by an internal fuse. This switches the battery off temporarily. There are also chargers that do not recharge deeply discharged batteries.