Lithium-ion (Li-ion) Battery Protector

Protection Circuit
Commercial Li-Ion battery packs contain redundant protection devices to assure safety under all circumstances. Typically, a FET opens if the charge voltage of any cell reaches 4.30V, and a fuse activates if the cell temperature approaches 90°C (194°F). In addition, a pressure switch in each cell permanently interrupts the charge current if a safe pressure threshold is exceeded, and internal voltage-control circuits cut off the battery at low and high voltage points.

The Li-Ion typically is discharged to 3V/cell. The lowest low-voltage power cutoff is 2.5V/cell. During prolonged storage, however, a discharge below this voltage level is possible. Manufacturers recommend a trickle charge to raise such a battery gradually back up into the acceptable voltage window. Not all chargers are designed to apply a charge once a Li-Ion battery has dipped below 2.5V/cell.

Some batteries feature an ultra-low voltage cutoff that permanently disconnects the pack if a cell dips below 1.5V. This precaution is done to prohibit recharge if a battery has dwelled in an illegal voltage state. A deep discharge causes copperplating, which can lead to a short circuit in the cell.

Most manufactures do not sell Li-Ion cells by themselves but make them available in a battery pack, complete with protection circuit. This precaution is understandable when considering the danger of explosion and fire if the battery is charged and discharged beyond its safe limits.