Li-Ion Protection Circuit

When a lithium-ion battery is discharged below the minimum recommended cell voltage its life expectancy is dramatically reduced. The circuit described here can avoid this by disconnecting the load from the battery when the cell voltage reaches a set level.The voltage at junction A may be set to 3 V, for example, by selecting the correct ratio of R1 and R2. When the battery voltage drops below the minimum value, the voltage at junction A will be smaller than that at junction B. The latter voltage is equal to: VB = 1.25 V + I R4 = 1.37 V where: I = (Vmin. – 1.25 V) / (R3 + R4) = 800 nA (Vmin. = minimum value)