NiCad batteries are excellent rechargeable power sources for portable equipment, but ~care must be taken to ensure that the batteries are not damaged by overdischarge. Specifically, a NiCad battery should not be discharged to the point where the polarity of the cell is reversed and is reverse-charged by the higher-capacity cells. This reverse charging will dramatically reduce the life of a NiCad battery. This circuit both prevents reverse charging and also gives a low-battery warning. A typical low-battery warning voltage is 1 V per cell.
Since a NiCad, 9-V battery is ordinarily made up of six cells with a nominal voltage of 7.2 V, a low-battery warning of 6 V is appropriate, with a small hysteresis of 100 m V. To prevent overdischarge of a battery, the load should be disconnected when the battery voltage is 1 V x (N-ll, where N = number of cells. In this case, the low-battery load disconnect should occur at 5 V. Since the battery voltage will rise when the load is disconnected, 800 m V of hysteresis is used to prevent repeated on-off cycling.