Design Ideas: March 14, 1996
To prevent battery damage, the circuit in Figure 1 disconnects the load at a predetermined level of load voltage. This load voltage, VTRIP, is closely proportional to the battery voltage. R1 and R2 determine the level of VTRIP that corresponds to voltage of 1.15V at pin 3 of IC1. A voltage of 1.15V at pin 3 of IC1 causes the internal comparator to trip. Thus, VTRIP=1.15(R1+R2)/R1.The load-battery connection remains open until the system receives a manual reset command. You may not desire automatic-reconnect circuitry; such circuitry may not be effective in many cases, because the battery voltage rises so much when you remove the load. If you must remove the load before full discharge or if the difference in terminal voltage from charge to discharge is small, then the necessary hysteresis (including the effects of component tolerance) may be too great to ensure an automatic reconnection after you recharge or replace the battery.
Pressing reset or pulling pin 3 above 1.15V with a transistor reconnects the load. Battery drain with the load disconnected is 5 µA, so the circuit can remain with the load disconnected for an extended period without causing a deep discharge of the battery. Choose Q1 for a minimal source-to-drain voltage drop at the required load current. (DI #1846)