A low-battery voltage-cutoff circuit prevents overdischarge of a rechargeable battery. An obvious requirement of this circuit is extremely low power consumption. Figure 1a's simple circuit has a measured current consumption of approximately 1.2 mA and uses only two components to perform the low-battery cutoff function for a four-NiCd battery.
IC1 is a 3.9V voltage detector with a maximum hysteresis of 0.3V. When the battery is charged, the 5V power supply exceeds this IC's threshold such that its output goes high to turn on Q1, an IRLZ14 MOSFET switch. The IRLZ14 is a logic-level device with an on-resistance of 0.2 Ohm. When the battery voltage drops to below IC1's threshold, the output of IC1 is zero, which turns off Q1.
If the load is heavy, the circuit may turn on and off when the battery voltage reaches the threshold. When the circuit cuts off the load, the battery voltage rises again; this higher voltage may exceed IC1's turn-on threshold. To prevent this problem, the circuit in Figure 1b uses a flip-flop to provide a clean cutoff. Pushing S1 turns on the switch. When the load has a large capacitance, R1 and C1 provide a delayed response to prevent the turn-on in-rush current from triggering the circuit. The power consumption of this circuit is in the same range as that of the circuit pictured in Figure 1a.
All the parts for this idea are available from Digi-Key (
www.digi-key.com). For a lower switch resistance, you can use the IRLZ44, which has an on-resistance of 0.022 Ohm.
I found it it is 1.2 μA 
