Battery-Saving Disconnect Switch

  
At a predetermined level of declining terminal voltage, the circuit disconnects the battery from the load
Battery-Saving Disconnect Switch - schematic

and halts potentially destructive battery discharge. Ql, a high-side, floating-source MOSFET, acts as the switch. The overall circuit draws about 500 when the switch is closed and about 8 when it"s open. The values of ii, R2, and R3 set the upper and lower voltage thresholds, Vv and VL, according to the relationships For the circuit to start, V+ must exceed VO. The voltage detector IC1 then powers IC2, but only while V+ remains above VL. Otherwise, IC2 loses its power, removes gate drive from Ql, and turns it off. IC2 is a dual chaige-pump inverter that normally converts 5 V to + 10 V. Capacitors CI, C2, and the two associated diodes form a voltage tripler that generates a gate drive for Ql that is approximately equal to two times the battery voltage. With the values in the schematic, the circuit disconnects 3-cell Nicad battery from its load when V+ reaches a VL of 3.1 V. Approximately 0.5 V of hysteresis prevents the switch from turning on immediately when the circuit removes the load; V+ must first return to Vu, which is 3.6 V. The gate drive declines as the battery voltage declines, cause the ON-resistance of Ql to reach a maximum of approximately 0.1, just before V+ reaches its 3.1-V threshold. A 300-mA load current at that time will cause a 30-mV drop across the disconnect switch. The drop will be 2 to 3 mV less for higher battery voltages. Resistor R4 ensures that Ql can adequately turn off by providing a discharge path for C2.




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