Above circuit diagram is a battery monitor circuit which detaches the load when the battery is discharged, anticipating a deep discharge which could cause permanent battery damage. The battery is automatically reconnected when a battery charger or other DC source is connected across the load. Select a relay which requires less than 100mA coil current and with contacts capable of handling the load and charging currents.
The TL431 is three terminal adjustable shunt regulators featuring great temperature stability and output current handling capability up to 100mA. The output voltage may be set to any chosen voltage between 2.5 and 36 volts by selection of two external divider resistors. This component can be utilized as a replacement for zener diodes in many applications requiring an improvement in zener performance.
C1 should be about 100uF and C2 may be near 1 uF. C2 simply delays the closing of the relay long enough for C1 to charge and C1 maintains the relay closed long enough for the battery voltage to climb above the cut-off point (about 22V for the circuit as shown). The 210 K-Ohm resistor may be reduced to 92 K-Ohm for 12V batteries. These R1 values may be replaced with smaller value if a lower drop-out voltage is desired. The circuit may cycle on and off several times if the battery is deeply discharged with a delay proportional to the value of C1. The load sees the full voltage of the charger before the relay connects the battery in this circuit.