This circuit reminds you when the battery enters into the deep discharge state. It always monitors the terminal voltage of battery, and activates a buzzer when the terminal voltage drops below 11.5 volts due to self discharge. This is an ideal circuit to monitor the standby battery which is charging occasionally.
When the battery is fully charged and its terminal voltage is above 11.5 volts, Zener diode conducts and forward bias T1. Green LED remains on indicating the normal voltage level. When T1 conducts, base of T2 will be pulled to ground and it remains off. When the voltage drops below 11.5 volts, T1 turns off and T2 conducts. Red LED turns on and Buzzer beeps.
Connect the circuit to a variable power supply set as 11.5 volts and slowly adjust VR till buzzer beeps. Increase the voltage to 12 volts. Buzzer should stop beeping.