For deep discharge protection, we need to identify the cut-off voltage of the battery. After that, we need to design a circuit in which, when the battery reaches the cut-off voltage level, a Relay disconnects the load from the battery.
For cut-off voltage identification, we will choose a Zener diode 9.1Volt. A Zener diode in reverse bias condition will act as an open circuit when its cathode is applied with voltage below breakdown voltage .we used a 10k variable resister with diode’s anode point to set the cut voltage as per our need. When the cathode voltage is above breakdown voltage, it starts conduction.
We will use a Zener diode with Zener voltage equal to the cut-off voltage of the battery and 10k variable resistor to do some adjustment in cut off voltage of battery. When the battery voltage drops below the cut-off voltage, the Zener diode will cut off the conduction and obstruct the flow of the base current of NPN transistor, thus disconnecting the battery from the load.