cutoff function that will disconnect your vehicle's battery with your selected electrical appliances when the battery becomes half empty and safe the battery to become fully discharge or flat.
The circuit can be used to automatically disconnect the battery from the headlights at times when you forget to switch off the headlights. You can also connect other lights and appliances of your car through the relay of this circuit, to disconnect them automatically with the battery when the battery capacity falls to 50%.
This circuit cannot only used with car, you can also use it with any 12V battery system or device. For example it can also be used with uninterruptible power supplies (UPS) systems, solar systems etc.
When the circuit is completed it requires a little tuning/adjustments for the first time. For this purpose a variable power supply will be required. You can use just about any variable power supply; I have adjusted it with my LM317 power supply.
For adjusting the circuit set 12.5V in power supply and connect it in the place of the battery in the circuit, now slightly adjust the VR1 until LED1 goes off. Now set 12.3V in the power supply and slightly adjust the VR2 until the LED2 goes off. Next set 12.2V in the power supply and adjust VR3 and stop when the LED3 goes off. At last set 12V in power supply and adjust VR4 and stop where the LED4 goes off, at this point the 2N4401 transistor will also goes off and hence deactivate the relay.
The figure below shows a project of a car battery monitor with low battery cutoff function. The circuit can be used with any vehicle; it monitors the battery of your vehicle and shows visual indication of the battery level through the four LEDs used in the circuit. The circuit also contains a low battery
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