When the battery is new it will drop only about two volts during cranking but as the battery time passes and the battery become old this dropping voltage slowly increases and reaches to 5V or 6V due to which the car engine does not start.

This failure can be happened sudden and can cause very mush hassle. The circuit shown here will be ideal to use to get rid of this situation. It will measure the voltage drop of the car battery and produces an alarm when your car battery voltage drops to 8V while cranking.

The circuit is easy to build and uses a 555 timer IC; the IC is working in comparator mode here. After building the circuit it require some adjustments / tuning for the first time. For doing these adjustments a variable power supply will be required. When you have completed the circuit do not connect it with the cigarette lighter connection and set 8 volt in the variable power supply and connect the circuit with this supply and slowly adjust the 10K variable resistor until the buzzer will sound. That's it.

After the above adjustments your circuit will be ready to perform now remove the variable power supply and connect it with the cigarette lighter connection of your car. For best performance fit the circuit in a suitable enclosure.

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This circuit provides an early warning or indication of the failure of your car battery by activating a buzzer for few seconds, so you can understand that the battery is now near to failure. Your car owns a lead acid battery that helps to start the engine, this battery normally lasts for 4 to 6 years.
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Early Car Battery Failure Detector Circuit Diagram