Car Battery Voltmeter with LED Indicator

The circuit was developed to create a voltmeter that will be used to test car batteries while showing an indication using LEDs.

  • Voltmeter – a device or an instrument used for measuring the electrical potential difference between two points of either alternating current or direct current electric circuit.
  • LM324 – has internal frequency compensated for unity gain, large DC voltage gain, wide bandwidth, wide power supply range, very low supply current drain, low input biasing current, low input offset voltage, large output voltage swing and differential input voltage range equal to the power supply voltage.
The voltage of a car battery can be measured with the use of a voltmeter as well as the charge left. A typical car battery voltage delivers around 12.6 V under no load condition and will require charging if the voltage reading is at 11.6 V. The measurement of voltage is best recommended during a high current like running the car head lights into high beam. In case the battery rapidly drops its voltage significantly under load, it would require a replacement.

This circuit will function as a comparator and will measure the car battery voltage with an interval or step of 1 V. The voltmeter will be connected across the battery terminals then starting the car. The voltage of the battery should not be measured below 10 V or else it will be considered as low in charge or low in water, since the water level of the battery should be about ¼ of an inch above the plates.

Car Battery Voltmeter with LED Indicator Schematic


Car Battery Voltmeter with LED Indicator

By applying the voltage of the battery in the inverting inputs of the amplifiers, the indicated voltage on the voltmeter is compared with the reference voltages that are produced by the Zener diode D1. The Zener diode is a special kind of diode that permits the flow of current in just one or forward direction as a normal diode, but will also allow in the reverse direction if the voltage is above or larger than a certain value of the breakdown voltage. The measured value is just enough to provide good thermic stability.

The presence of 10K trimmer RV1 is to adjust the degree of voltage that is required or desired while the visual indication will originate from the four LEDs.

R1=1K2
R2-3-4=680R
R5=15K

R6=10K
R7-8-9-10=1K
D1=5V6 /0.5W Zener

D2-3-4-5=LED
IC1=LM324
RV1=10K trimmer

The main use of the car battery voltmeter is to monitor the life and performance of batteries. It can be mounted on the dashboard that shows the battery condition to easily monitor the electrical system voltage while driving. The measurement is done by switching off the engine as well all lights and accessories and switching on the key without starting the engine. The battery is full charge if the voltmeter reads 12 V or more while a voltmeter reading of much less than 12 V signifies the battery is either discharged or failing.
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