Car Battery Voltmeter Circuit

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

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.

Voltmeter with LED

Voltmeter with LED

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.


D1=5V6 /0.5W Zener

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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