A properly selected and connected voltmeter in a car is a very useful multifunctional diagnostic tool. The device allows you to control the voltage of the on-board network, according to which you can draw a lot of conclusions about the state of automotive electrical equipment. This article tells you how to connect a voltmeter in a car, which one to choose, and how to navigate according to its readings.

Features of a voltmeter in a car

A voltmeter in a car only at first glance seems to be a simple and useless device. But one has only to choose and connect it correctly - according to its indications, one can clearly see:

  • the degree of battery charge (in %);
  • battery charge voltage from the generator;
  • Why is the battery undercharging?
  • cause of electrolyte boiling;
  • on-board voltage drops when powerful consumers are turned on;
  • How much does the starter take on?

Also, when you constantly have voltmeter readings before your eyes, you can understand in a timely manner that it is time to change the battery to a new one.

Choosing a car voltmeter

Any device will not work. Analog or pointer disappear immediately. They look cool, but there is very little sense from them. You need a digital voltmeter with the following parameters:

  1. Measurement range - up to 30 V. The closer this figure is to the measured voltage, the more accurate the device. For example, voltmeters that can measure up to 100V in the 10 to 15V range are less accurate than models up to 30V.
  2. Number of decimal places - 2. Mandatory at voltages above 10 V. Cunning Chinese often photograph their voltmeters at a voltage of 5-7 volts, and there really are 2 decimal places after the decimal point. But one has only to apply more than 10 V, and one decimal place disappears, since there is no longer room for it.
  3. The update rate of indications is up to 25 milliseconds. A quick voltmeter will allow more extensive diagnostics.
  4. Separate food. Cheap models are powered by the same voltage that they measure. And it jumps. So that the readings on the display do not walk, the voltmeter needs to be provided with a separate, stabilized power supply.
  5. Availability of calibration. Either the Chinese volts are different from ours, or there are some other reasons, but without calibration it is almost impossible to find two equally showing voltmeters.
Decent voltmeter for car
Decent voltmeter for car

If the model you have chosen does not meet any of the requirements listed above, then advanced diagnostics will not work. Although such voltmeters in the car can show something.

How to connect a voltmeter in a car: 3 ways

A car voltmeter can be connected in three ways:

  1. Insert into cigarette lighter.
  2. Run 2 wires to the battery terminals.
  3. Apply separately stabilized power to the battery + terminals.

Let's consider all three options.

Voltmeter in cigarette lighter

Typical voltmeter in cigarette lighter
Typical voltmeter in cigarette lighter

This is definitely a bad way to connect a car voltmeter. But it's simple. Auto shops are full of display meters that are designed to be installed directly into the cigarette lighter socket. Fast. Conveniently. But it doesn't work, and here's why.

Firstly, such voltmeters do not take into account the voltage drop on the wires between the cigarette lighter and the battery. Therefore, the readings will always be underestimated, inaccurate.

Secondly, it is difficult to find a voltmeter in the cigarette lighter with two decimal places.

Thirdly, it is impossible to calibrate them without interfering with the design.

Fourth, they are slow.

And such voltmeters are prone to wild overheating, especially if they are with USB connectors and are used to charge gadgets. There were cases when such devices became the cause of a fire in a car.

Voltmeter two wires to the battery

The normal way to connect a voltmeter, but with some drawbacks. The most obvious is the lack of a separate stabilized power supply. This means that the readings of the device will be guaranteed to walk, and you will not know the exact voltage of the on-board network of the car.

However, this option is much better than a cigarette lighter toy. The main thing is not to be lazy, and connect the voltmeter directly to the battery terminals. At the same time, it is not necessary to take very thick wires, since the currents flow through them are low. Ideally, if the model you purchased has a calibration screw. After pulling the wires and connecting the voltmeter, you can compare its readings with the multimeter at the battery terminals, and correct them if necessary.

Voltmeter to battery terminals + separate power supply

Popular amateur radio voltammeter
Popular amateur radio voltammeter

This is the most competent and complete way to connect a voltmeter to a car. To supply a separate power supply in such models, there are two additional wires. Some even have a built-in voltage stabilization circuit. If it is not there, or it could not be found out, then the supply voltage can be stabilized with the simplest module from China. They are inexpensive, as they consist of three to five radio components.

Diagnostics according to voltmeter readings

Now let's move on to the most interesting and useful - what can be learned from the readings of a car voltmeter.

Battery state of charge

Beautiful voltmeters, but inaccurate
Beautiful voltmeters, but inaccurate

You can find out how much percent your battery is charged by the voltage that it gives out in a rested state. The latter means that before the measurements the battery was not charged or loaded for at least 8-12 hours. This is a prerequisite. Usually, the degree of battery charge on a voltmeter is optimally assessed in the morning, when the car has stood motionless all night.

Immediately after a trip, charging the battery from a stationary charger, trying to start the engine, and in similar cases, it is useless to assess the degree of charge. The readings will either be too high or too low.

The percentage of battery charge by quiescent voltage can be estimated from this plate.

Quiescent voltage (V)

Charge (%)

< 11.90























Incorrect measurement


Battery charge voltage from the generator - undercharging and overcharging

When the car's engine is running, the battery is charged by the alternator. Moreover, there should be a voltage in the region of 14.4-14.5 V on board.

If the car voltmeter shows less, then there is not enough voltage to fully charge the battery. It will only partially charge, but it will never be restored to 100%. In such cases, the battery has to be regularly charged from a stationary charger. This must be done, as the constant operation of the battery in an undercharged state is guaranteed to end in sulfation.

About battery desulfation.

If the voltmeter shows a voltage above 14.5 V, the battery will overheat and boil. If the electrolyte in it is liquid, then its level will be constantly underestimated. In the case of AGM and GEL technologies, voltages up to 14.9 V are allowed, which can usually be found on the label or in the attached instruction manual.

There is a separate article about battery boiling on the Auto without service station website.

Voltage drops

They are clearly visible on the voltmeter, if it is accurate and fast enough. Almost any device is able to show a voltage drop when such powerful consumers as headlights, a stove, and an air conditioner are turned on. A stable voltage in the vehicle's on-board network must be maintained by the relay-regulator on the generator. What drawdowns are allowed, and which ones are a symptom of a malfunction - read the article on low on-board voltage on a Car without a service station.

Starter and general condition of the battery

When the starter is running, there is a hard voltage drop. However, if the battery and starter are good, the voltmeter readings should not fall below 10 V. If you see a drop to 9-6 V, then this means one of two things: either the starter takes on a lot; Or the battery is not holding the load.

Problems with the starter are described in detail in the article “Why the starter turns badly” on Auto without a service station. If you suspect that the battery is not holding the load, read the material "How to check the battery with a load plug".

VIDEO: voltmeter install


How to connect a voltmeter in a car should now be clear. There are three ways, only one of which maximizes the functionality of this device on board. If you have any questions, comments or thoughts on this topic - write everything in the comments below.