Checking voltage on a car battery with a voltmeter

If the battery is down on charge these readings will be lower and can be anywhere from 6. If so the battery will need to be load tested , charged or replaced before testing can begin. Have a helper start the engine and hold the RPM's just above idle at about and observe the meter. It should read between If the voltage on the meter stays the same or drops when the engine is started the alternator is not charging. You should then check the fuse panel for blown fuses and inspect the wires in the harness which lead to the alternator.

If everything checks out the alternator is bad and needs replacing. The next step in testing is to load test the alternator to see if it can hold up under heavy usage. While the voltmeter is still hooked up and the engine still at a raised RPM turn the headlights and air conditioner on.

How to Use a Voltmeter on a 12 Volt | Sciencing

This will demand an electrical system load which will force the alternator to perform to its maximum output. If the voltage starts to drop the unit is weak and needs replacement. When the alternator diode pack fails it can leak AC voltage which can cause disruptions in the electrical system including causing premature battery failure.

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It can also cause other electrical components such as the radio and gauges to act strangely. To test for this condition switch the voltmeter to AC voltage with the engine at about RPM, the meter should read 0 volts.

If voltage is present one or more diodes has failed and the alternator should be replaced. Please watch the following video below to watch the test being performed by one of our mechanics.


The following information pertains to all internal combustion engines vehicles. For this test you will need to wait until evening so you can see the headlight brightness which is best at night.

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How to Check AA/AAA Alkaline Battery Using a Voltmeter

Then turn the vehicle headlights on with the engine off and then observe the headlight brightness, then start the engine. The brightness level should momentarily fade as the engine cranks but then get brighter than when the engine was not running telling you the alternator is charging. If you still cant tell, you will need a voltmeter. This test will not work for LED headlights. An alternator cannot sustain maximum output for long periods of time, it will overheat and fail. A shorted or old battery can cause premature failure of the charging system so it's a good idea to replace the battery at the same time as the alternator if it is three years or older.

Measuring Voltage

Avoid unnecessary sparks near the battery. This is to stop accidental ignition of hydrogen gases present inside the battery causing it to explode. This conditioner is typically present when you can smell sulfur causing hydrogen gases. If you need more information please visit our forum where our mechanics have answered thousands of alternator questions or ask one of our mechanics , we are happy to help.

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What Should Be the Voltage of a Car Battery? (Car Battery Voltage Range)

How to Test an Alternator. Need to test your alternator? What Goes Wrong? Ask a Car Question. The dial selections on a multimeter are divided into three groups. The one you want is voltage, denoted by V. You'll find several settings in the voltage group representing a progression in sensitivity from millivolts mV to volts.

The dial selections may also include settings for DC and AC voltage. DC voltage is denoted by a straight line, or no line at all, while AC voltage is denoted by a wavy line. To measure voltage in a 12V system, you want to turn the dial to select DC voltage in the 20V range. Before measuring voltage on a battery, it's a good idea to disconnect it from the circuit by removing the battery cables.

This minimizes the chances of a stray current draw affecting your reading. Note the reading on the meter.

How to Test an Alternator

It tells you the voltage between the battery terminals. If the battery is good, the reading should actually be higher than 12 volts. It should be somewhere between The battery is not charged, and could be bad, if the reading is You can conduct two other voltage tests on a 12V battery that powers a motorcycle or lawn mower. To do this, you'll need alligator clips on the voltmeter leads or a helper.