Testing Your Alternator With the Battery Connected

Alternator performs a crucial function to make the vehicle usually operate. In most cases, the alternator uses both the engine and supplies the battery with power for future usage, especially when engine power shuts off. The absence of the alternator in the car system results in the vehicle’s failing to turn on or function usually. Therefore, it is advisable to troubleshoot any alternator issue before seeking help from skilled personnel.

Sometimes, the alternator may become faulty and affect the correct functioning of the car. Breaking of the alternator requires immediate fixation after testing it for any faults. Some people prefer testing the alternator with the battery disconnect, and the method is considered unsafe, especially for the alternator parts. Luckily, the article below shows different methods of connecting the alternator with the battery connected.

Different ways of testing the alternator with the battery connected

1. Using a multimeter

Test Alternator With Battery Connected

A multimeter is the best tool to test the alternator and is usually simple and easy as long as you follow the recommended steps. The steps below describe how to test an alternator with the battery connected or guidelines.

  1. Charge the battery until complete using the correct charger before testing it with a multimeter. To ensure the battery is in the correct state, use an advanced trickle charger that suits the battery model. Avoid using faulty batteries as they may damage the battery, draining energy quickly, or switching off at a specific percentage.
  2. Stop the car and position it in a well-ventilated area. Ensure you park it correctly and leave it for the engine to cool down completely to get accurate results.
  3. Connect the multimeter to the car or vehicle battery. Ensure the terminals of both the multimeter and the battery match.
  4. Read the recording on the multimeter, and it must be approximately 12.4 to 12.6v. This is because the multimeter records the reading in voltage; hence, it is essential to know the standard voltage for your car or vehicle battery.
  5.  Turn on the engine and re-record the battery voltage to get accurate results. The readings differ when the battery is working and when it cools down. The readings must be approximately 14.2 and 14.8 volts. If the readings are less or more than expected, the alternator is in bad condition.
  6. Switch on the whole vehicle, including the radio, lights and again repeat the process of reading the voltage measurements. It must read up to 15V but not fall below 12.9 volts. If the voltage drops, the battery is not charging, or the alternator is not in good condition.
  7. Turn off the whole engine and connect it with the multimeter before cooling down and recording the voltage. It must be approximately 12.9 volts but if it falls below the recommended voltage, check the battery, charger, and power supply source to see if they are functioning.

After testing the alternator, consider the following tips to maintain and take care of the car battery

  • Clean the battery and multimeter terminals of dirt to improve the functioning of both. Accumulation of rust and dirt hinders the battery from charging correctly. When cleaning the, use appropriate cleaning tools such as sandpaper or wire wool to leave the terminals clean.
  • Inspect the battery connection and ensure the wires are connected well. For example, the ground wire connects all negative terminals, and the other terminals connect positive terminals.

Ensure the battery, and multimeter cables are in a good state, and if not, consider replacing them with new ones and testing the battery alternator. Also, ensure the wire connection is tight for the continuous flow of energy from the power source to the battery. Finally, avoid overtightening the wires to avoid breaking them.

  • Replace the multimeter with worn-out brushes.
  • If you don’t have a multimeter, use an oscilloscope to test the alternator of the battery.

After testing the alternator with the battery connected, records incorrect voltages, inspect the battery, power source, and the connection between the battery and power source to detect any faults. Then, attempt troubleshooting the faults by replacing each component with the new one that suits the type of your car model. But, if the problem persists, consider seeking help from a professional mechanic to help you diagnose the problem and fix it. Also, you may consider buying another car if you are financially stable and if the troubleshooting methods fail.

2. Using a “full-field” methodUsing a “full-field” method

The method functions by forcing the alternator function at its total output. The method is easy and limits the alternator’s output when the battery is still connected, hence preventing electrical sparks or protecting the car battery from any damages such as electric sparks. When applying the “full-field” method, ensure you have a complete understanding of the steps necessary to avoid damaging the car battery and its parts, such as wires, etc.

The following steps describe how to test an alternator while connected using the “field-test” method.

  1. Stop the car and park it in a well-ventilated room.
  2. Connect the ground terminal to the alternator’s field terminal. To carry out the activity, pass the battery regulator until the highest voltage records. If the voltage fails to reach maximum, the car battery is not charging or dead.

 Some alternator field terminals of some batteries are connected to the positive terminals of the regulator. Therefore, ensure the regulator records the maximum voltage unless the regulator or the battery is at fault and requires instant fixation. If the regulator is at fault, consider following the recommended steps to fix it or consider replacing the regulator before testing the alternator to get accurate recordings.

3. Applying the “alternator magnet” methodApplying the alternator magnet method

The alternator magnet method is suitable in places with no multimeter, and a screwdriver is essential in such situations. The following steps describe how to test the alternator with the battery connected without using a multimeter.

1) Clean the screwdriver and remove all dirt and rust to avoid incorrect voltage reading using a recommended cleaning tool such as a damp clean cloth.

2) Locate the position of the alternator within the battery and open the battery hood.

3) Place the screwdriver tip near the nut that holds the alternator pulley wheel in position. Ensure the connection is good and there are no electrical sparks produced as a result.

4) Switch on the car’s ignition to activate the dashboard light but avoid starting or driving the car.

5) Touch the alternator pulley wheel again using the same screwdriver.

6) Check the magnetic attraction between the screwdriver and the terminal. The connection is strong, but if the magnetic attraction is weak or absent, the alternator is damaged, or the car battery is not charging.

Frequently asked questions:

  • Can I test my alternator by disconnecting the battery, and is it safe?

No. it is indeed unsafe to check the condition of the alternator with the battery disconnected due to the following reasons; first, charging the battery while disconnected results in the production of excess heat that may damage the electronic appliances and reduce the lifespan of the battery alternator.

Also, connecting the alternator with the batteries increases spark production, thus affecting the entire electric device. Therefore, it is advisable to test the alternator battery without disconnecting the battery to protect your equipment from any form of damage.


Testing the alternator is essential as it aids in knowing a fault car battery and alternator, and the best method of testing the alternator is by using a multimeter. Luckily, the article above shows steps on how to test the alternator using a multimeter.

Also, the article shows different methods of testing the alternator while the car battery is connected without using the multimeter. Therefore, read all the methods above and apply one that suits your preference or is readily available.

For more information you should watch this video below, Enjoy!

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