Testing car battery vs. alternator

Written by arthur barnhouse
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Testing car battery vs. alternator
Diagnosing your vehicle's electrical problems isn't as difficult as you may think. (dead battery image by Katrina Miller from Fotolia.com)

If your car battery and/or alternator are not working correctly, you stand a high risk of being stranded with a car that refuses to start. Thankfully, diagnosing battery and alternator problems is fairly simple, even for a novice mechanic. Obviously, if your battery is constantly dying and in need of recharging, you can assume that something is amiss. However, do not immediately assume that you need a new battery. Often, the charging system, i.e., the alternator, is at fault.

Skill level:
Moderately Easy

Things you need

  • Voltmeter (one with a digital display will be easiest to read)
  • Battery charger (optional)

Show MoreHide


    Testing the Battery

  1. 1

    Turn off your engine and all accessories, radio, air conditioning, etc.

  2. 2

    Open your vehicle's hood and remove any shields/covers from your battery and terminals.

  3. 3

    Touch the positive lead of your voltmeter to the positive battery terminal. Positive leads and terminals are most often red in colour and/or marked with a plus sign.

  4. 4

    Touch the negative lead of your voltmeter to the negative battery terminal. Your negative lead and terminal are most likely black and designated with a minus sign.

  5. 5

    Look at the display on your voltmeter. You should see a reading somewhere between 12.5 and 12.8 volts. If you see a reading lower than 12.5, your battery is not holding a full charge. This is a sign your battery may be bad. To further test it, recharge the battery completely using a battery charger and wait a day or two, without running the vehicle or using the battery. Test the battery again. If it is low once more, you have a bad battery and it needs replacement.

    Testing the Alternator

  1. 1

    Open the bonnet of your vehicle and remove any guards or shields from the battery. Turn off any and all accessories and start the car or truck.

  2. 2

    Allow the vehicle to idle while you touch the positive lead to the battery's positive terminal.

  3. 3

    Touch the negative lead of your voltmeter to the negative terminal.

  4. 4

    Look once more at the display on your voltmeter. You should be seeing a reading somewhere between 13.9 and 15.1 volts (on an average summer day at about 25 degrees C). On extremely cold days, the volts may be slightly greater. On very hot days, it may read slightly less. However, anything less than 13.5 volts is a sign of a bad alternator. In this case, you may need to take your vehicle to a qualified mechanic in order to have the alternator replaced.

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