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At many auto parts stores they will test - often for free - a vehicle's electrical system by attaching a device to the positive and negative terminals of the battery. That's all they do. Then you turn on your vehicle, and, like magic, they can tell you if your battery, starter, or alternator is faulty. Not only that, they can provide very specific information regarding any faults.

Is the information produced by these systems meaningful?

What if more than one component is actually faulty?

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This test is rudimentary, but efficient.

  • Testing the battery voltage with the car off tells you the current state of the battery (can it hold a charge)
  • Testing the battery voltage with the car on tells you the state of the alternator (can it recharge the battery)
  • Measuring the battery draw while you are starting the car tells you the current state of the starter (is it drawing the correct amount of current)

Any deviation from normal, and by how much, provides well-educated guesses.

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    A small nit, but on the third point you're measuring the voltage drop while cranking rather than the current drawn. – dlu Jul 20 '16 at 3:26
  • @dlu How would you rephrase "is it drawing the correct amount of current"? – RockPaperLizard Jul 20 '16 at 5:33
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    (If the vehicle won't start, it's a problem with the starter or the electrical connections. The alternator doesn't factor into the equation, at this point.) – Mathieu K. Jul 20 '16 at 7:19
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    Once the vehicle is running, the voltage across the battery is the voltage the alternator is putting out, which is higher than the voltage measured in the first step—so that it recharges the battery. If it's not higher than the voltage from step 1, the alternator isn't working, so the battery is being discharged but not recharged; the car won't start with a discharged battery. – Mathieu K. Jul 20 '16 at 7:25
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    But those are all tests you can perform with a load tester, a voltmeter, and a battery charger. What the store's actually doing, I'd be interested to find out. – Mathieu K. Jul 20 '16 at 7:29

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