Battery & Alternator Diagnostic

Your battery and your alternator are critical components: your car won't start without them, and it won't keep running without them. Other than waiting for your car to not start or to shutdown while driving, there's a simple test you can do to crudely determine the state of health of your alternator and battery.

Function

There's a bit of a voodoo aura that surrounds these 2 components. At it's most basic level, the alternator charges the battery while the engine is running, and the battery provides the energy to start the car. It is possible for a vehicle to keep running without a battery (or with a dead battery), as healthy alternators provide more than enough electrical power.

Symptoms

With a weak or dying battery, the engine won't turn over at all when trying to start, or will turn over slowly when trying to start. Any attempts to jump the battery will fail.

A weak or dying alternator, on the other hand, will produce weird electrical things: slower wipers, dimmer lights, or my favourite, dimming dashlights when the turn signal or rear defogger are turned on. On the extreme side of the scale, the alternator will not charge the battery and you get the symptoms listed above for a weak battery.

Diagnostic & Troubleshooting

It's important to tell which of these 2 components is causing your trouble. As stated above, a weak alternator can cause you to suspect you have a weak or dead battery instead. Fortunately, there's a simple test you can perform to gauge the state of health of these 2 components separately, and all you need is a multi-meter ($15 at a department store).

Testing the Battery

  1. Keep the engine off
  2. Set your multimeter to "20 VDC"
  3. Touch your positive lead to your positive post, and your negative lead to your negative post
  4. Interpretting the voltage reading
    1. A voltage reading above 12.6 VDC indicates a well charged battery, skip to testing your alternator
    2. A voltage reading below 11.8 VDC indicates a poorly charged battery, continue with the next step
  5. Repeat the voltage reading by touching your leads to their respective battery posts directly (not the clamp or connector), and press your leads in a little bit
  6. Repeat the voltage reading one more time by touching your leads to their respective battery cables (not the connector or the post), and press your leads in a little bit
  7. Interpreting the voltage reading
    1. If you get a proper voltage reading from the battery posts and a lower reading from the battery cables or connectors, there is a poor connection between your battery cables and your battery posts. Disconnect the battery cables and clean both the cables and the posts using a steel brush, then repeat the voltage test.
    2. If you get the same voltage reading (too low) from both the battery terminals and the battery cables, your battery truly has a low voltage. Continue to the next step
  8. If the vehicle has been stored below freezing, you may have a frozen battery. Disconnect the battery cables, remove the battery and take it some place warm for several hours. Repeat the voltage test on the battery posts after a few hours. If the voltage recovers, this may have been your issue.
  9. If something electric may have been left on or if the vehicle has not been started in several days, the battery may simply have drained itself. Give it a boost either from:
    1. Another vehicle (quickest and most reliable)
    2. A battery jumper (quick, but you need to make sure the jumper is actually healthy)
    3. A battery charger (takes hours)
  10. Repeat the voltage test after the jump to see if the battery recovered. If it did, try to start the engine. If it didn't, take it to be tested more thoroughly at a parts store.

Testing the Alternator

This one's simpler, but you do need the engine to start.

  1. Turn the engine on
  2. Set your multimeter to "20 VDC"
  3. Touch your positive lead to your positive post, and your negative lead to your negative post
  4. Interpretting the voltage reading
    1. A voltage reading around 14.0 VDC indicates the alternator is charging the battery, continue to step 5
    2. A voltage reading below 13.0 VDC indicates the alternator may not be charging the battery. This may be caused by:
      1. Poor electrical connection
        1. Turn the engine off
        2. Disconnect the battery cables
        3. Clean them using a steel brush
        4. Reconnect them tightly
        5. Start the engine and repeat the voltage test. If the voltage is now reading normally, dirty or loose battery cables may have been your issue. If not, continue to the next step
      2. Poor mechanical connection
        1. Turn the engine off
        2. Check the tightness of the belt driving the alternator. It should be taut enough to require some effort to deflect it more than 1/4 inch. If it's loose, tighten it
        3. Inspect the belt for wear. A worn belt may slip
        4. Start the engine and repeat the voltage test. If one of these solved your problem, this was likely your issue. If not, get your alternator tested more thoroughly at a parts store
  5. Turn on everything electrical you can think of in the vehicle: turn on the headlights, set the blower fan to high, turn on the defogger, turn on the radio
  6. Repeat the voltage test, a healthy alternator will not drop below 13.0 VDC under load. If it does, get it more thoroughly tested at a parts store

How Do Batteries Drain Themselves?

There's 2 main things that can drain a battery while the engine is off.

You Left Something On

Leaving something on is the most common cause of a drained battery.

There's a Parasitic Drain

All vehicles have a small parasitic drain. That means that leaving the vehicle sitting with the battery connected but without starting the engine will lead the battery to slowly drain itself over time. Batteries and vehicles are designed to be able to handle this situation for several days, maybe even a few weeks. But if something increases the parasitic drain, the battery will not be able to withstand this for very long. A vehicle that frequently needs a boost for no apparent reason (didn't leave anything on) may have a high parasitic drain. This is how you test for it.

  1. Keep the engine off
  2. Disconnect the negative cable of the battery (we are going to be using it)
  3. Set your multimeter to "20 Amps DC" (10 Amps DC will do if you don't have a 20). Most multimeters need you to plug in one of your leads into a different port than for a voltage reading. Make sure you do that
  4. Touch one of your leads to the negative battery post, and touch your other lead to the negative battery cable, observe the reading
  5. Interpreting the current reading
    • A normal parasitic drain is less than 50 mA. On your 10 Amp or 20 Amp scale, this would read as 0.05 A (or less)
    • If your parasitic drain is greater than 50 mA, you need to find the cause of it.
      1. Make sure everything electrical is turned off (all lights are off and nothing is connected to the electrical ports, USB ports or cigarette lighter)
      2. Locate your fuse box under the hood and start removing fuses 1 at a time until your parasitic drain drops to a normal level. If a fuse eliminates your parasitic drain, you've found the electrical circuit that is causing your issue. Inspect electrical wires and elecrtrical connections on that circuit, remove electrical components 1 at a time and repeat the test until you find the cause
      3. If you can't find a fuse that will eliminate your parasitic drain, look for another fuse box under the hood (there may be more than one) and continue. If not, locate the fusebox in the cabin and continue there (there may also be more than 1 in the cabin)
  6. When you think you are done, repeat the current test, make sure you are getting a parasitic drain less than 0.05 A. Switch scales on your multimeter (200 mA is good) and get the actual number. Make sure it's less than 50.

If a vehicle isn't going to get started for several days, it would be safer to disconnect the negative cable of the battery.