my car battery is flat. I did some measures with a multimeter. I will explain them below. Can anyone please tell me what can be the issue? Note: My car is a VW Polo 9N3 2007

  1. Symptom

For the last 2 weeks, the battery light on the dashboard does not turn off when I start the car. After I drive for 1-2 min, it turns off. However, 2 days ago, the battery was completely flat, with all electrical components (interior lights, radio, power steering, etc.) on the car stop working (except the headlights, I was driving at night).

  1. The alternator was replaced 1 year ago, all the gears and belts seem to spin fine.
  2. Tests with a Multimeter
  • Voltage when the engine is off, key is off: 11.6V
  • Voltage when the engine is off, key is on: 0V (in fact, few mV is displayed, same for the 0 results I mention below)
  • Voltage when the engine is on, the battery light on the dashboard is on: 11.6V
  • Voltage when the engine is on, after I ramp-up the engine, the battery light on the dashboard is off: 0V

After these tests, the battery is completely flat, so I cannot do any test anymore :D Please give me some possible reasons.

  • How are you starting the engine with a flat battery? Jump leads? Push start? – HandyHowie Dec 20 '20 at 16:06
  • Where exactly are you connecting the multimeter probes? – HandyHowie Dec 20 '20 at 16:08
  • @HandyHowie: The battery had not been completely flat when I start my tests. I was also surprise that I could start the engine to do the tests. However, after these tests, it seemed that the battery was completely empty, so I couldn't start the car any more. I still can lock/unlock the car with electrical door lock thought. – thenewasker Dec 21 '20 at 12:21
  • @HandyHowie: I connected the multimeter to 2 terminals of the battery. – thenewasker Dec 21 '20 at 12:23
  • 2
    Something doesn’t appear correct about your tests. I can’t see how you could start the engine using the battery and then have the battery read 0v. Either your multimeter leads are faulty, or you were not connected to the battery correctly. Did you put the probes on the battery posts, or on the connectors that are clamped to the posts? – HandyHowie Dec 21 '20 at 12:50

The battery isn’t charging properly - if the charging light was switching off after some time then it could be an intermittent electrical fault or perhaps the drive belt to the alternator is slipping, which is easy to check.


First disconnect the battery from your car and get the battery fully charged.you need to clean the connectors on the battery and in your car to get optimum contact.

When the battery is fully charged re connect it to your car and start your engine,you can now measure the voltage on your battery(the voltage with the engine running should be 13-14V if the alternator charges your battery.

When you turn your engine off the voltage should drop to about 12,7-13,8V and it should not drop below 12V even if you do not use your car for a couple of days.

If your alternator does not charge your battery you need to test if it gives off the correct voltage(with the engine running),measure the voltage on the +out on your alternator to ground connection,next measure the voltage on the battery it should show the same voltage if it does not you have a poor connection.

To find a poor connection start by measuring the voltage drop from +out on the alternator to +on your battery(with the engine running)the voltage drop should be less than 1V if the voltage drop is larger you have a poor connection but we do not yet know if it is on the +ve or -ve side so we need to measure the ground connection and to do this you measure the voltage drop from your engine block to battery -ve,if the voltage drop is more than 1V you have a poor ground connection if not you have found the ground connection to be fine and can focus on locating the poor connection on the +ve side.

All of the above is if your alternator is functioning correctly if it is not you will have to replace your alternator ,or diagnose the controll circuit of your alternator(some cars has a digital charging controller that might fail but you can most likely diagnose this via the obd2 port).

Your battery is probably in poor shape after beeing undercharged for some time.

Most of the measurments above is done by setting the multimeter to its 12-20v range.

  • Thanks for your long advice. I will try them in the following days and tell you the result then. – thenewasker Dec 21 '20 at 12:24

you can check from the youtube video how to check if your alternator is dead. CLICK HERE

But I will add more. Turn on your engine and (this should be done if you know what are you doing.) and try with your steel screwdriver close enough to your alternator body (do it from behind the belt, not from the front, your screwdriver will be splashed to your head), if you feel the screwdriver attracted to your alternator is mean your alternator coil working fine. but the diode could be dead inside or the regulator inside is worn.

If you check 11.6V while turning off and turn on the engine only gives you nothing that is not normal.

Check fuse (alternator need firing cable or voltage starter for rotor) if that is blown you will not getting anything from the alternator, check the cable from the alternator.

  • Thanks. I will try the video you posted above. The test you recommended with a screwdriver seems to be too dangerous for me :D, so I will skip it :D – thenewasker Dec 25 '20 at 14:23
  • yes it is dangerous 😊, you may skip that part, because i know that from serviceman that handle it like everyday, he doesn't flip an eye with the fan blades running on full throttle lol it's a bit crazy to see him doing that.. i did it too lately because it faster that way without any tools :D – Cubic273.15 Dec 26 '20 at 15:43

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