How to Test Battery Failure

You get in your van, turn the key, and nothing… It can happen to anyone for multiple reasons, the question is:

How do you know if it’s a knackered battery, parasitic drain, or a faulty alternator?

Before you take the vehicle to the garage, or run out and buy a new battery, there are some really easy diagnostics you can do, and we thought it might be useful to give you a little rundown; after all, what good is your lovely camper/van if you can’t drive it away? So here goes, to do these test you will need two things:

  1. A battery charger (or some other way of getting your battery charged)
  2. A multimeter

Follow these simple tests and you’ll soon know where the problem lies.

1. Charge your battery

The first thing you want to do is charge your battery, you need it to be charged in order to do the other tests.

To test if it’s charge, get out your multimeter, plug the black cable into the com port and the red cable into the voltage port, set the dial to dc 20 volts, then touch the diodes to the corresponding battery terminals and check your reading:

If your battery is not testing above a minimum of 12.5 volts (ideally above 12.7) after charging, you know straight away that you need a new battery. If all is well so far you can move on to the next steps.

2. Test whether the battery is holding the charge

If you’ve taken your battery out of the vehicle to charge it, you can simply leave it out (wait a few hours for any surface charge to dissipate) and test it with the multimeter (as above) to see that it has charged, then wait a further length of time (no less than 12 hours) to see if it’s losing charge (dropping below 12.7) with nothing attached.

A quicker way to test is to see if the battery has the cold cranking amps to start the car. Get someone to start the vehicle while you hold the voltmeter across the battery (as above), the reading will drop and then (hopefully) rise as the alternator kicks in, if it drops below 10 when you start the vehicle this indicates the battery is starting to go (below 5-7 and you know the battery is knackered).

Here’s a video for you visual learners:

3. Test for alternator problems

Test across the terminals as before, once your engine is running you should see an increase to around 13.5-15 volts, this indicates your alternator is charging the battery, if you don’t see an increase, it’s likely you have an alternator issue.

Assuming your alternator is working, next you want to check the alternator regulator; get someone to rev to around the 2000 rpm range, your meter should not exceed 15 volts, if it does, you may have an alternator regulator issue which needs further investigation.

Finally, do a load test. Keeping the engine running, turn on your lights, fans, radio, etc. What you want is for the reading to stay above 13 volts. If it does not, you will want to investigate further.

Other things you can check are your alternator drive belt and pully, visually inspect the wires, and do a drop test from the positive terminal on the alternator to the positive terminal on the battery. Here’s a video covering the above alternator tests:

If you don’t have a multimeter, a different – very basic – way you can test your alternator is by removing the positive terminal from the battery once the vehicle has been started. If the vehicle continues to run, your alternator is working.

4. Test for parasitic drain

If your battery and alternator have both tested fine, you then want to move on to test for parasitic drain. To test whether there is a drain:

  1. ensure everything is off, your vehicle doors are closed, the key is out of the ignition, etc.
  2. remove the negative terminal clamp from the post, switch your multimeter to read amps (put the black multimeter cable to the com port, and the red cable to 10 amp port), then bridge the gap between the battery post and the removed clamp. Like this:
  3. it is normal to get a reading of under .50 amps, but if you are getting anything above 1 amp you have something draining your battery that shouldn’t be.

So how do you work out what is draining your battery?

What we want to do now is test what is drawing power from the battery. We will do this by checking the fuses so you need to discover the location of your fuse board (or boards), frequent locations include under the bonnet, around the glove box area, near the steering wheel, or in the boot.

Don’t forget to keep everything off, key out, and doors closed to avoid false readings.

Many people will recommend pulling the fuses one by one until the amperage draw stops showing on your multimeter. Pulling out fuses is, however, something to avoid if not necessary. A more accurate (and in our opinion quicker, easier, and safer) way to do what is essentially the same test, is to use your multimeter to test the voltage drop on each fuse. So rather than pulling out each fuse, simply set your multimeter to millivolt mode and put your meter across the fuse, like this:

you should have a small reading which goes down within a few seconds. If you come across a fuse with a higher reading that doesn’t go down, there is your culprit. You now know what is draining your battery and can concentrate on fixing it.

Here’s a video covering the above parasitic draw test:

And that’s it! Hopefully, this has helped you identify the problem and you can be on your way to fixing it.



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