2 Easy ways to check if a car’s alternator is draining the battery

Alternators can be quite reliable. So, should your car’s battery drain, it is better to investigate other possible causes first such – appliances -radio, ignition switched on, lights left on first and check the alternator as a last resort.

car alternator
Car alternator (see red arrow)

Once you have ruled out other causes, you can then proceed to check if the alternator is draining your car’s battery.

I have listed below two easy steps you can follow to confirm the alternator’s performance.

Step 1. Check that the alternator can charge the car’s battery

It is important that the alternator is able to charge the car’s battery. So, first check the car’s alternator charge function – Using an alternator tester or digital multimeter, are you able to measure a voltage of 14V DC across the car battery with the engine running?

When the engine is running, take care to always connect the alternator terminals to a battery. In some cases, the higher than usual voltage output from an alternator not connected to a battery can damage the inbuilt diodes.

Note: This is not a conclusive test, it is possible for the alternator’s charge function to work yet you still have an alternator draining the battery.

Faulty diodes – open or shorted – can affect an alternator’s power output and hamper its ability to charge the battery and power the car’s electrical system

Step 2. Measure the current flow from the battery to the alternator (Alternator battery drain test)

The alternator battery drain test may require a qualified mechanic. If however you feel up to it, using a fully charged car battery, disconnect the rest of the car’s electrical circuit and leave the alternator alone connected to the battery overnight.

1. Do you find the battery discharged in the morning? Or

2. Using a Digital multimeter, with the rest of the car’s electrical system disconnected and car’s engine switched off, measure the current flow from between the car’s alternator and battery – there should be no current flow.

Hopefully, these two steps help you confirm or rule out the possibility of an alternator draining the battery.

In summary, a faulty alternator can drain your vehicles ‘battery.

To test whether an alternator is draining the battery, with the car’s engine switched off disconnect the car’s electrical system leaving the battery only.

Using a multimeter set to measure current (amps), should you have current flow between the battery and alternator (with the rest of the ca’s electrical system disconnected) then yes – your alternator is draining the battery.

To resolve the issue, you may replace the faulty diodes or replace with your car manufacturer’s recommended alternator.

Related Questions

# 1. How to fix an alternator that is draining the battery

If your are technically inclined and are comfortable taking the alternator apart, you can test the diodes and replace with the OEM recommended diodes.

Alternatively, you may replace the OEM recommended alternator – Make sure its fits and provides sufficient power to the car’s electrical systems reducing stress on batteries.

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