Skip to main content

There are quite a number of issues that can cause parasitic battery drain.


Causes of parasitic battery drain

1. Weak battery

2. Poor/dirty/corroded termimal

3. Faulty grounds

4. Drain from car accessories

5. Security system of the car

6. Interior lights

7. Faulty wiring

8. Faulty alternator/diode

Some of these causes will be explained towards the end of this article.



Parasitic drain also known as phantom drain is undesirable excessive current draw from the battery when the car is turned off and resting. This battery drain can be very frustrating because it can send several car batteries packing if it's root cause cannot be determined on time.

Usually, when the car is resting, any battery current draw above 500 milliamps is regarded as Parasitic but this can vary between cars, especially with modern cars. This shall be a topic of another day. I prefer to tell you how to use a clamp meter to troubleshoot Parasitic battery drain today.


Different ways of measuring current:

1. Connecting an ammeter in series circuit to measure current.
2. Using a clamp meter by clamping a current carrying conductor (commonly wire).Clamp meters have wide jaws that clamps around an electrical conductor to provide non-contact current measurement.
We can measure current without disconnecting the circuit to create a series connection with an ammeter. The meter does not have to physically touch the circuit that we want to measure its current.

Most clamp meters can also measure voltage, capacitance, frequency, resistance, temperature, continuity and more...

It is safer and more convenient to use a clamp meter because there won't be fear of passing high current through the ammeter or fear of making errors in connection.

There are different types of clamp meter but the type that we need is hall effect type which can measure both direct and alternating currents and are more sensitive.

In electromagnetic principle, when current flows through a conducting material, it causes the generation of magnetic flux around that conductor. It is through this magnetic flux that the clamp meter measures current because the amount of flux generated is directly proportional to the amount of current passing through the conductor.

I recommend that a clamp meter with detachable/remote screen should be used if it is available. With this type of clamp meter, it is easier to measure currents of individual circuits because you will not have to disconnect each circuit. For instance, you can clamp the wire that powers the radio amplifier. You can also camp a circuit in the boot, close the boot and use the remote screen to monitor it.


Things to ensure before carrying out battery drain test

1. The car's battery is a known good one and is fully charged.
2.The battery terminal can be washed with water and soda to ensure a clean terminal with good electrical contact.
3. Put off all electrical accessories like radio, wiper, heaters, lights and so on...
4. Close all doors and boot
5. Leave car to seat for about 30 minutes because some modern car's electronics do not go into sleep mode immediately the car is turned off.

The battery parasitic drain test procedure

Without disconnecting any terminal of the battery, clamp the negative wire of the battery and set the clamp meter in the range to measure current with the jaw of the meter and begin to read the current displayed.

Note: If the battery of the car is seating in the boot, you can still clamp it there taking note of the little current draw from the boot lights. Alternatively, there will likely be a positive battery cable in the engine bay that supplies the kick starter power.

If there is excessive current draw, you can begin to pull fuses to identify which one is causing high current draw. Once that fuse is identified, you will need to know the accessory or accessories that fuse is protecting. Check the owners manual of the car for this information.

The clamp meter can also be used to test individual circuits like electric power steering, radio, electronic control modules and so on.

Note that some cars have multiple fuseboxes in their engine bay, cabin and boot areas.

Ground connections should also be checked for Parasitic current drain because poor contact points can create a region of high resistance which can cause current draw.

Major power cables from alternator to battery and from battery to kick starter should be checked for parasite drain. This can be done effectively through voltage drop test that I will treat in another different post.

If parasitic battery drain cannot be traced to any accessory, ground or major power cables, it is high time to check the alternator diodes because faulty diodes allow current to leak in the opposite direction. A diode when operating normally allows current to pass in only one direction except a very tiny amount of reverse current that it can pass. Do not let the reverse current confuse you. Just ignore it and know that a diode allows current flow in one direction. A faulty diode will allow current to flow from the battery to the coil of the alternator causing parasitic current drain.

You need to have a good information about the car that you are testing before you will know what level of current drain is parasitic. Some cars with keyless entery can have their modules searching for the key signal and these modules draw certain small amount of current which can be mistaken as parasitic drain. That is one of the reasons why some of those cars are equipped with large batteries like 100 amps battery. It's good to consult repair manual for information on current draw of the different accessories if provided.

You also need to ensure that the charging system of the car works well and fully charges the battery because a battery that does not attain full charge drains faster.