How to perform a battery drain test

What is a parasitic load?

This is when the battery is being drained when the vehicle is inactive – something is drawing current when everything should be switched off. It’s worth noting that all modern vehicles have parasitic loads as many systems never truly shut down.

How do I test for a parasitic load?

There are a few ways to do this, but it’s important to avoid disturbing any electrical system unless absolutely necessary. You can attach an amp clamp to the lead connected to the battery’s negative terminal and test the current.

Alternatively, you can measure the voltage drop across the fuses (a volt drop indicates that current is flowing).

Lastly, you could use an ammeter. Remove the negative terminal at the battery and connect the ammeter to complete the circuit. Then test the strength of the current. Don’t switch on the vehicle’s lights or engine as this could cause the ammeter to blow its internal fuse

How much current is a parasitic load?

When testing, we’re not looking for a result of zero as some systems will always be running in the background and never truly switch off. You can refer to the manufacturer's data for the expected current. Anything over that would indicate a parasitic load.

How do I diagnose the cause of a parasitic load?

Keeping the amp clamp or ammeter connected to the battery, start isolating individual systems by removing their fuses to shutdown active circuits on the vehicle. If the current reduces when you pull the fuse, you can narrow down your search by seeing what that system is connected to (using topology and wiring diagrams).

Additionally, you can use an infrared camera to detect any fuses that have a current running through them, or are much hotter than the others.