Parasitic drain — why the battery goes flat while parked

If the battery goes flat over a few days’ standing despite being in good condition, the cause is often parasitic drain — a parasitic draw by electronics that didn’t go to sleep properly.

What parasitic drain is

After you lock the car, some systems go into standby and draw a small, acceptable current (usually tens of milliamps). When a module doesn’t sleep, the draw rises and drains the battery in just a few days.

The most common causes

• Lights that don’t switch off (boot, glovebox, interior)
• A faulty module, radio or amplifier that won’t sleep
• A badly fitted alarm or extra electronics
• A damaged relay or contact
• A worn battery that simply won’t hold

How to diagnose it

Parasitic drain is measured with an ammeter connected in series, once the car has “gone to sleep”. Then, by pulling fuses one by one, you locate the circuit responsible for the excess draw. It’s a methodical but effective procedure.

If the battery dies after a few days’ standing, it isn’t always the battery’s fault — sometimes something in the car “isn’t asleep”.

First rule out a worn battery

Before you go hunting for a drain, it’s worth checking the battery itself — because the symptoms can be identical to wear. Only a healthy battery that still goes flat points to parasitic drain.

We’ll help rule out a worn battery and assess the cause of the discharge as part of our other mobile services.