What can drain a car battery when its off?

Uncover the Culprits: What Drains Your Car Battery When the Engine is Off

Few things are more frustrating than getting into your car, turning the key or pressing the start button, and hearing nothing but silence. A dead car battery is an inconvenience that can happen unexpectedly, leaving you stranded and searching for a jump start. While car batteries naturally lose charge over time, certain factors can cause them to drain more quickly, even when the engine is off. Understanding what drains a car battery when the car is off can help you identify and prevent these issues, ensuring your vehicle is ready to go when you need it.

Understanding Parasitic Drains

When your car’s engine is off, some electrical components continue to draw a small amount of power from the battery. This is known as a parasitic drain, and it’s normal to some extent. For example, your car’s clock, alarm system, remote keyless entry system, and electronic control units (ECUs) require a minimal amount of power to maintain their functions when the car is off. However, when the parasitic drain is higher than usual, it can quickly deplete the battery and leave you with a dead car.

1) Computer parasitic battery drain is the most common cause of what drains a car battery when its off

Computer modules draw power when they’re running. When you shut the vehicle down, the modules are programmed to enter into sleep mode after a set period. When in sleep mode, all the modules in your car should draw less than 50 milliamps. With that small of a current draw, your car battery can maintain its starting power for around 4-6 weeks, depending on its condition. However, if your battery is old, that 50-millamp draw can drain it in about 2 weeks.

If any one of the computer modules in your car fails to enter low-power sleep mode, it can draw several amps, which can drain your battery overnight.

2) A stuck relay can drain a battery when its off

Carmakers use electro-mechanical relays to switch power to the fuel pump, radiator fans, blower motor fans, and even some computer systems etc. The relay includes an electromagnet that, when energized, pulls two electrical contacts together to send power to the motor. However, if the electrical contacts develop pitting, they can sometimes stick together even though the electro-magnet has shut off. That keeps the motor or computer running long after you’ve shut down the vehicle. That kind of power drain can kill your battery overnight.

This type of parasitic battery drain is a little easier to diagnose. You can sometimes get a problem relay to shut down simply by tapping it with the handle of a screwdriver.

3) A short or interference on the data bus can prevent a module from entering sleep mode.

Computer modules talk to each other digitally on the data bus. A shorted module or interference on the data bus can prevent a module from entering sleep mode, keeping the data bus active and draining the battery.

3) A stuck relay can drain a battery when your car is off

Carmakers use electro-mechanical relays to switch power to the fuel pump, radiator fans, blower motor fan, and even some computer systems, etc. The relay includes an electro magnet that, when energized, pulls two electrical contacts together to send power to the motor. However, if the electrical contacts develop pitting, they can sometimes stick together even though the electro-magnet has shut off. That keeps the motor or computer running long after you’ve shut down the vehicle.

This type of parasitic battery drain is a little easier to diagnose. You can sometimes get a problem relay to shut down simply by tapping it with the handle of a screwdriver.

4) A short-to-ground diode in an alternator can cause a battery to drain when the car is off

A diode is a one-way electrical valve. They fail most often in the open mode that prevent power from flowing. But sometimes, they can fail in the short-to-ground mode and that will quickly drain a car battery.

5) Battery plate shedding can drain your battery even when your car is off

Over time all battery lead plates shed plate material. The degraded

Short circuit caused by battery plate shedding

plate material collects at the bottom of the battery, and when it builds up, it can touch the good plates and cause them to short out and drain the battery. This is often what causes a battery that works fine when you park it at night to be completely dead the next morning.

For more information on what causes a battery to die quickly, see this article.

6) Dirty battery case

If the top of your battery is covered with battery acid or even water, you can create an electrical path from the positive post to the negative post. This can create a short that slowly drains your battery.

The fix is to clean the top of your battery case.

©, 2023 Rick Muscoplat