What drains a car battery when the car is off?

What components drain a car battery when the car is off?

1) Leaving lights on is the most common cause of battery drain

This one is pretty obvious, but it’s also the most common. Leaving your headlights or parking lights on all night will completely drain your car battery by the next morning. If you jump start it and just drive it for a short drive, that’s not long enough to fully recharge your battery.

If you’re serious by bringing your battery back to full charge and eliminating the damage caused by a full discharge, the best way to do that is with a real battery charger that has a reconditioning mode. The reconditioning mode will dissolve the sulfation crystals that formed when it discharged.

2) Computer parasitic battery drain is the second most common cause of what drains a car battery when the car is 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 of time. 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.

What causes parasitic draw

A module that doesn’t enter sleep mode

A module might not enter sleep mode if it’s in a digital endless loop (locked up). In that case, the only way to get it reset is to remove the battery cables to reboot it.

Another way a module stays in power mode is if the system it controls has an open or short. If you leave a door open, for example, the body control module might not go into sleep mode for a long time, draining your battery in the process.

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

Modules talk to each other on the data bus. A shorted module or interference on the data bus can prevent a module from entering sleep mode.

Not shutting down your car completely

This happen more often on push-to-start vehicles that don’t use a key. In those vehicles, you push and hold the start button until the engine starts. But a quick push puts the car in to accessory mode, so you can listen to the radio or operate the windows without running the engine. If you don’t hold the start button in long enough to shut down the engine, you can accidentally put the vehicle into accessory mode and that can drain your battery when the car is off.

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 shorted-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 worked 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 a 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.

7) Extreme weather can drain your battery

A battery makes power through a chemical process. All chemical processes speed up at higher temperatures and slow down in cold temperatures. In fact, contrary to popular belief, heat is the #1 killer of car batteries. That’s one reason why carmakers often insulate the battery with a battery insulation cover. If you replace a car battery and don’t reinstall the insulating cover, you will actually reduce the life of the replacement battery.

©, 2023 Rick Muscoplat