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What drains a car battery when the car is off?

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

Leaving your headlights or parking lights on all night will completely drain your car battery by the next morning. A short drive is not long enough to fully recharge your battery.

The best way to bring your battery back to full charge 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

Computer modules draw power when they’re running. When you shut the vehicle down, the modules are programmed to enter sleep mode after a set period of time. At that point, all the modules in your car should draw less than 50-milliamps. With that small 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.

3) A stuck relay

Electro-mechanical relays switch power to the fuel pump, radiator fans, blower motor fan, etc. The relay includes an electro magnet that pulls two electrical contacts together to send power to the motor. 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 easy to diagnose. You can sometimes get a problem relay to shut down simply by tapping it with the handle of a screwdriver.

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

Over time all battery lead plates shed plate material. That material collects at the bottom of the battery and 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 parked it at night but is completely dead the next morning.

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.

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, 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 reduce the life of the replacement battery.


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