What Drains a Car Battery? (5 Common Causes)

The main function of a car battery is to help start the engine. After that, the alternator takes over by converting mechanical energy into electrical energy for all the vehicle’s electrical components.

But the car battery needs to be functional or else this entire process doesn’t even start. A car battery should last for a good 2 to 5 years, depending on how much you drive your vehicle and the climate you are in.

However, if you notice that your car battery is losing too much power or going dead a lot sooner than it should be, then something is causing the car battery to be drained of its power too quickly.

Related: How Long Do Car Batteries Take to Charge?

Common Causes of a Car Battery Draining

There are lots of reasons for a car battery’s power to be drained. Below are five of the most common reasons that should help you figure out why your car battery is dying prematurely.

1) Bad Alternator

how long do alternators last

When people get electrical problems, they often don’t know whether it’s the car battery or the alternator that’s gone bad. If it’s the alternator, this can actually cause the battery to lose power and go bad soon afterward.

If the alternator is failing to deliver power to the vehicle’s electrical components, then the vehicle will depend on the battery to supply this power instead. Since car batteries don’t have enough power to satisfy all these components, the battery’s power will be drained quickly.

2) Old Battery

dead car battery

Alternators are responsible for recharging the car battery with power. But if the battery is old or too worn out, then it won’t be able to get recharged properly and hold power like it used to.

This means you’ll constantly have problems trying to start your vehicle with jump starts becoming a regular occurance. This is the case with any rechargeable battery. You probably notice your 3-year old iPhone doesn’t last as long on a charge as it used to on the day you bought it.

See Also: Car Battery Replacement Cost

3) Faulty Charging System

bad ground strap symptoms

It is possible to have a good battery and alternator, but to still have a problem when it comes to charging the battery. This problem lies within the charging system and it can cause power to be drained from your battery as the vehicle is being driven.

To make matters worse, the battery power will drain even more if you’re using things like headlights, air conditioner, or radio. These types of problems can be a huge headache diagnosing and should be left to electrical diagnostic experts.

4) Left Something On

car interior light

A lot of newer vehicles will automatically shut off its electrical components when you turn off the engine, get out of the vehicle, and then shut the door. However, if the door does not completely close or you didn’t completely close the trunk, then it could still leave the headlights or cabin lights on all night long.

This will drain the battery and then you’ll find that your car does not start up in the morning. Hopefully, your vehicle has features that will prevent this from occurring.

5) Electrical Problem

multimeter test vehicle wiring

If you turn your engine off and the vehicle’s electrical components continue to operate, this is called a parasitic drain. Sometimes this can be normal when it’s just powering the security system and clock of your vehicle after it’s turned off.

But if it’s powering other things like dashboard computer screens or headlights after the car is turned off, then you have some kind of electrical problem that is draining your battery.

A battery warning light will often illuminate on your dash, or in case of newer GM vehicles, a “Battery Saver Active” warning message.


6 thoughts on “What Drains a Car Battery? (5 Common Causes)”

  1. my 1998 CLK320 Mercedes Benz had check engine light on prior to that a new battery and Alternator been put in worked few days now battery drained again will not even turn key nor lock or unlock doors nothing no lights sound or nothing prior to check engine electronic light on it went off after new Alternator now back on and code was read for Bank2 Sensor 2 Downstream went bad so in the mist of alternator being put in Saturday crank fine now its Friday battery dead as a doorbell what can be draining the battery like that please someone help me

  2. Obviously I need someone who can diagnose this problem with sophisticated electronics. I have a 2004 Chevy Sil. 2500hd. I have discharged 3 new sets of batteries so then had a my alternator replaced. Now it has happened again. So it probably was not the alternator. We have automatic running boards and they have been repained recently and they are working irratically, but that could just be a symptom of the current problem. What else should we look at?

    • Electrical issues can be tricky. I would do a parasitic draw test to see if you can narrow down what is drawing power.

      If it were me, I would start testing at the automatic running boards since you said they were acting up.


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