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What Can Drain a Car Battery When The Car Is Off? 5 Reasons


A car battery is the most important piece of equipment necessary for your vehicle to start and be driven. It sends power to the spark plugs from the starter motor that ignites the car’s fuel and gives other system power, such as air conditioning, radio, lights, etc.

However, you might wonder what can drain a car battery when the car is off, or you might already experience your car battery quickly draining overnight. This is especially frustrating if you need to attend urgent events and your vehicle is not working, and you have to deal with a battery failure.

If your car’s battery dies when the car is off for the first time, you might not worry as much. However, if it happens a couple of times, you might start wondering what went wrong with its electrical system, or perhaps damaged battery terminals, or it’s a battery discharge. Keep reading to know the answer and what you should do!

10 Reasons Your Car Battery Drains Even Your Car is Off

Having a drained car battery repeatedly can lead to other issues. Ideally, the car battery must fuel the engine to work once you start your vehicle. Then, the alternator will kick in and start to charge the car battery while you drive fully.

When you park your car and turn it off, the car battery must be full and ready to start the engine for your next time. However, if it is fully charged during the night and you cannot start the engine in the morning because it is drained, here are things that can drain your car battery overnight.

#1: Old Battery 

When was the last time you replaced your car battery? If you are still using old car batteries, there is a big chance that they will drain more quickly than usual.

Using a car battery for more than four years can cause draining when your car is off. An old battery already fails to hold a full charge, unlike before. Generally, this scenario requires car battery replacement. You can replace a car battery between even 3-5 years of usage.

#2: Electrical Glitches 

Different factors, such as faulty wiring, faulty fuse, and poor installation, can cause car and car battery electrical glitches. These glitches can lead to typical parasitic drains on the car battery. But they can also result in excessive drains when your car is off.

Parasitic drains are the energy supplied by the battery to particular car components to keep them running even after the car is turned off. These components include the clock, radio presets, and security alarm systems.

On the other hand, if the car battery comes with electrical issues, the normal parasitic drains are extended to the normally off car components. And this is how a car battery gets drained when your car is off.

#3: Poor Maintenance 

How often do you maintain your car battery? Do you keep it properly?

If you fail to ensure that the car battery is well-maintained and clean, it can result in battery draining when your car is off. Remember, proper maintenance can prevent dirt or acid accumulation at the top of the car battery.

When a car battery is not properly taken care of, it starts to leak the charge. These leaks grow until they cause massive drains, leaving the car battery with little charge. So, it cannot power the car engine effectively.

If you suspect a car battery charge is leaking, you can clean it with a cleaning solution. Mix 250 ml of warm water with one tablespoon of baking soda. Dip a clean cloth into the solution and wipe the battery. You can also use a soft brush to scrub the car battery with the solution.

If your car battery features removable caps, be extra careful in cleaning it, and make sure that the solution will not seep into the car battery to prevent any damage.

#4: Corroded Alternator Diode 

If your car battery dies when the car is off but is not old, you can inspect the condition of the alternator. Keep in mind that positive and negative terminals connected to your car battery can also jostle loose in the long run.

Your car’s alternator is designed to recharge the car battery and power the electrical systems, such as dashboard lights, ignition, lights, radio, etc. A corroded or defective alternator diode faultily continues charging the circuit even after you turn off the car. As a result, this can drain your car battery, and the car does not start.

#5: Ineffective Charging System 

Is your car’s charging system not charging the battery as it should? The reason can be a drained battery when your vehicle is off or the car battery draining while driving, resulting in a dead car battery.

Having an ineffective or faulty charging system can lead to corroded tensioners or loose battery connections. In other words, the car battery does not charge properly, and the output is below the 13.5-14.5 expected volts.

#6: Driver’s Error 

As mentioned above, you might not worry if your car battery drains overnight for the first time. And here is the reason.

In most cases, you might think that a big underlying issue has caused a battery drain after realizing that it is dead after the vehicle has been off throughout the night or off for several hours. However, it is not always the case.

The driver’s mistakes can also cause a drained car battery when the car is off. These errors include:

Leaving a cell phone or other electronic item plugged on and operated after shutting off the car.

The driver can also forget to close the car door fully or leave the interior lights when the vehicle is parked.

If you leave the headlights on after turning the car off, this can cause your battery to drain as well.

#7: Extreme Temperature Changes 

Extreme temperatures in car batteries refer to below 10°F and above 100°F. If you leave your car in extreme weather conditions for an extended period when it is off, it results in lead sulfate crystals buildup.

And this buildup causes the charge in the car battery to be drained. It also increases the battery’s charging time. Please note that lead sulfate buildup also damages the bar battery and reduces its lifespan.

#8: Frequent Short Trips 

Taking your car for too many drives can make the battery wear out. While it is convenient to use your car when going somewhere, even if it is a short distance, it can cause damage to your battery.

Your car battery will exert a lot of power when starting your car. If you shut off your vehicle before the alternator recharges, your car battery will continue to die or not last long.

#9 Parasitic Drain

A fully charged battery can be drained by a “parasitic draw.” You can perform a parasitic draw test by disconnecting the negative battery cable from your battery and using a multimeter to measure the current draw.

Find out if any of the following are not turned on:

  • Door lights
  • Glove box lights
  • Alarm system
  • Dome light or other interior light
  • External electronic devices

Once you have verified that none of the above-listed items is an issue, you can locate a battery charger to jump-start your vehicle. If your vehicle does not want to start, then most likely, you have a bad battery that needs to be replaced.

#10 Corroded Battery Terminals

Corrosion in your battery terminals can cause the battery to drain. When these are corroded, the battery itself does not receive enough power from the alternator to charge, resulting in a dead battery.

Your battery relies on your alternator to keep it charged and ready to go, but if the terminals are corroded, it will not be able to receive that charge properly. A bad alternator can also lead to a dead battery, as it will be unable to produce enough charge to keep the battery running.

 What Are Some Car Battery Drain Preventive Measures? 

After discovering the potential causes of having a drained car battery when the car is off, you might wonder how you can avoid them. Here are some valuable care tips to prevent car battery draining and other battery-related issues:

Take the car battery to the garage to perform regular maintenance checks.

Consider replacing the car battery if it is over five years old.

Consider investing in a new battery maintainer to keep the car battery well-charged and prevent overcharging.

Prevent corrosion and loose battery cables through proper cleaning and tightening of areas around the battery terminals.

Always keep the battery clean.

Before leaving the car, make sure all the electronic components and internal lights are all.

Make sure there is correct latching of the glove box, trunk, and door to avoid parasitic drain.

How to Test for Car Battery Draining 

Perhaps parasitic drain is the leading cause of car battery drain when the car is off. And you can use a digital multimeter to test for a parasitic drain. Here are some simple steps:

Connect the digital multimeter to the negative battery terminal to determine the culprit. Please make sure that the battery is fully charged before the test.

Remove the fuses one at a time while looking out for changes in the device’s reading.

Return the fuse if you do not notice any changes after removing it.

Lastly, remove the next fuse until a drop in the multimeter reading occurs.

Before replacing the battery, use a trickle charger or jumper cables to try to jump-start the battery. Let it sit for 10 minutes or so, then check the reading on the multimeter again.

When to Replace a Car Battery?

On average, the car battery’s lifespan or service life is between 3-5 years; you need to replace it after this period. Several signs indicate that you need a new car battery before that period arrives.

Modern vehicles that come with special battery packs that are located under the hood provide more efficient, secure, and long-lasting performance than regular car batteries.

It is essential to have your battery checked regularly in order to ensure it is working optimally and will not leave you stranded in the event of a breakdown.

How to Do a Quick Charging System Check

You must ensure that both the positive and negative cables are clean and correctly connected. Then, follow the steps below:

  • Set the voltmeter to 20 volts on the DC scale.
  • Start the car’s engine and run it at 2000 rpm.
  • Connect the meter leads across the car battery posts.
  • The meter should read between 13.5-14.5 volts.
  • Verify that the voltage regulator is not malfunctioning.
  • The positive battery post and the negative battery post need to be cleaned.

Your car relies on the battery, fuel pump, and alternator to provide the necessary power for it to start and run. A battery check or inspection is important to ensure that the battery has the correct amount of voltage and that the voltage regulator is working properly.

No matter what car model you are driving, keeping up with the maintenance of the battery is essential. Leaving any of the interior lights on can drain your car battery, so try not to forget to turn them off when you leave the vehicle.


Like other things in the world, you can have your car battery for extended periods; it has a given lifespan. And if you experience battery drain when your car is off, make sure to find the reason immediately.

After that, you can proceed with solving the dead battery and other related issues, such as having your battery power or alternator tested. Plus, do not forget to take preventive measures so that you can enjoy the car battery based on its lifespan.

Last Updated on: February 1, 2023

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