What Can Drain a Car Battery When the Car Is Off?

Your car battery was full and functional the last time you parked your car. But now, the car won’t start. What could have drained the battery overnight?

Many things can drain your car battery even when the car is off. This article explains seven of them, plus their possible solutions.

7 Possible Causes of Battery Drain When a Car Is Off

Although your car can be off, a lot of things may cause its battery to drain. Here are seven of these causes that you should know.

a. Parasitic Drain

You remember turning off your car completely, not even a single door was left open, but the car battery was still dead by morning. That’s quite unusual, but it happens.

Electronic components like the radio, amplifier, power mirrors, and glove box lights could be slowly draining your battery.

This process is called a parasitic drain, but it shouldn’t go beyond 75 milliamps. A parasitic drain beyond the normal range becomes a problem that can adversely affect the performance and lifespan of your car battery.

b. Temperature Extremes

Extreme weather conditions can drain your car battery even when the car is off. Any temperature below 10 or above 100 degrees Fahrenheit doesn’t sit well with your car battery.

Your car shouldn’t stay turned off for long in such weather conditions since lead sulfate crystals will accumulate.

The crystals will drain whatever percentage was left on your battery. Charging it also becomes a problem since it will take too long before it can get fully charged.

c. Your Own Doing

Human beings are prone to errors, which is also one of the leading causes of car battery draining when the car is off.

You may have forgotten to do several routine checks before leaving your vehicle. Perhaps you didn’t turn off the headlights, or you didn’t close the trunk. When these happen, the battery will still be sending power to these components until it runs out of charge.

You’ll receive an alert if you leave any lights on in most cases, but this doesn’t always cover all the components.

d. Your Car Battery Is Worn Out

A car battery can last between 3-5 years, but this can vary depending on several factors, including the weather.

Any car battery beyond five years old is probably worn out, so it won’t hold a full charge like it used to before.

Over time, a car battery’s ability deteriorates until it can’t provide enough power needed to start your car. An old battery will discharge by itself when you park your car for a long time.

e. The Battery Cables Are Loose or Corroded

The two battery cables usually connect to the positive and negative terminals of the battery. When these cables are loose, it may become difficult for your car to start due to constant shaking, even with a fully charged battery.

Cable corrosion is also another problem you have to deal with to ensure your car battery remains effective. Look for the signs of corrosion around the battery terminals, such as blue or dirt-like substances.

f. Have You Checked on The Alternator?

Perhaps your car battery is still new, but you’ve noticed that it drains when the car is off. Then it’s about time you check on the state of the car’s alternator as it could be corroded or faulty, putting you in this mess.

The alternator’s work is to recharge your car battery and power other electrical parts, including the AC and radio. However, with a corroded alternator diode, the alternator will continue powering the circuit even after turning the car off. This fault will, in return, drain your battery.

One way to know if the alternator is faulty is when you can’t start the car on the first ignition attempt. Failure to ignite after several attempts could indicate a defective alternator that doesn’t supply enough electricity to the battery.

g. Having Many Short Drives

Having way too many short drives can shorten the lifespan of your car battery and make it unable to hold a charge after you’ve parked.

Your car uses a lot of power from the battery when starting, and so you need to drive it long enough for the alternator to recharge the used energy.

Remember, the alternator can only recharge a car battery when you’re driving. The short drives can only mean that you always shut your car before the alternator can finish its work.

Car Battery Drain Solutions

Now that you’ve seen the possible causes of your car battery drain, you probably need a lasting solution. Thankfully, there are things you can do to stop your car battery from draining when not in use.

How To Keep Car Battery From Dying When Not In Use

Below are vital care tips to prevent a dead car battery.

  • Before leaving your car, ensure all the internal lights and electronic components are off. Also, ensure correct latching of the doors, trunk, and glove box to prevent parasitic drain.
  •  Keep your car battery clean.
  • Avoid loose battery cables and corrosion by properly tightening and cleaning the areas around terminals.
  • Consider buying a battery maintainer to keep your car battery charged and also avoid overcharging.
  • Consider replacing your car battery if it’s above five years old.
  • Take your car battery to the garage for regular maintenance checks.

How To Test for Battery Drain

A parasitic drain is perhaps the leading cause of battery drain when your car is off. But how do you test for a parasitic drain? A digital multimeter could be of great help in this case.

Connecting a digital multimeter to the negative terminal of your car battery will help you find the culprit. But first, ensure that you fully charge the battery before you put it to this test.

Once connected, remove the fuses one after the other as you look out for the changes on the multimeter reading.

If you remove one and don’t notice any changes, return it and remove the next until a drop in the reading happens. That right there is the culprit.

When To Get a New Battery

Since a car battery’s average lifespan is between 3 to 5 years, it’s only fitting that you replace yours after this period.

That said, several signs might indicate you need a new battery, even before this period elapses. Below are some of the tell-tale signs.

  • The car struggles to start
  • You get dim headlights
  • There’s a foul odor every time you’re driving
  • You’ve jump-started it severally now
  • Your car won’t start even with the engine cranking

You may consider buying a car battery from these recommended brands for a change, Odyssey, EverStart, DieHard, Braille, and Deka.


What causes a new battery to drain?

Several things can make your new battery drain. These are internal lights left running, a faulty charging system, extreme weather, or a worn-out alternator.

How long should a car battery hold a charge?

A car battery in perfect condition can last for about two weeks without the need to charge. A new, fully charged battery can hold its charge for two months.

Can a bad alternator drain a battery when the car is off?

Yes. That’s because the alternator will continue to power the circuit, draining your car battery in return.

How much car battery drain is normal overnight?

A car battery should only lose up to a tenth of a volt overnight. Anything beyond that is not normal.

Will a car battery drain if disconnected?

Your car battery will not drain if you disconnect the terminals. You may decide to disconnect one or even both.

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Author: Dave Johnston

Dave Junior is a hands-on automotive technician with experience in performing service, diagnostics, and repairs on domestic and imported vehicles. He enjoys writing and sharing his knowledge far and wide.

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