What Drains A Car Battery While It Is Off?

Have you ever had issues with your car battery? It’s very frustrating when your car doesn’t start and you have places to be. So you ask yourself, what drains a car battery while it is off?

The most common things that drain a car battery while it is off are headlights on overnight, a parasitic draw (which all cars have), corroded battery terminals, extreme temperatures (both hot and cold), battery not being charged due to a faulty alternator, or damaged battery cells.

Let’s dive deep into the issue and see what are the common causes of drained battery and how to identify what is draining your battery.

What Drains A Car Battery While It Is Off?

Almost all electronics on your vehicle can drain your battery. And as you know there are many electric-powered features on your car. However, these devices should not completely drain your car battery. If that happens, there is something going on. Here are the six most common features that drain a car battery while it is off:

1. You forgot to turn the headlights off


Leaving headlights on while your car is off is one of the most common ways a car battery can drain. This is particularly true when drivers forget and leave their lights on overnight, unknowingly draining the battery.

In addition, though not as common, door-ajar warning lights activate if a door is still open after a driver shuts off their vehicle. The light is usually located near the instrument panel in many cars and while this may not sound like much of an issue, it actually causes an ongoing electrical draw on the car’s battery which eventually leads to depletion.

Thankfully, modern cars are equipped with multiple warning systems that alert you when either of these issues arises.

2. Parasitic draw (alarm, locks, clock, etc.)

A car battery can experience a parasitic draw even when the engine isn’t running. This occurs when an electrical system remains powered, such as a power trunk release, an alarm system, or a radio so it can be used immediately upon starting the car.

Even with all of these features turned off, there are still components that require power and will draw from the battery even when not in use. This is why you need to check and adjust any significant level of a parasitic draw on your vehicle’s battery lest you find yourself with a dead battery on your next start-up.

If you’re suspicious that a parasitic draw is impacting your car, one of the best ways to check is by using a digital multimeter. A digital multimeter, or DMM, will allow you to measure the various parameters in your car’s electrical system – such as voltage and current – with accuracy and precision.

To test for a parasitic draw, start the car and ensure all accessories are off, then set the multimeter to its lowest amperage setting and attach it in series between the negative battery terminal and any unused metal area on your vehicle.

Then observe the reading on your instrument while waiting 20-30 minutes – if there’s still an active current from the battery after that timelapse, then it’s safe to assume there’s a parasitic draw occurring in your car’s circuitry.

3. Corroded battery terminals

What Drains A Car Battery While It Is Off

One of the most common causes of a drained car battery while it is off is corroded battery terminals. When the metal parts at each end of a car battery become corroded over time, it can impair the flow of energy to different components, resulting in a weak or dead battery even when it isn’t being used.

Corrosion from moisture and other elements in the air can form quickly and spread rapidly, leading to these unwanted consequences. Fortunately, regular checks and maintenance done on your car battery will ensure that no corrosion builds up at the terminals and keeps your car running efficiently.

Cleaning corroded battery terminals is essential for optimal car performance. It may look daunting but in reality, it is a simple and straightforward task that requires basic tools found around the house. The process involves unclamping the terminals’ negative and positive ends from the battery and using a wire brush or sandpaper to scrape away corrosion from the terminals.

After cleaning the terminals, you should apply a thin film of petroleum jelly to prevent further corrosion. Lastly, be sure to have everything tightened securely before you start your vehicle again. With some time and patience, taking care of your car’s battery terminals can extend its life immensely.

4. Extreme temperatures (hot or cold)

What Drains A Car Battery While It Is Off

Extreme temperatures – either hot or cold – can have a major impact on car batteries. In hot climates, the heat can cause corrosion on the battery terminals and other electrical connections, rendering it ineffective. The same thing happens when temperatures drop: extreme cold can reduce the flow of electrons within the battery and prevent it from providing sufficient charge to power your vehicle.

In extreme heat, car battery cells are at high risk of failing. Warmer climates in particular make it difficult for car batteries to stay charged and run smoothly. Heat increases the rate of corrosion on the battery cells and can lead to an irreversible breakdown of the voltage that powers the vehicle. If a car’s battery gets too hot, it can cause permanent damage and require complete replacement.

To reduce the risk of damage, drivers need to pay attention to signs such as slow engine cranking or difficulty starting and get their car checked by a professional immediately.

In extremely cold temperatures, car battery cells suffer greatly. The cold reduces the amount of cranking power that the battery can generate, making it harder to start your car. It also decreases the life span of your battery, as its effectiveness and efficiency tend to drop off in cold temperatures.

What happens is that deposit buildup on the plates inside the battery causes electrical resistance to grow, resulting in decreased performance over time until eventually, you need to replace it altogether.

5. Improperly charged battery (most common due to faulty alternator)

What Drains A Car Battery While It Is Off

Alternators are an extremely important part of modern vehicles. They are the mechanism responsible for keeping the battery and electrical components of the car running. Alternators generate electricity by using power from a car’s crankshaft, which turns an outer rotor attached to a pulley connected to an inner rotor with a series of coils.

As the rotors spin in opposite directions, a magnetic field is created that interacts with the coils and creates an electric current. This direct current is then inverted into an alternating current, which can be used to power the vehicle. It is this alternating current that allows accessories like headlights, air-conditioning, and other electrical components in a car to function properly.

If a car alternator is not working properly, it can slowly drain the car battery over time even when the vehicle isn’t running. Other components such as interior lights, clocks, and radios may also contribute to a slowly draining battery if they are left on while the ignition is off.

6. Old battery that can’t hold a charge

One of the main causes of a drained car battery when it is off is an old battery that can’t hold a charge. This can happen when the battery has been in service for some time, whether due to regular wear and tear or other everyday driving factors.

As car batteries age, their performance and longevity will start to decline. Over time, a car battery will not be able to hold as much of a charge, meaning that cars may need to be jumpstarted more often. Additionally, older car batteries will take longer to recharge after being used. Eventually, it gets to the point of no return where an old car battery needs to be replaced in order for a car to function properly.

It’s true that many car batteries today are high-tech and last longer than older batteries, but eventually, all batteries become less efficient at holding a charge. The best way to prevent an old battery from draining while it is off is to keep track of its age, or if noticeable signs of deterioration emerge, replace it proactively right away.

How To Identify What Is Draining Your Battery?

I will show you the easiest way to troubleshoot a vehicle that has a battery that drains down overnight, or over a few days of not using the vehicle. Let’s get started.

1. Inspect the battery

The first thing you’re going to do is rule out the battery as being the cause of your problem. As batteries get older, the plates could start to short out, causing the battery to drain down when it sits for hours at a time. So, the thing to do is when you’re done driving the vehicle for the day, the alternator was running charging up the batteryOpens in a new tab.

Reconnect the negative battery terminal and try to start the vehicle. If the vehicle starts right up and you don’t have the problem with the battery being drained down anymore, then clearly there is something in the vehicle that is draining the battery. If you still have the same problem where the battery is not strong enough to turn the engine over, then definitely you need to replace your battery.

2. Check the voltage on the positive battery terminal

When the positive battery terminal is initially connected to the battery, you’re going to notice a spark. The reason for that is there’s a surge of current going into the vehicle charging up capacitors, and powering circuit boards, and usually that current is under 1 amp (usually 300 to 750 milliamp), and then after a set duration maybe around 15 seconds or 20 seconds, that current should drop to a level between 25 and 50 milliamps.

If the current being drained from the battery is well above 50 milliamps, then the battery can drain down over time from sitting.

When you do this test make sure that there’s no light on your hood. If there is a light on your hood, remove the bulb because that’s going to interfere with this test.

You need an inexpensive digital meter. They are around $5. Set the meter to DC current, and have it on 10 amps. Disconnect the positive terminal clamp and touch it with your RED plug from the digital meter. With your black plug touch the positive terminal of the car batteryOpens in a new tab.

New Car Battery Price

If you ran the tests that I told you above, and you came up with a conclusion that your battery is just old and needs replacement, here is how much it will cost you.

The price of lead-acid car batteries is from $50 to $300. You will find a decent car battery for $150.

The price of lithium car batteries is a little bit higher, starting from $200 to $400.

The labor for battery replacement is obviously $0 because you will do the work. Just connect the positive terminal with the positive clamp and the negative terminal with a negative clamp. Secure your battery with a bolt (if your model has a bolt).

Best Car Batteries

  1. Odyssey PC680 Battery: This battery has excellent efficiency. Has a strong and rugged construction. Can work in harsh environments. Has 3 times more lifespan than regular AGM batteries.
  2. XS Power D6500 Battery: Great product. I have used this one on my 2007 Hyundai Santa Fe. It’s leak-proof, with no external vents. Has a 3-year warranty.
  3. Optima 34/78 RedTop: Lifespan twice longer than lead-acid batteries. Loaded with a tightly coiled column of pure lead. Strong 5-second starting burst. 3-Year warranty.
  4. VMAX857 AGM Battery: Heavy-duty battery. Best bang for the buck. Very small and easily transportable.
  5. ACDelco 94RAGGM: Has about 80 AMP hours of energy and 800 cranking amps. Can work in harsh environments. Ideal for all types of vehicles.

How To Dispose of Your Old Car Battery

It is very important to properly dispose of your old car battery. Improperly disposing of your batteryOpens in a new tab.

Take your old car battery to an auto parts retailer or hazardous wasteOpens in a new tab.

Bob Semana

Hi there, I am a Mechanical Engineer that specializes in AC, Alternators, Batteries, Cooling systems, and Drive Train issues.

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