Maintenance Tips, Tricks And Hacks For Your Vehicle

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?

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.

Key Takeaway

  • A car battery is a rechargeable device, typically lead-acid type, that provides electrical energy to start the engine and power the electrical systems in a vehicle when the engine is not running.
  • A car battery can be drained while it is off due to factors like interior lights left on, faulty wiring, bad fuses, corroded alternator diode, electrical glitches, ineffective charging, and parasitic power loss from devices such as the clock, radio, and alarm system.
  • Signs of a dying car battery can include a “check engine” light on the dashboard, slow engine crank, ignition clicks, dim headlights, general electrical problems, difficulty starting the car, a swollen battery size, an odd smell from the battery, and poor performance in cold weather.

What Is a Car Battery?

A car battery is an essential component of an automobile’s electrical system. It’s a rechargeable battery that primarily supplies electrical energy to the vehicle.

The primary role of a car battery is to provide the electrical power necessary to start the engine. When you turn the ignition, the battery sends a burst of energy to the starter motor, which then turns the engine.

Once the car is running, the alternator takes over to power the vehicle’s systems and recharge the battery.

In addition to starting the engine, the battery also supplies electricity to the various electrical components in the car, such as the headlights, radio, air conditioning, and onboard computers when the engine isn’t running or when the alternator can’t supply enough power.

A car battery is typically a lead-acid type, consisting of six cells that produce a total of 12 volts. Over time, due to usage and chemical reactions within the battery, it will eventually need to be replaced.

In essence, a car battery plays a critical role in the vehicle’s performance and functionality, powering the start-up process and providing the necessary electricity for various systems.

What Drains A Car Battery While It Is Off?

What Drains A Car Battery While It Is Off
  • Self Discharge: A standard lead-acid battery naturally drains at 0.1V per month, even when not in use.
  • Continual Power Draw When Parked: Even when your car is off, your battery provides power to systems like the clock, radio, and alarm system, among others.
  • Opening Doors/Trunk: Each time you open the doors or trunk, the interior and door lights drain the battery.
  • Leaving Key Close to Car: If the key fob is too close to your car, it can continue to communicate with it, potentially draining the battery.
  • Frequent Short Drives: Short trips drain the battery as the alternator doesn’t have enough time to replace the charge used for starting the car.
  • Parasitic Drain: This refers to constant battery drains from headlights, dome lights, or other electrical systems.
  • Alternator Fault: If your alternator isn’t working correctly, it won’t charge your battery properly, making it hard to start your car.
  • Cold Weather: A battery can lose up to 35% performance at freezing temperatures and 50% below that.
  • Sulphation: If your battery falls below 12.4V, a chemical reaction called sulphation begins, degrading the battery and reducing its capacity.
  • Forgot to turn off your headlights: 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.

Signs of a Dying Car Battery

  • Difficulty Starting the Car: This is often the first sign. The engine may take more time to start than usual, or you might have to try several times before it finally starts.
  • Dimming Headlights and Dashboard Lights: If your headlights and dashboard lights are dimmer than usual, especially while you’re idling, this could indicate a weak battery.
  • Electrical Component Issues: Problems with electrical components such as power windows, windshield wipers, power seats, radio, or air conditioning could signal a dying battery.
  • Swollen Battery Case: If the battery case appears swollen or bloated, it could mean the battery is bad. This can happen due to excessive heat causing the battery to overcharge.
  • Corroded Connections: Look for blue or white corrosion around the battery terminals. Corrosion can interrupt the connection between the battery and the rest of the car.
  • Frequent Jump Starts: If you need to jump-start your car frequently, it’s a clear sign that your battery may be failing.
  • Old Age: A car battery typically lasts between three to five years. If it’s older than this, it may be nearing the end of its life.
  • Bad Smell: A rotten egg smell coming from the battery could indicate a leak, which is a sure sign of a bad battery.
  • Warning Light: Many vehicles have a dashboard warning light, usually shaped like a battery, that illuminates when the battery is not charging properly.
  • Slow Cranking: If your engine cranks slower than usual when trying to start the car, it could be a sign of a weak battery.

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 battery, you’re going to disconnect the negative from the battery and you’re going to leave it disconnected overnight.

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 battery itself. You should see a reading of 0.01 which is 10 milliamps.

How To Fix a Parasitic Battery Drain

Fixing a parasitic battery drain involves several steps to identify and resolve the issue. Here’s a general outline of what you’ll need to do:

1. Identify the Parasitic Drain: The first step is to confirm that a parasitic drain is causing the battery problem. You can do this by performing a parasitic draw test using a multimeter. Disconnect the negative battery cable and connect the multimeter between the negative battery post and the negative cable. The reading on the multimeter will indicate whether there’s a higher than normal current draw.

2. Locate the Source of the Drain: If a parasitic drain is confirmed, the next step is to locate which circuit is causing it. With the multimeter still connected, start pulling fuses one at a time and monitor the multimeter’s reading. When the reading drops significantly after pulling a fuse, you’ve identified the circuit that’s causing the drain.

3. Investigate the Problematic Circuit: Once you’ve identified the problematic circuit, you’ll need to investigate all the components on that circuit to determine the specific cause of the drain. This could involve inspecting lights, radios, sensors, or any other components powered by that circuit.

4. Repair or Replace the Problematic Component: Once the specific component causing the drain has been identified, it can be repaired or replaced as needed.

Remember, dealing with electrical systems can be complex and risky if you’re not familiar with them. If you’re uncomfortable performing these steps yourself, it’s best to bring your vehicle to a professional mechanic who can diagnose and fix the problem says AutoTrader.

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 battery may lead to fire and dangerous chemical leaks. While waiting at your disposal, keep your battery in a cool and dry place, away from kids and pets.

Take your old car battery to an auto parts retailer or hazardous waste collection site. Car batteries cannot be thrown into the trash because they contain lead acid. If you are unable to do this, get in touch with, Home Depot, or Auto Zone. They will be more than happy to help you with the disposal.


Q: How long does it take for a car battery to drain?

A: The time it takes for a car battery to drain completely depends on various factors such as the battery’s capacity, the age of the battery, and the power requirements of the components drawing power. In general, a fully charged battery can last several weeks without starting the car, but it can drain much faster if there is a parasitic draw or a malfunctioning electrical component.

Q: What is a parasitic draw?

A: A parasitic draw refers to the electrical power that is drawn from the car battery even when the engine is not running. It is usually caused by faulty electrical components, such as a malfunctioning alarm system, a sticking relay, a short circuit, or a faulty switch. A parasitic draw can drain the car battery over time if left undiagnosed and unresolved.

Q: How can I test for a parasitic draw?

A: To test for a parasitic draw, you will need a multimeter capable of measuring DC current. Start by turning off all electrical components in the car and disconnecting the negative battery cable. Connect the multimeter in series with the negative battery terminal and the negative battery cable. If the current reading is higher than the manufacturer’s specification (usually in milliamps), then you likely have a parasitic draw that needs to be identified and addressed.

Q: Can a faulty alternator drain a car battery?

A: Yes, a faulty alternator can drain a car battery. The alternator is responsible for charging the battery while the engine is running. If the alternator is not functioning properly, it may not be able to provide sufficient charge to the battery, causing it to drain over time. If your car battery frequently dies even after a jump-start, it is a good idea to have your alternator checked by a qualified mechanic.

Q: How long can a car battery be disconnected before it loses its charge?

A: A car battery can typically be disconnected for a few weeks without losing its charge significantly. However, if the battery is already weak or old, it may lose its charge more quickly. It is always a good idea to recharge or use a battery maintainer if you plan on disconnecting a car battery for an extended period of time.

Q: Can extreme temperatures affect car battery drain?

A: Yes, extreme temperatures can affect car battery drain. Cold weather can reduce the cranking power of a battery and make it harder to start the car, while hot weather can accelerate the chemical reactions inside the battery and lead to faster self-discharge. It is important to take proper care of your car battery and ensure it is properly insulated in extreme temperatures.

Q: Can a short circuit drain a car battery?

A: Yes, a short circuit in the wiring or an electrical component can cause a car battery to drain quickly. A short circuit creates a direct path for electrical current to flow, bypassing the intended circuit. This can result in excessive power draw from the battery, leading to rapid drainage. It is important to have any electrical issues in your car inspected and repaired by a professional to prevent further damage.

Q: Does jump-starting a car damage the battery?

A: Jump-starting a car should not damage the battery if done correctly. However, it is possible to cause damage if the jump-starting process is not performed properly. It is important to connect the jumper cables correctly, ensuring that the positive and negative terminals are properly aligned. Additionally, it is recommended to avoid repeatedly jump-starting a car without properly addressing the underlying issue, as this can put additional strain on the battery and potentially shorten its lifespan.

Q: How often should I replace my car battery?

A: The lifespan of a car battery can vary depending on factors such as usage, climate, and maintenance. On average, a car battery can last anywhere from 3 to 5 years. However, it is a good idea to have your battery tested regularly and replace it if it shows signs of weakness or if it is more than 3 years old. Additionally, extreme temperatures and frequent short trips can also shorten the lifespan of a car battery.

Q: How can I prevent my car battery from draining while it is off?

A: To prevent your car battery from draining while it is off, make sure to turn off all electrical components such as lights and the radio when the car is not in use. Keep an eye out for any warning lights or signs of electrical issues and have them addressed promptly. Regularly check the battery’s terminals for corrosion and clean them if necessary. If you plan on leaving your car unused for an extended period, consider disconnecting the battery or using a battery maintainer to keep it charged.

Conclusion and final thoughts

In conclusion, several factors can cause a car battery to drain while it’s off.

These can range from internal issues like a faulty sensor, a bad starter solenoid, or any other electrical component that might be stuck in the ‘on’ position and causing a parasitic drain.

External factors such as leaving headlights or interior lights on, or not fully closing the car doors or trunk can also cause battery drain.

Even the natural process of battery self-discharge can slowly deplete battery power over time. Diagnosing a battery drain issue can be complex and may require professional help.

Regular maintenance and inspection can help prevent such issues and ensure your vehicle’s electrical system remains in good health.



Vide Polowenski, Senior Mechanic

The information in this article is current and up-to-date in accordance with the latest mechanic SOPs.

Please take the time to leave a comment if this article has helped you in any way, you need additional help, or you have a suggestion.

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