So you turn off the engine, take out your belongings from the car, lock the car, and walk away only to realize that you forgot to turn off the lights of the car! Let’s face it! We’ve all done it at some point in time!
But, how does leaving the lights on in the car affect your car battery?
Your car battery can die if you forget to turn off the car lights. Leaving the headlights switched on when your car is off can discharge your battery in usually 30 minutes to 4 hours whereas the cabin light can take longer to discharge the car battery (8 to 18 hours).
But, why exactly does the car battery discharge, can this have permanent damage to your car battery and how should you start a car when your battery is dead? We will cover all these topics and more in this article.
So, without further adieu, let’s get started!
What Happens If You Forget to Turn Off Your Car Lights?
Your car battery will discharge over time if you forget to turn off your car lights after you switched off your car. Eventually, your car battery will get completely drained if you don’t remember to turn off the lights in time and you may face trouble starting your car again.
Some cars have a built-in system that will sound an alarm if you remove the key from the key fob without turning the headlights off in the car. However, this alarm doesn’t sound off specifically for the cabin lights mounted inside the car.
Furthermore, you can also set up your car battery to automatically cut off the current to the lighting system (or other electric components) after 10 or 15 minutes once the key has been removed from the key fob in order to avoid inadvertent battery drainage.
However, you will need to pay a visit to the mechanic for this solution in most cases.
Why Does the Car Battery Die from Leaving the Lights On?
Your car battery may die if you forget to turn off the lights as the electric components draw power from your battery. When the car is turned off, the battery is unable to recharge itself as the alternator is also off. This leads to your car battery losing charge.
The only source of power for the car battery is usually the alternator of your car. The alternator charges your car battery only when the car is running. However, when the car engine is turned off, the alternator in the car is also turned off. This prevents the car battery from charging.
The combination of both events i.e. car battery unable to recharge and discharging simultaneously due to the lights being on drains your car battery. Eventually, your battery will run out of juice and it will be unable to start your car.
What’s also interesting to note is that variance in temperatures can also affect your car battery. For instance, if you leave the lights on during the winter season, chances are that your car battery may end up freezing faster as it discharges!
To understand how the car battery capacity affects its freezing possibility, check out my other article on “Can a Car Battery Freeze” where I cover this topic in great detail!
How Long Does It Take For a Car Battery to Die from Leaving the Lights On?
Generally speaking, your car battery can last in the range of 30 minutes to 18 hours before it dies from leaving lights on. The duration varies greatly based on your battery’s age, health, capacity, and the lights that were left on. Headlights tend to drain the battery much faster than cabin lights.
Most car batteries usually come with a 44 Ah to 48 Ah rating with a 12 V capacity. Assuming that you’ve left a couple of 10 W light bulbs on (cabin lights), it will take about 26 to 30 hours for your battery to completely discharge when it is fully charged.
Related Article – Can a Car Battery Die If Not Used? [Here’s What to Know!]
However, this is an ideal scenario when your battery is new and at full capacity. This may not always be the case.
Furthermore, the power required to crank your car engine is usually at least one-third of your car battery capacity. Thus, your car may refuse to start well before your battery dies completely! This means, even if your car battery may last 30 hours in an ideal scenario, your car will not start even after 18 to 20 hours (again, an ideal scenario).
It is difficult to pinpoint exactly how long will it take for your car battery to last from leaving the lights on as the factors can vary greatly from one car to another. Additionally, the power consumed by the headlights of the car is far more than the cabin lights.
Let’s explore this further in the next section.
Will the Car Battery Die if You Leave the Lights On Overnight?
In general, your car battery will die if you leave the headlights of the car on overnight. However, the battery may survive and your car may even start if you leave just the cabin lights (lights inside the car) on overnight.
As mentioned in the previous section, the power consumed by the bulbs in the cabin lights varies greatly from those used in the headlamps. After all, the goal of the cabin lights is to illuminate a small area within the car whereas the headlights need to be much brighter!
Let’s understand the range of your car battery in the two scenarios i.e. when you leave your headlights vs the cabin lights
The cabin lights or the lights used inside the car usually use 2 to 4 bulbs of 5 to 10 Watt each. These bulbs are enough to just about illuminate the inside of the car when switched on.
Your car battery can last about 8 to 18 hours if you forget to turn off the cabin lights of your car. This range can vary greatly depending on the age, health, and capacity of the car battery, and the number of bulbs used in the cabin lights.
Thus, even if you were to leave the car overnight with the cabin lights on, chances are that your battery will have enough power to crank up the engine the next morning and you will be able to charge your battery once again!
Your car battery can last from 30 minutes to 4 hours if you forget to accidentally turn off your car’s headlights. This range can vary based on age, health, and the capacity of your car battery.
The headlights of your car generally use a 50 W or 60 W bulb. Thus, the power consumed by the headlights is usually 5 to 6 times more than that of the internal lights of the car. This number can be lower if your car headlights are equipped with LED lamps. Even so, the difference in power consumption is marginal.
Check out this video that showcases an experiment on how long it takes for the battery to die with the lights on –
The experiment done in the video is using the headlights (instead of the cabin lights) on a fully charged battery. This is the key reason for the battery to last a little over 2.5 hours instead of much longer.
Does Leaving the Car Lights On Damage Your Car Battery Permanently?
If your car battery died from leaving the lights on, you might be wondering if this has done permanent damage to your car battery and whether you need a new car battery!
In most cases, the car battery does not get damaged permanently from leaving the car lights on accidentally. However, if your battery has aged or if the battery has experienced a deep discharge in the process, it may fail to charge again and will need to be replaced.
One thing to note is that you might be tempted to think that your battery has gone bad if your car fails to start from leaving the lights. However, the reality may be that the battery simply needs to be charged. Although a car battery can die completely if it gets over-discharged, it is usually a rare occurrence.
Will the Car Battery Recharge if It Dies from Leaving the Lights On?
Your car battery should usually recharge just the way it usually does even if it has discharged from leaving the lights on when your car was off. Try to start the car first to recharge the car battery.
If you are able to start the car, leave the car idle for 30-45 minutes. The alternator will charge the battery in the process. You can also take your car out for a drive. However, make sure that you go for a long drive and not turn off the engine for at least 30 minutes.
This will be the easiest and the most convenient way to charge the car battery.
Sometimes, however, it may be a couple of days before you realize that you left the lights on your car switched on. This can drain the car battery beyond an acceptable limit and affect the life of the battery. It is possible that your car battery may even refuse to recharge!
In such a scenario, you are left with no other choice but to replace the car battery!
How to Start the Car if the Battery is Dead?
If your car battery has discharged such that you are unable to start your car, there are still a few things you can do. As long as your battery has not been over-discharged, the following techniques should work –
- Jump-start your car using jumper cables – If you have access to another person’s car who is generous enough to let you jump-start your car, then you can easily plug your car battery into their car battery using a set of jumper cables [on Amazon].
In fact, I recommend buying this NOCO Boost Plus Battery Starter Box [on Amazon]. It comes with a power bank and a set of jumper cables attached. It is slightly more expensive than a simple jumper cable but the best part of this jumper set is that you don’t need to depend on any other car battery to jump-start your car!
- Push-start your car – If your car won’t start due to a dead battery and you don’t have access to a jumper cable set, you can use some muscle power to crank your car!
You will need 2 or 3 friends who can push your car. Before they start pushing, put the car in the first gear and release the handbrake. Press and hold the foot paddle for your clutch and ask your friends to start pushing the car. Once the car reaches certain momentum, release the clutch. This will start the car engine.
- Charge your battery outside your car if at home – If your car has battery has been dead from your car sitting at home for a long time, one of the best ways to revive your car battery is to use charge them!
You can easily buy a car battery tender like this one [on Amazon] to connect with your car battery and charge the car. A battery tender will ensure that your car battery gets fully charged and ready to go when you are. You don’t need to remove the battery out of your car with this battery charger.
- Call for road-side assistance – If none of these are options for you and your car battery died in the middle of the road while you were driving (which is entirely possible as it has happened to me once!), the best option would be to call for roadside assistance offered by your car servicing company.
Credit to Sara Kurfeß on Unsplash for the featured image of this article