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Will a Phone Car Charger Kill Your Car’s Battery?

Charger plug phone on car

After finding out how expensive a replacement car battery is, you probably want to do everything in your power to avoid that cost.

Does this mean that you have to unplug your phone during long road trips? Did you realize that the phone is charging due to your car’s battery? Since there are a lot of questions about this topic, I’ll go ahead and answer your questions once and for all.

Charging your phone in a running gas-powered car is no issue. Your car’s battery will be fine. If the car is turned off, the alternator is dead, or you’re just idling, you might wind up killing your car’s battery. In the case of an electric car, there is no alternator, so any electricity used is stolen from the car’s battery. The incoming voltage supply to a car’s charger and USB ports is typically 12 volts but is reduced to roughly 5 volts in order to safely charge devices.

In this piece, I’ll talk about whether or not a phone car charger will kill your car’s battery. In the end, I’ll give you some tips to help you charge your phone during a car ride.

What Goes into Charging a Phone?

There’s a lot of electricity going on when you look at charging a phone. Your wire that gets plugged into the wall has a corresponding value of “amps.”

This is a measure of current. What’s current? It more or less describes “how much” electricity can flow from the wall to your phone. The higher the amps, the larger the stream of electrons that can come through.

The phone’s battery, on the other hand, is only designed to accept a certain number of amps. Too many amps come with added heat, and we all know that adding heat to a battery shortens its life.

Understanding Where the Power Comes from in a Car

The available plugs in your car can easily charge up a phone. The only question is, where does the power come from? The car’s battery.

As long as your battery is alive and healthy, the charging ports in your car will charge your phone. There’s a part called an alternator that makes it work even better.

The alternator’s main job is to keep recharging your car’s battery so there’s enough juice. Without it, your car’s battery will deplete whenever it’s used and eventually die. Think of it like the charger that’s plugged into your phone.

The battery is plugged into all the electrical components in your car. As long as the battery is charged, it will provide the power that your car needs. As long as the car’s moving, the alternator is spinning and regenerating your battery.

It’s worth mentioning that an idling car’s battery doesn’t charge that fast. Without a load on the engine, the alternator doesn’t create a ton of reserve power.

Car battery in the engine bay

Comparing a Phone to an AC System

If you want a clearer picture, look at a big energy user in your car — the AC system. Your battery is fueling this taxing system all the time and it never complains.

If that’s the case, what’s the damage to a little phone plugged in? In gas-powered cars, there is no damage.

Your AC might be using upwards of 30 amps. Comparatively, your phone is almost always using less than 2.5 amps.

If you look at these side-by-side, you’ll realize how minimally impactful a charging phone is in the grand scheme of things.

Charging a Phone in an Electric Car

What about in a Tesla? As you probably know, a Tesla doesn’t have an alternator. The only time it’ll recharge its battery is when it’s plugged in, going downhill, or using some regenerative systems while you drive.

In this case, plugging in a phone absolutely can kill your car’s battery over enough time. It’s a dramatic answer because it will take forever to kill your Tesla battery just with a charging phone. After all, that same battery is used to move a multi-ton car super fast down a highway.

Tesla Model S Electric Vehicle charging

Will a Phone Car Charger Kill Your Car’s Battery?

In almost every case, a phone car charger will not kill your gas-powered car’s battery. In the case of an electric car, there is no alternator. This means that theoretically, charging a phone can kill your car’s battery.

Times When a Charging Phone Can Kill Your Car’s Battery

The only time when a charging phone can kill your car’s battery is when the alternator isn’t making enough juice. The two major examples are:

  • You’re idling for an extended period of time while charging
  • Your phone is plugged into a car that’s turned off

Ever forgotten to turn off your headlights overnight and wake up to a dead car? The lights are demanding electricity that your car’s battery can’t regenerate. Once the battery is out of power, it’s dead since the alternator isn’t bringing it back to life.

Tips to Charge Your Phone in the Car

Give me a second to outline some tips to charge your phone in the car. These will look to minimize the energy siphoned from your car’s battery and help you prevent using your car’s USB port altogether.

Use an External Battery

My favorite solution is an external battery. These little guys can get charged in your house and then carried around with you.

When your phone is getting low, you plug it directly into this external battery. There’s no need to use a wall outlet or your car’s ports to charge your phone anymore.

Most options are really small and carry a charge for a long time.

Smartphone wireless charging portable

Bring a Back-Up (Non-iPhone Solution)

If you don’t have an iPhone, see if you can easily pop the back off of your phone. If you do and it exposes your phone’s battery, this is great news.

You can simply buy a second battery for your phone. When the first one is dead, take it out and replace it with the second one. Don’t forget to charge the dead battery before this new one dies.

This is the preferred option for people who often travel for work and don’t know when their next charge will occur.

What About a Self-Charging Phone Case?

If you have an iPhone or don’t want to get a second battery, then you can always buy a self-charging phone case. This is an integrated battery right into your case.

You can charge your phone directly from the case. It’s a clever piece of technology. You just have to charge the case independently of the phone, but it works just like an external charger.

Wait Until You Arrive

This answer might be a no-brainer, but can you wait? If you can survive the car ride without charging your phone, you can tap into your destination’s power outlets and charge your phone then.

This answer is useful if you’re driving home or to a hotel.

Turn Off Your Phone to Charge Faster

If you need to charge your phone in the car, there’s a simple trick to use: turn off your phone. This helps your phone charge a lot faster since battery power won’t be wasted doing different tasks.

Smartphone car

Overcharging a Phone Battery is Never Good

When your phone hits 100%, you should disconnect it. Unless you have a smart charger plugged in, your phone will accept all the incoming juice.

As a result, your phone will overcharge, heat up, and hurt the battery life. Keep in mind that most smartphones nowadays will only draw a specific amount of current from the charger in order to preserve the battery life and avoid damaging the smartphone.

In addition, your car’s battery doesn’t know any better. It will keep tossing electricity to your phone and also deplete itself.

Underpowered or Overpowered Chargers Do More Harm

If you’re using a charger that plugs into one of those cigarette lighters in your car, you should know that 12 volts are constantly being supplied to the charging port ie cigarette lighter, however, the actual car charge adapter you use will usually step it down to 5 volts to safely charge any device you connect.

That’s typically way too much power for a simple phone charger. Alternatively, the designated USB ports built into your car might be underpowered.

In either case, it’s not great for your phone. It will cause undue harm to your phone and actually kill its battery quicker.

Whether the rate is too fast or too slow, your phone isn’t designed to accept the charge. The result is always damage to your battery.

Not Using the Charger? Unplug it

The simple act of having your charger plugged into your car can siphon away your car battery’s power. Whenever you’re not actively charging your phone, just disconnect the charger.

The impact might be pretty small, but it adds up over the hours of driving.

Car charge with phone charging 2

If You Drive an Older Car, Beware

Older cars with older batteries might not have the strength to constantly charge your phone. As I mentioned, there’s a certain level of draining that comes with charging your phone. If your battery isn’t at “full health,” it could be the straw that breaks the camel’s back.


Now you know about charging your phone in your car. In most cases, it won’t kill your car’s battery. I also reviewed some tips to help minimize the impact if you’re a little skeptical. For more car questions answered, explore my blog. Make sure you check out my page of recommended car products.

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Ernest Martynyuk

An automotive enthusiast who's been tinkering with vehicles since I was 15-years old. Repairing automotive electronics has been my main job for over a decade now and have a passion for everything technical regarding cars.

2 thoughts on “Will a Phone Car Charger Kill Your Car’s Battery?”

  1. Electrical engineer here. 12 volts has nothing to do with how much power the charger uses. 12 volts is simply the voltage your car runs at, like how usb runs off 5 volts, and your remotes at mostly 3.3 volts. If you were to make that point, you should rather make it about how the conversion from the cars 12 volts to your phones 5 volts wastes about %40 of the power (voltage step has nothing to do with power loss)

    • Thanks for the insightful information! I updated the article’s short summary at the top, as well as the bottom section highlighted in bold where I discuss the constant voltage supplied being reduced to charge a device. Most smartphones will draw a specific amount of current often times based on the charging behavior of a user in order to preserve the battery’s life span.


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