When you plug your phone into your car to charge it up — especially when the car’s engine isn’t running — a feeling of dread can sneak into your mind. “Can my phone kill my car battery?” this voice whispers. The short answer is “yes.” The long answer, however, imparts some electronics smarts.

Most of today’s smartphone batteries use lithium polymer technology, and their capacity is measured in milliamp hours (mAh). The iPhone 6, for instance, has a capacity of 1,810mAh. According to Joshua Duffy, an electrical engineering manager with smartphone accessory company Scosche, the average smartphone battery is typically discharged — or used up — at a rate of .21 times the battery capacity. That means a 1,810mAh battery loses about 181mAh of charge over an hour of typical use.

At that rate, eventually your phone’s battery will run down and you’ll want to recharge it. So you pick up the phone and plug it into a random charger. At this moment, many people make a big mistake, says Duffy. “For every smartphone on the market, there are three main charging standards,” he says. “You should know what type of charger you need to purchase.”

First, there’s the Apple standard, which supports from 1 amp to 2.4 amps worth of charging. Amps are a measurement of electrical current. Think of a water pipe with different numbers describing the size of the opening. This pipe helps electrons flow from one part of the circuit (in this case, the power source, like the cigarette port in your car) to another (the battery in your phone). Samsung also has its own standard, says Duffy. The Korean device maker uses up to 2 amps for charing some of its products. And finally there’s also a generic USB standard which allows for up to 1.5 amp of charging.

There’s also the question of voltage. If amps represent the pipe size, volts act like the water pressure, pushing electrons along. The majority of smartphones use a standard operating voltage of 3.7-3.8 volts, but some will allow all the way up to 4.3 volts to maximize a battery’s capacity, says Duffy.

Still, he says, there’s only so much a lithium ion battery can do to charge faster. That’s because they power up in three phases. The first phase is the pre-charge, which occurs if you deplete a battery all the way to 0%. This is the slowest part of recharging your battery because the device had to shut itself completely off so the battery wouldn’t sustain any damage. If you begin charging your battery at this stage, it will only get a trickle of electrical power.

But once it reaches a certain threshold — it could be 3.1 or 3.0 volts — then the charging process switches to a constant current mode. “This is where you see the biggest difference between battery chargers,” says Duffy. If you have a good charger, it can fill your battery’s capacity very quickly, drawing in power at higher currents.

Constant current mode continues until your phone reaches about 70%, and then it switches into a constant voltage mode. At this stage, which runs from 70% to 100% on your battery meter, says Duffy, it doesn’t really matter much what kind of charger you’re using. “As long as you can maintain that voltage, it’s basically like topping off the tank,” he says.

Keeping lithium ion batteries well-charged can help extend their life, says Duffy. While it was recommended to drain older batteries (like nickel metal hydrid or nickel cadmium cells used in 2000-era cameras) to 0%, today’s batteries — because of the three charging phases — shouldn’t be allowed to dip too low.

And if used frequently, the fast-charging constant current phase can actually reduce your battery’s capacity more quickly. For instance, after 200 charge cycles (a cycle is the cumulative equivalent of bringing a battery from 0 to 100%, or charging your half-dead phone twice, for instance), your smartphone battery will only have 95% of its original capacity left.

“If someone charges their phone every day, and they keep that phone for two years, they’ve only got 80% of their initial capacity left,” says Duffy. “If you think of it like a car, it’s almost like your gas tank gets smaller every year.”

And speaking of cars, there’s still that pesky question to answer. Charging your phone via your car is “simply energy transfer from your car’s battery to your phone battery,” says Duffy. So, if you had a 100Ah car battery, and your phone’s battery was 1.8Ah (which is equal to 1800mAh), a completely dead handset would pull less than 2% of the charge out of your vehicle.

Depending on the circumstances, that could be enough to drain your car battery dead. That’s because car batteries suffer from the same discharge problems that smartphone batteries do, just on a much different level. If your car battery is old and only holds a 2% resting charge (which is unlikely, but possible), your phone could easily kill your car. What’s more likely, says Duffy, is that charging your tablet or laptop would drain your car battery first, since they have higher-capacity batteries, and would draw a higher percentage of your car battery’s reserved charge.

But don’t be alarmed — yet. Just leaving your phone plugged into your car doesn’t mean your battery will drain. Many cars require that the keys be in the ignition and turned to the “accessory” position for the battery to power the cigarette port. Other vehicles might have an always-on port, an option that automakers are increasingly adding to their models. Ultimately, with all the variables at play, there’s no universal answer to this question. After all, it’s situations like that that led the auto industry to coin the phrase “your mileage may vary.”

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