Here’s a myth that no one has quite really been able to bust. And that’s saying a lot, because Motor Mythbusters, the spiritual successor to the classic Discovery Channel show, tried. They concluded that charging your phone does have an effect, and you can see their segment here.
“I reject your reality and substitute it with my own.”
Having watched this episode when it aired, I didn’t really feel like it was saturated with scientific accuracy. In other words, it was sloppy. There were too many other factors in play to really say for sure if charging your phone will drain your battery, whether you have an electric, hybrid, or gas car.
In order to really test this, you’d need a rig like what the EPA uses to test gas mileage. It would have to be totally repeatable, no variables. Why? Because the amount of current used by charging a phone really isn’t that much.
Since I don’t have that kind of rig either, all I can do is answer with science.
The basics
Here’s what you need to know.
- Charging a phone requires 5 volts at 2 amps, in other words 10 watts.
- It very rarely takes more than an hour to charge a phone.
- After your phone charges, it will signal your charger to step down the current until it’s about .1 watt.
- Gas cars have 12 volt batteries at about 500 amps, in other words 6 kilowatts. However, the battery can only supply that amount of current for about 10 minutes.
- Some electric cars also have 12 volt batteries with about 350 amps, in other words 4.2 kilowatts.
- Other electric cars just use the main battery and step down the voltage. A typical electric car battery can supply 60 kilowatts for an hour.
Question 1: Will charging your phone drain your gas car’s battery?
If your car is running, you do not need to worry about charging your phone. Yes, the power will be supplied by your car’s 12 volt battery. But, if the battery runs low, the car will engage the alternator and recharge the battery. It’s possible that you may see a 1mpg difference due to the drag on the engine from the alternator, but it’s within the margin of error.
If the car is turned off, and your car has USB ports that stay on after the car is off, is that a problem? I’m going to say no, based on what I said above. Yes, there will be some drain for the first hour. Then, the phone will stop drawing current and only draw a tiny bit over time. It’s not likely to cause any serious degradation of your battery.
Question 2: Will charging your phone drain your EV’s battery?
Here the answer is very similar, but we can look at things with more precision because it’s all electricity. The ultimate measure we use is “kilowatthours,” which is the same measure your home’s electric meter uses. A kilowatthour is exactly what it says. It’s using one kilowatt continuously for an hour, or the equivalent amount of energy in any time frame. If you use two kilowatts for 30 minutes, that’s one kWh.
As I said, there are two possibilities with an electric vehicle. If the car has a small, ancillary battery, it has a capacity of about 4.2 kilowatts for about 10 minutes, in other words .7 kilowatthours (4.2 x 10 / 6). The regular battery has a capacity of about 60 kilowatthours, sometimes more.
Since we know the phone will really only draw significant current for an hour, and that it’s using 10 watts (.01kW), we can say that its total draw is .01kWh. What this means is that somehow you would need to charge 70 phones at the same time to even drain the little battery. This is, on the face of it, stupid.
Question 2a: Will charging your phone have an effect on your EV’s range?
Motor Mythbusters says yes, but the math just doesn’t add up. A typical trip in a typical EV uses about .3kWh per mile. Some are better, some are worse. But it doesn’t matter. Your driving style has a lot to do with that number. Jackrabbit starts and high speeds can drive that number up to .9kWh per mile. Coasting and going downhill actually charges the battery, adding as much as .2kWh per mile. At any rate, you’re not going to notice a draw of .01kWh. Simple as that.
While we’re on the subject…
You can use the same math with virtually any other device that plugs into your car’s USB or accessory port. Unless you’re talking about a fairly large inverter, the draw on any device such as a cell booster or radar detector is also probably going to be about 10 watts. You can generally get this information from the manufacturer. If all they list is volts and amps, multiply one by the other to get watts.
Trick your car out with stuff from Solid Signal
Solid Signal doesn’t try to compete with auto parts stores, but we do have the best selection of cell phone signal boosters and chargers for your car, truck, or RV. Shop the great selection at our web site, or call us at 888-233-7563 with any questions. We actually answer the phone! We’re here during East Coast business hours. If it’s after hours, fill out the form below and we’ll get back to you, usually within one business day.
