Science Questions

Is it free to charge mobiles while driving?

Mon, 14th Apr 2014

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Alan McNamara asked:

For some time I have been trying to find an answer to my question; if I charge my mobile devices at home it costs me money (power bills). If I recharge whilst driving, does this cost money?


Thank you


Alan McNamara




Mobile PhoneWell, for a start, it depends if he’s paying for the petrol because basically, that’s what it boils down to.  So, when you're driving along, any electrical item you run inside your car, is running off the alternator or the battery, and the battery is recharged using your petrol when you're driving, the alternator uses the petrol when you’re driving.  So, when you plug stuff in, there is more drag in a nutshell and your fuel consumption is affected accordingly.  However, a mobile phone costs about 50 p to charge over the entire course of the year, so I really wouldn’t worry about it.  If he wants to save on his electricity costs, he should probably look at things in his home like his plasma TV perhaps which might be 100 pounds over the course of the year, maybe something like a washer/dryer.  The dryer part of a washer/dryer is going to cost many dozens of pounds a year and stuff like that.  I think my favourite one is a microwave oven.  So, the clock on a microwave oven uses more electricity over the course of his lifetime than cooking food does.  So, if you want your microwave oven to cost the least amount of money, don’t use it as a clock.  Just turn it on when you want to use it.


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Charging the phone will mean the engine has to work slightly harder and that will mean you use more fuel. Bored chemist, Sun, 13th Apr 2014

You are talking about really trivial amounts of money your mobile phone charger would cost you at the most if left running for a week £0.50 as for charging during driving the most minor variation in driving technique would far exceed in cost the cost incurred in charging. syhprum, Sun, 13th Apr 2014

Yes, taking energy from the electrical system of the car means that the engine has to work harder.

Car (petrol) engines are about 20% efficient, whereas power stations are more like 30-45% efficient and the car fuel is more expensive than the fuel used in power stations.

So it's usually much cheaper to charge from a wall socket.

However, diesels are much more efficient, so if you charge it in a diesel engined car, it should be more similar in terms of energy efficiency; but there is tax on diesel, so it may still be more expensive.

However, as others have noted, the actual amount of energy used to charge a phone is very, very small; a very few watt-hours; whereas electricity is sold in kilowatt hours (which costs 20p or so); and the energy in petrol or diesel is very high, so, practically, it makes very little odds which you do. wolfekeeper, Sun, 13th Apr 2014

Most countries now have regulations on the minimum efficiency of chargers; these regulations typically require that chargers are at least 85% efficient while active, and consume < 1 Watt when inactive. This basically requires them to shut down when they aren't actively charging.

Regulations on car petrol consumption set a much lower target (after all, the long-term efficiency of your average motor vehicle is almost precisely 0.00%).

Mobile phones really are amazingly energy efficient (but much greater efficiency is possible in theory).
People have also tried charging mobile devices from a generator in their shoe (lose weight while while charging your phone), or from solar cells.... evan_au, Sun, 13th Apr 2014

"(after all, the long-term efficiency of your average motor vehicle is almost precisely 0.00%)."
Same ball park as the efficiency of a phone .
Of course, if you have a company car and someone else pays for the fuel... Bored chemist, Mon, 14th Apr 2014

I believe that you are guys are sarcastically referring to the fact that a car may well end back where it started with an empty tank; even so, the efficiency of a car is not zero in any normal sense at moving around, or charging your phone; it's about 20-30%; the fact that you got to go where you wanted, and were able to talk implies a non zero percentage. wolfekeeper, Mon, 14th Apr 2014

Perhaps you should just fork out the dough for a solar charger. CliffordK, Mon, 14th Apr 2014

Alan McNamara asked the Naked Scientists: If I charge my mobile devices at home it costs me money (power bills). If I recharge whilst driving, does this cost money? Thank you Alan McNamara What do you think? Alan McNamara, Tue, 15th Apr 2014

Efficiency is defined as: (energy output)/(energy input).

It is an axiom that energy can't be created or destroyed (only changed from one form to another), so energy output excludes:

Heat (the most common waste product) - unless the device is a room heater

Wind - unless the device is a fan

Light - unless the device is a torch or a TV

By this measure, a motor vehicle mostly produces heat in the engine & brakes; much of the remainder is dissipated in air turbulence. In a Physics sense, the "Energy Out" of a vehicle involves lifting mass against Earth's gravity - but when it returns to the same garage in the evening, the vertical rise is zero but the energy consumption is non-zero, so the average efficiency is zero.

In contrast, the function mobile phone is:

To take weak electromagnetic signals from the antenna, and turn them into more powerful sound signals in the earpiece, using power from the battery

To take weak sound signals from the microphone, and turn them into more powerful radio signals into the antenna, using power from the battery

If it is a smartphone, it will have a large display, which is intended to produce a moving pattern of coloured light using power from the battery

If it is a smartphone, it will play music and videos from onboard storage

All of these have a finite, non-zero efficiency.

Of course, the difficult question is about the long-term efficiency: What is the long-term efficiency of playing "Angry Birds"?

evan_au, Tue, 15th Apr 2014

Can I rephrase the question somewhat - I'm asking not to save money, but just out of curiosity. If I charge an item using the car's battery while the car is running, am I using waste energy? I'm thinking the car's battery probably is capable of outputting more power than the car actually needs (as long as the battery is healthy) - so when I plug into it, I'm not really taking anything from the car. Bored chemist suggests that it will make the engine work harder but I wonder if that's really true. I guess the question really is - does a car battery produce more power than the car needs while the car is running? Alison, Fri, 11th Mar 2016

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