[Alan McNamara]

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?

[Bored chemist]

No.
Charging the phone will mean the engine has to work slightly harder and that will mean you use more fuel.

[syhprum]

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.

[wolfekeeper]

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.

[evan_au]

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....

[Bored chemist]

"(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...

[wolfekeeper]

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.

[CliffordK]

Perhaps you should just fork out the dough for a solar charger.

[evan_au]

the long-term efficiency of your average motor vehicle is almost precisely 0.00%.
Same ball park as the efficiency of a phone.

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"?

[Alison]

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?