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I have a number of small devices that can be charged in my car (using a charger that plugs into the car's electrical system and is powered by the car's gasoline engine) or my home (using electricity supplied by my local grid.)

Does charging a phone or other device in the car change the amount of gasoline the car uses, or is the charging done using energy that would otherwise be wasted? If it does use gasoline, is the process of making electricity like this more or less efficient than whatever my provider uses (where I live, this is a mix of nuclear, fossil, and hydro-electric plants, supplemented with some insanely expensive wind and solar)?

Given the choice between charging while I'm driving home (say I'm at 20% battery) or leaving it until I get home and charging with a wall unit, which is cheaper for me or better for the planet? To what extent does this depend on how my home electricity is generated?

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Yes, it matters, a little, but it also depends on your location.

In your car, the process of converting fuel to movement is pretty inefficient (about 25% to 30%, according to this). That movement (kinetic energy, if you're a physicist) is then converted to electrical energy with an efficiency of 75% to 80%, giving you an overall efficiency of 18.75% to 24%. You might also consider the inefficiency of getting the fuel to the service station that you bought it from.

Using more electricity in your car means a higher physical resistance experienced by the engine when it tries to turn the core of the alternator. ie it's harder to turn the alternator when more electricity is being drawn from it, and thus more fuel is used to keep the vehicle at the same speed, unless the vehicle is idling, or overrunning the engine downhill.

In your home, electricity is generated with varying efficiencies depending on the generation that supplies it. Transmission losses in the US are apparently about 6.5% (ie 93.5% efficient). It looks like an average for generation efficiency would be around 39%. This last link also says the overall efficiency of getting power to the consumer is about 33%, which seems about right, given the previous two figures. The efficiency is somewhat higher than when you use electricity from your car.

It's better to charge your phone from home.

Of course, it very much depends on how your electricity for your home is generated. If you use an electricity retailer that leans towards renewable energy, it's going to be much better for the planet. It's clear there would be very few cases where the electricity grid generation was so inefficient that your car did a better job.

All that said, cellphones don't use much power. It's probably not worth worrying about too much. For most people, there are much bigger energy efficiency improvements that can be made through actions like improving home insulation and heating/cooling systems, reducing use of hot water, and analysis of home appliance electricity use.

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    Just to be clear, because you zoomed straight into efficiency, charging something in the car does cause the car to use more gasoline than it otherwise would? – Kate Gregory May 29 '14 at 21:46
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    @KateGregory: yes, charging causes the car to use more power. Unless it's idling, but in that case the efficiency is already zero (no distance/some energy=0). The transmission losses in the car are broadly similar to the conversion losses to electricity and then into chemical energy in your battery. Broadly meaning "between 5% and 50% losses" (there are lots of variables) – Móż May 29 '14 at 23:11
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    @KateGregory, Moz is correct. Using more electricity in your car means a higher physical resistance experienced by the engine when it tries to turn the core of the alternator. ie it's harder to turn the alternator when more electricity is being drawn from it, and thus more fuel is used to keep the vehicle at the same speed, unless idling. – Highly Irregular May 29 '14 at 23:29
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    @HighlyIrregular Well, even if the engine isn't running, driving downhill in gear will jump-start the engine. The reason there is always some consumption is that, even in idling, the engine needs fuel to keep the engine turning at around 900 r/min. When the car is driving downhill in gear, you don't need any fuel to maintain 900 r/min. Hence, you don't use any fuel. The "instantaneous consumption" displayed in newer "board computers" should confirm this. – Earthliŋ May 31 '14 at 0:52
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    @Earthliŋ, looks like you're correct where there's a smart engine computer to figure it out, which is likely true in most modern cars. forums.tdiclub.com/showthread.php?t=226400 With older engines, I understand fuel flows when the ignition is on and the engine is turning, even if running downhill. Perhaps a good test is to idle your car downhill in 1st gear with the engine on, and the engine off, and compare speeds. – Highly Irregular May 31 '14 at 9:01

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