Now that gasoline is practically being given away, I’m wondering about my situation WRT charging my electronics. For convenience, I usually charge my iPhone, iPod, and iPad while driving. I have two in a cig-lighter and one in an inverter, if that matters.
Given TNSTAAFL, and forgetting about convenience, is it cheaper to charge my electronics in my car or in my home? Has anyone here done the math?
(Although posited as a general question, there are a number of unknowns and I have no problem with the thread being moved)
My guess is that the power you are using to charge your electronics in your car is trivial compared to the amount of current your alternator is generating… so the cost is minimal. Someone will be by shortly to prove me wrong…
Well, if you only charge your stuff in the car when you would be driving it anyway, you may be saving a few kWh per year off of your house electric bill. Driving your car just to charge your stuff would just waste gas. Charging your stuff in the car with the engine off will run down the car’s battery, but how fast would depend on the current draw of your devices.
A gallon of gasoline contains 33.7 kWh of chemical potential energy. An average automobile engine is 20% efficient, although Toyota recently announce with pride that they’ve build an engine which is 38% efficient. Let’s assume 20% efficiency for your car, so you get 6.74 kWh of kinetic energy out of the engine. Now we’re going to use that kinetic energy to spin an alternator, turning it into electricity. Let’s assuming the alternator is 90% efficient. Now we’ve got 6.07 kWh of electricity. Then we have to run it through the inverter. Let’s assume the inverter is also 90% efficient. So you’re getting 5.46 kWh at the outlet. As of today, the average price of gasoline in the US is $2.477 per gallon. That’s 45.4 cents per kWh.
Here in Oregon, I pay about 9 cents per kWh. Other parts of the country it might cost anywhere between 10 and 20 cents. In Hawaii it’s 38 cents, by far the highest in the US.
So it’s a lot cheaper to plug your electronics into a wall outlet than in your car.
Charging in the car isn’t free: when the phone is charging, it’s drawing more current from the alternator, which puts more of a mechanical load on the crankshaft, so you need to burn more gasoline to go the same speed. But either way, the cost is pretty minimal.
My not-very-fancy phone has a 5.5 W-hr battery, and if I’m using it hard I charge it once a day (don’t usually need a full charge, but we’ll be pessimistic here). I’ll assume the charger from an outlet is maybe 90% efficient (again pessimistic).
So that means 6 W-hr of electric use per day, or, over a year about 2.2 kiloWatt-hours. I happen to have an electric bill in front of me, and the charge is around 30 cents per kW-hr.
So a year of charging your phone at home is maybe seventy cents.
Now for your car: Gasoline is, let’s pessimistically say $3.00/gallon, and it looks like that gallon has about 33 kW-hr, so the base cost of gasoline energy is nine cents per kW-hr. But your car needs to turn that gasoline energy into electrical energy, and that’s very inefficient – maybe 25% efficiency at turning the gasoline energy into turning the crankshaft, and I’ll make a WAG of 90% efficiency through the alternator and charger. Which gets us to 40 cents per kW-hr for the car. Alright, that means almost a dollar per year for charging your phone in the car.
Yeah, the iPad/iPod will be a little more, but I think we can agree that while you theoretically save money by chargin at home instead of the car, if your time is worth literally anything at all to you, the economically efficient thing to do is whatever saves you time.
Look not only at the cost of gas vs electricity but the total system. More electrical drain on the system could cause to premature alternator failure, also may very well shorten battery life if the battery does not always charge itself fully from start to stop at times.
Your cheapest solution may be to charge it at work, but no one really desires to live as a parasite, so I believe the cheapest would be to charge at home. However convenience plays a factor also.
There is never any surplus electricity, either it is used or stored in the battery or not produced. As the alternator produces more the alternator becomes harder to turn, taking more power from the engine.
It’s designed to maintain a target system voltage, typically about 14.5 volts, and it pushes out only the amount of current necessary to achieve that.
If some accessory gets switched on and starts pulling current, system voltage will drop a bit, and the alternator’s automatic on-board regulator automatically adjusts the current in the field windings to increase the magnetic field strength, so that the alternator puts out more current at its present RPM, restoring the system voltage to 14.5.
If that accessory then gets switched off, the alternator is now putting out too much current, so the regulator decreases the current in the field windings, and the alternator’s current output decreases, bringing system voltage back down to the 14.5-volt target.
Occasionally the regulator dies, and the field windings then receive no current, resulting in zero magnetic field. Now the alternator doesn’t generate any voltage or current. Your engine will continue to run, but only for as long as the battery can continue to power the electrical systems before it runs out of charge.
In some older cars, if you had the headlights on and the HVAC blower set to max (drawing a lot of current, then when the engine went to idle speed the lights would dim and the blower would slow down. This was because the field windings were receiving max current, and at idle RPM, the alternator still could not put out enough current to keep everybody happy, so system voltage flagged a bit.
The biggest battery I can find off the top of Google’s head for any portable device is an iPad battery at 42.5 Wh. Electricity prices vary from place to place around the country, but the average is somewhere in the vicinity of 10 cents per kWh. That means that charging up even the beefiest iPad from dead empty to completely full costs less than a half a cent worth of electric energy. Now, a car’s electrical system is drawing off of a more expensive fuel than your house does, and I don’t know off the top of my head what that cost is, but let’s say that it’s four times as much. Fully charging that device now costs less than four cents. Either way, the cost is not something you should bother worrying about.