Cars generally (and specifically a Honda Accord 2002) are not very energy-efficient.

Given all the inefficiencies, is it "free" to plug in, say, a smartphone into a car to charge while I am driving?

By "free" I mean, if I don't plug anything in, would the energy that would've gone into my phone wasted anyway -- converted into heat, noise, or exhaust?

If it's not free, where specifically does the energy come from? Does the alternator put a higher physical load on the engine?

(When answering, please keep in mind that certainly I am aware that there's no "free energy". My definition of "free" mirrors the idea of regenerative braking, for example. Sure, that power came from your fuel to begin with, but it would've come from your fuel anyway -- you're just capturing what would've been vented out as heat in your brake pads.)


4 Answers 4


Given all the inefficiencies, is it "free" to plug in, say, a smartphone into a car to charge while I am driving?

Short answer: no (but it's so hard to measure the impact that it be hard for you to tell).

Remember, the alternator has to power everything connected to the electrical system. Charging the battery is a big load at first but drops off as the battery reaches a full charge. Your wonderfully beefy stereo also draws a bunch of power. Lights are another surprisingly large load: remember all those horribly inefficient incandescent bulbs that everyone is replacing in their houses? In the right circumstances, I can observe my engine idle dropping a bit using only the lights (if I go from full dark to high beams).

Worse, the alternator isn't particularly efficient itself. Let's look at the Wikipedia article for a summary of why:

Efficiency of automotive alternators is limited by fan cooling loss, bearing loss, iron loss, copper loss, and the voltage drop in the diode bridges. At partial load efficiency is between 50-62% depending on the size of alternator and varies with alternator speed.[6] This is similar to very small high-performance permanent magnet alternators, such as those used for bicycle lighting systems, which achieve an efficiency around 60%.

So, even before you plug in your phone, the alternator is already coping with a variety of much larger loads including its own internal inefficiencies. So, if you hook up a 2.1 amp charger to an alternator that is rated for 50-70 amps, yes, you are drawing more current. You are also putting more inductive load on the alternator which, therefore, puts more of a load on the entire mechanical system.

Are you going to be able to tell? Not without a careful experimental procedure.

If it's not free, where specifically does the energy come from? Does the alternator put a higher physical load on the engine?

Yes (but not nearly as high a load as actually moving the car). Again, using a wikipedia article as a starting point:

Alternators generate electricity using the same principle as DC generators, namely, when the magnetic field around a conductor changes, a current is induced in the conductor. Typically, a rotating magnet, called the rotor turns within a stationary set of conductors wound in coils on an iron core, called the stator. The field cuts across the conductors, generating an induced EMF (electromotive force), as the mechanical input causes the rotor to turn.

So, at an incredibly simplified level, you can think of an alternator as something similar to a water pump except that, instead of creating a water current, it's creating an electrical current.

This current then alternates direction very fast, which you'd never do with a water pump as it would cause cavitation. The analogy breaks down pretty fast but let's hand wave our way past that part....

Anything electrical connected to the system needs that current to flow. If you add more load to the electrical system, that will eventually translate into more mechanical work for the alternator to create that original current. As the engine turns the alternator, eventually this all results in the engine doing more work.

  • @supercat it seems like you are trying to start an open-ended discussion that is only loosely based on the original question. Please feel free to bring that discussion to the chat: chat.stackexchange.com/rooms/340/the-pitstop
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The alternator is there to charge your battery. It's the battery that provides power to the electrical components inside the cabin. A voltage regulator determines whether your battery is fully charged or not and will allow the alternator to charge it if not. So if you don't use the energy, the regulator will discard the energy coming from the alternator. Probably as heat energy.

The alternator is driven by a belt running off the engine, so there is parasitic drag that reduces the efficiency of the engine somewhat. That is why many racing cars have a crank-fired ignition system that eliminates the need for an alternator. They may still carry a battery to power the lights and/or management system. These batteries need to be charged before each race.


No, it's not free. It's just that compared to everything else going on in the car, the power to charge your phone is insignificant. You could ask a similar question: "where does the power come from to drive the speakers when I turn up the volume on the car stereo?". In both cases, the answer is: all other things being equal, the extra power comes from the alternator, which places extra load on the engine, which either burns a bit more fuel (maintaining the same RPM), or slows down a bit (thereby losing less idle power to all the things an engine loses power to when it idles).

This is sometimes observable. When a car is idling, such as at a stop light (auto trans in drive, brakes applied to hold it), or possibly in park (or neutral if a manual trans). Activate a turn signal and listen carefully to the pitch of the engine. You may notice it rise and fall very slightly, in time with the turn signal. This is because the extra power to illuminate all the turn signal bulbs is imposing an additional load on the alternator, which in turn is imposing an additional load on the engine. This demonstration might not work if the turn signals use LEDs instead of incandescent bulbs (they use less power, so the effect may be too small to notice).

Bottom line: "there is no free lunch". The power to charge your cell phone has to come from somewhere - ultimately a little more fuel would have been burned that would otherwise have been applied to something else.


It depends upon the vehicle and how regulation occurs.

A) It is "free" if the the regulator draws 100% of the capacity 100% of the time, and shunts the excess to ground. You are drawing from spilled energy.

This is very common on older motorcycles and can shorten the life of the stator windings. On the bright side it is free to run the headlight.

B) It is not free otherwise, there will be greater resistance under load. Your heater blower is a good load to put on the alternator to test this, but not AC, as your throttle may be nudged whilst the compressor clutch is engaged.

Some specialised alternators/generators have gearboxes, and some very modern systems have buck/boost/or Split-Pi systems that are much more efficient than traditional regulation and can help to reduce losses.

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