Motor Vehicle Maintenance & Repair Stack Exchange is a question and answer site for mechanics and DIY enthusiast owners of cars, trucks, and motorcycles. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

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.)

share|improve this question
Not an answer, but a related question that doesn't show up as being related:… – Scott Mar 19 '14 at 2:26

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.

share|improve this answer
So, where is your source for this? How exactly do you come to the conclusion that the same old alternator uses more mechanical power when supplying a higher electrical load? – cnst Dec 8 '14 at 18:55
@cnst, are you asking if conservation of energy continues to apply? – Bob Cross Dec 8 '14 at 22:03

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.

share|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.