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My understanding is that a car’s engine drives an alternator, which generates electricity to charge the car’s battery, and that all electrical power is then taken from the battery.

If so, does this mean that using electricity has no effect on how much fuel the car uses, because the alternator is being driven at the same rate anyway?

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  • \$\begingroup\$ Check out some of the engine computers, when full engine power is demanded, some cars will cut the aircon and the alternator to prioritize engine power to the wheels. The aircon can take about 10 bhp... so makes a difference in acceleration. \$\endgroup\$
    – Solar Mike
    Jan 28, 2023 at 18:09
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    \$\begingroup\$ Back in the old days, I used to use A/C regenerative braking. Whenever I had lifted off the gas and was slowing, I would punch A/C to max. \$\endgroup\$ Jan 28, 2023 at 23:05
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    \$\begingroup\$ The title and the question body ask opposite yes/no questions. Now that there is an accepted answer that answers "No" to the body question, it is nonsensical for the title. ("Does X?" "No, X.") \$\endgroup\$
    – shoover
    Jan 29, 2023 at 16:08
  • \$\begingroup\$ Good point shoover, thanks. It’s helpful then that the body of the answer elaborates. \$\endgroup\$ Jan 29, 2023 at 19:37

4 Answers 4

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No. Using electricity from the alternator makes the car consume more fuel.

The battery is only a "buffer" for the electrical energy generated by the alternator, so we can ignore it for our purposes. In the end, all the energy comes from the alternator, regardless of whether it's temporarily stored in the battery or not.

The alternator might always spin at a constant rate; however, its mechanical resistance increases as more power is drawn from it. This means that it becomes harder for the engine to spin the alternator the more electrical power you use in your car. As a result, the engine has to produce more torque, and therefore consumes more fuel.

If it didn't consume more fuel as you drew more electrical energy from the alternator, the world's energy problems would be instantly solved: Just take a single car, let it idle, and draw infinite electrical power from it. Unfortunately, that's not how this works, and we can't extract more electrical energy from a fuel than the thermal energy contained in the fuel.

Let's take a 100 horsepower car, for example: Assuming the alternator can handle an arbitrary amount of power and is 100% efficient (which it's not), you could draw up to 100HP of electrical power from the car, which is about 73kW. When you do this, the engine is at 100% load, so you'd have to keep the gas pedal floored to prevent it from stalling, and it'll consume just as much fuel as when you drive on the highway with the pedal to the metal.

Alternators also don't just always run at full power; instead, they're regulated to only generate as much power as is needed by the connected loads at any given time (lights, radio, battery charging, etc). In particular, car alternators are regulated in such a way that their output voltage remains roughly constant to prevent over-charging or over-discharging the battery. This regulation is done by varying the current through the alternator's excitation coils, which changes the strength of its internal magnetic fields, and therefore the amount of mechanical power it converts into electrical power at any given RPM. Stronger fields mean that the alternator consumes more mechanical energy and consequently generates more electrical energy. If you want all of the details, here's the datasheet of an alternator regulator IC by Infineon.

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    \$\begingroup\$ Thank you for your answer. I understand now that the alternator resists the engine more when a greater load is demanded of it. And I wasn’t suggesting that we could get electricity for nothing! An analogy that my help is a water pump (the alternator) pumping water up a hill to a reservoir at the top of the hill (battery), and the reservoir having a waste overflow for when it’s full. My question was whether the pump keeps pumping at the same rate irrespective of how much water is removed (electricity) from the reservoir. \$\endgroup\$ Jan 28, 2023 at 14:02
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    \$\begingroup\$ This is totally correct, but it's not a self-evident conclusion from first principles that using electricity must consume more fuel. The alternator could always consume a constant amount of energy regardless of electrical load, dissipating any excess as heat. This does happen to some extent in a real alternator, but the mechanical load does increase with electrical load. \$\endgroup\$
    – TypeIA
    Jan 28, 2023 at 16:36
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    \$\begingroup\$ @010110001000 Most of the time, everything is powered by the alternator, including recharging the battery. But certain high powered items (such as electric winches on off-road vehicles) may take more current than the alternator can provide. In that case, the battery would also provide current to the electrics. Kirchhoff’s Current Law sorts it all out without any special electrical devices being needed to direct the current to where it's needed. \$\endgroup\$
    – Simon B
    Jan 28, 2023 at 20:59
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    \$\begingroup\$ To further develop @TypeIA's point -- it's not necessarily the case that fuel consumption must always go up; for example at idle, the ECU may be delivering excessive fuel to keep the catalytic converter hot; or for various reasons the engine might simply be in a particularly inefficient part of its curve, and additional load exchanges heat dissipation for work without affecting fuel flow. Engine efficiency is low enough ~20%?) that there's a lot of room for this sort of trick. As it happens, real engines aren't so clever, so the fuel consumption most likely will increase. \$\endgroup\$ Jan 29, 2023 at 0:37
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    \$\begingroup\$ Another question is, by how much; you might expect, for every kW drawn, divided by efficiency, an equivalent amount of fuel is drawn. But by the same argument above, the incremental cost may differ; maybe only half as much extra fuel is needed. So, it's a very hand-wavy thing, and really requires the engine's empirical performance curves to know precisely. \$\endgroup\$ Jan 29, 2023 at 0:40
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When there is a load on a generator, the magnetic field produced inside the generator resists rotation. More energy is needed to spin the generator shaft. Since the generator (in this case an alternator) is mechanically coupled to the drive pulley with a belt, it creates more resistance on the driveshaft's rotation and the engine needs to work harder to maintain the same speed. Thus, more fuel is needed.

The Ontario Science Center had a similar setup when I was young: a stationary bicycle connected to an alternator powering a light bulb. When you switched the (100 W) light bulb into the circuit, the pedals became almost impossible to turn at speed. You could build something similar: just try hand cranking a DC motor with its leads connected to nothing, connected to a load, and then connected to each other.

For more information, see this question: Does the electrical load on a generator affect its mechanical resistance?

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Using electricity in the car uses more fuel. The energy has to come from somewhere.

However, so much, and very variable, power (up to a few 10s of kW) goes to driving the car forward that an extra percent or three of that, consumed by the alternator, is not noticeable to the driver. It would be challenging to measure that small difference on a test track with a meter on the fuel input.

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  • \$\begingroup\$ I was expecting it to be impossible outright for a car's normal electronics. Maybe, just maybe you could measure the headlights but everything else lost in the noise. \$\endgroup\$
    – Joshua
    Jan 29, 2023 at 22:43
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In a modern car, using a moderate amount of electric power from the car could amount to zero increase of the fuel consumption.

How this is possible?

The car alternator is kept off most of the time. It is switched on only when the engine is used in "brake" mode, when the wheels transfer power to the engine and the fuel is cut off. Most of the kinetic energy dissipation happens in the engine, but the alternator can make use of as much as it can.

In short, in this very moment the mechanical energy is for free.

Of course, this works only if enough engine braking is done, e.g. in an intensive urban cycle and by an experienced driver. Otherwise, when the battery depletes past certain point, the alternator is engaged when the engine is providing mechanical power.

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