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I don’t have much knowledge about cars but this is what I understand. Whenever the engine is on, the alternator will always be generating electricity to power all components while charging the battery, because it is connected to the engine via serpentine belt. Does this mean even if you have a low battery and power hungry accessories, it’ll still generate the same amount of energy as if you don’t have many accessories on, and a full battery? Or does the alternator somehow adjust itself for more power output if this happens? I am wondering this because a few times I’ve heard if you have more power hungry lights and accessories you get worse gas mileage, but the engine will rotate your alternator the same amount no matter what just depending on how you drive from what I’ve seen. Can anyone debunk or prove this?
 

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The alternator has a regulator built into it that gives it a range of charging capacity. It will usually charge at a bit higher voltage and amperage when the system demands it, but too much voltage actually damages electrical systems and too little means it has failed and needs to be replaced.
 

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Yes the alternator output is adjusted based on the load applied to it and how fast the engine is running. The rotational speed doesn't change to adjust the output, but the torque required to turn it varies. The regulation is done electronically by changing the voltage applied to the field windings. The regulation is crude and inefficient, but since you have a battery to smooth it out it's ok unless you connect unprotected sensitive circuits to it.

Power hungry is relative. A 100 watt laptop charger would take about .1 or .2 hp to run the alternator. To maintain 60 on level ground it takes something like 10 hp, depending on the car, so with the charger now you need to make 2% more power. Maybe your mpg goes from 30 to 29.4 on paper, but real world you probably won't notice.

Say you have 1000 watts of rally lights. That's a little over 1hp, say 2hp after inefficiencies are considered. You might notice that.

A 1000w sound system doesn't use 1000w continuously so it's not the same as lighting.
 
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