CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Alternator mileage drain

T&R today said that headlights caused a slight gas mileage decrease. Isn’t the alternator belt driven, requiring the same amount of engine power regardless of output? There’s no clutch in there, is there?

No. When the alternator is producing more current it will put more drag on the engine.

Think of it this way an alternator (or generator) is really a motor working in reverse. The more power it puts out, the more energy it needs to consume. To get that energy it puts a drag on the rotation which ends up at the engine. Sort of like putting the brakes on.

The voltage will be about the same, but the amperage (or is that wattage?) will rise according to the power needed. The fuses are rated in amps but they’re all 12 volt fuses. When jump starting a car, don’t touch the alternator or you may get burned from the heat. It won’t get hot unless it is putting out more power. You can put out 15 volts all day without any real power. When you have to make power, it takes energy, which the engine provides.

Just stop by your local Museum of Science and get on the stationary bicycle that powers a household lightbulb. See if and how long you can pedal it to keep a 100 watt bulb lit.