Does driving with your headlights on during the day shorten the life of your alternator or battery?
Daniel Becker, Patent Attorney and Early-stage Engineering (2013-present)
It is hard to say. Using any device consumes some of its lifespan, but many components decay with lack of use.
As to headlight bulbs themselves: My instinct is that headlights are something that have a lifespan roughly proportional to hours of use, so I would presume that the more often you drive with them on, the sooner they will need replacement. I seriously doubt that use during the day or night makes any difference, though.
As to alternators: Your car needs electricity to power many components other than the headlights. Headlights are probably among the lower drains on the electrical system. My instinct is that they are easily handled by the alternator.
As to battery: Batteries wear out because car batteries are typically designed to be continuously slightly discharged and slightly recharged while a vehicle is running. You could think of them as a sort of buffer for the generation of electrical energy by the alternator. Headlights are a continuous draw, though, and so they burn a steady amount of energy, whether the alternator is running or not. Certainly regularly sitting with the engine off, with headlights on, will deplete a battery rapidly. But I also would expect that headlights create a larger burden on an electrical system more than when at a steady roll, when an engine is able to better expend fractional energy toward accessory purposes.
An example of how I notice this frequently is that one of my cars is fairly old, and it struggles to lift its electric windows at a stop. To move them much more easily, I just rev the engine.
I do not know whether vehicles which are driven in stop and go traffic are more likely to wear out these systems more rapidly than cars which spend a greater amount of time at cruising speeds, but I bet that’s probably common.
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Daniel Becker, Patent Attorney and Intellectual Property Attorney 