Headlights dimming in Honda Civic

I drive a 2004 Honda Civic, and recently I’ve noticed that, when driving at night, my headlights will dim significantly when I step on the accelerator. Not only is it annoying, but actually is making it harder to drive at night.

I spoke with a mechanic who told me it would either be my battery or my alternator, but when he ran a test on them, found that the electric output was good on both. I’m pretty much at a loss, and really don’t have any knowledge when it comes to cars. Any suggestions?

It may be a SWAG but maybe you are having a problem with a ground wire. I would start by checking both ends of all wires attached to the battery and the wire that goes from the engine block to the car frame. Disconnect each end of each wire, make sure they are clean and tight. If anything looks bad, replace it, a wire can go bad internally.

I’ve seen alternators which had the right voltage output at idle (14 volts); but, when the engine rpm was raised, the voltage output would become very erratic.
If your mechanic checked the alternator at idle (or, low rpm), of course, it would appear ok. Take your car to an auto parts store and let them check the battery and alternator (for free). Be sure to tell the tester that there is a problem at higher engine rpm (3,000 rpm?), and be sure that s/he checks the alternator at the problem rpm.