“Sounds like” your alternator/voltage regulator has gone bad which in turn caused your battery to go bad because it isn’t getting charged. While your car is running, check across your battery poles with a voltmeter. You should read 14.6-7vdc while running. If it isn’t, your battery won’t charge and yes your lights will dim.

You’re right and yes, my 14.6v is high and only in a perfect world scenario. I stand corrected, thanks.

So I looked it up and Your Mechanic says, “The voltage should read a minimum of 13 volts. A good alternator should put out between **13.5-14.5 volts. Stress test the alternator - Place a load on the alternatorby turning on the headlights, the radio and the air conditioning. The voltage should remain high with these circuits on. If the voltage does not change when the engine is started, if it does not get below 13 volts, or if it charges above 15 volts then the alternator may be faulty.” https://www.yourmechanic.com/article...r-alternator_2

A charged battery should read 12.5 - 12.65vdc at rest, first thing in the morning. If an alternator can’t put out enough (12.5-12.65) voltage, it can’t run your vehicle and charge your battery at the same time. Since I suspect you drive way more than you idle, your alternator is able to compensate for it over the long haul and maintain a charge, but if your idle speed is right and no other power using items are turned on, your voltage shouldn’t drop below 12v.

But back to the original question; no the headlights shouldn’t dim if they’re all that’s on and a voltage check with the right readings in mind should be performed as a first step.