Whith only this amount of information and no codes there could be a emission problem.

First off, I think it could be low voltage of the system.

I would take a voltmeter reading of the battery engine off and running. If you do not have a dash mounted voltmeter you might want to get one anyway.

If you do not have access to a voltmeter, I would use the brightness of my headlights near darkness, shing on a wall. You want a nice brightness almost all the time or minor changes.

Have the lights on bright with the engine running at idle and then rev it up and see if they brighten up nicely stay that way the whole time.
Then turn on you heater fan and see if they slightly get dimmer but in general but stay bright.

If all stays the same or gets a lot dimmer after the heavier load, you might have a charging problem again.

Check cables especially the ground cable to the body of the car.

Pass that, we need codes, if you have no other symptoms.

Phil