UK Car Repair Questions? Ask a Mechanic for Answers ASAP
Hi Im David, are you talking about headlights? interior lights?instrument panel lights?
how long have you had the vehicle? and did this just start happening or has it been going on for a while?
if you are sitting idle, your accessories are all at the lowest rpm. if you have a load on the engine, i.e. heater on, radio on, phone charger, and if the heat is selected to defrost, the ac compressor is actually cycling on and off to keep humidity out of the cabin, its going to pull a load on the alternator up and down, hence bright...dim...and yes a weak batter would be the culprit. if it over 4 years old, its definitely time for a new one.
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