I'm having this problem right now. Car has been sitting for awhile (maybe 3 years). I just replaced the alternator and now the Brake, Battery, and door open lights flicker on when the car it idling under 2000 rpm. I'm guessing it's the battery. The battery is a Walmart brand maintenance free battery bought in June 2021. It's sat dead out in the cold for a long time. Going to a real parts store soon to replace the battery.