I have a 2009 Hyundai Accent. My battery light and seatbelt light comes on when I’m driving at low speeds (below 20mph) and my A/C is on. FYI my A/C is an after-market unit. Do you think my A/C could have been connected incorrectly and is draining my battery?
I’d more suspect that your alternator or voltage regulator is dying and is unable to provide proper voltage when the alternator is spinning too slow.
Maybe the belt is slipping. Check the belt tension.
When a car is bought from the factory with AC, it usually comes with heavy duty cooling and alternator. Adding it afterwards would put more load on the alternator and it maybe just can’t keep up at low speeds.