Blinkling battery and seatbelt dash light. 2007 Hyundai Sonata 4 door, 6 cylin

I would consider it quite odd that both lights would flash at the same time, and otherwise act normally. This could indicate a problem in the CAN network or instrument cluster circuit. This will require testing with a GDS diagnostic tool at your Hyundai dealership, for equivalent tool elsewhere. Have you verified charging system voltage and amperage output when the light is coming on? Even if you test it conventionally, it's best to see what is happening while the problem is occurring.