Problems with batteries and starters would probably top the list of questions here. I wonder why auto manufacturers don’t put OBD II diagnostic electronics into new car to test these systems on-board. Like they do for the fuel tank evap system, O2 sensor, cat, EGR, etc.
It seems like all they’d have to do is monitor the battery voltage, everything-off current, starter terminals voltage and current during cranking, and a function to load test the battery. Then the car could tell the driver when the battery was past it’s prime, or the starter is about to give out.
The OBD II system is already there right? And it already tests stuff more complicated than the battery and starter. I can’t imagine the electronics package to test these two functions as part of the on-board diagnostics would cost more than $100.