[shorod]

What you can try is to put your meter on the battery and monitor what the voltage dips to when the starter motor is engaged. If your meter has a "Min" setting that would be more accurate, but positioning it to where you can see it from inside the car would work or having a helper either read the meter or start the engine would work as well. If the battery voltage drops to something less than 10.2 volts when the starter is spinning the engine, you either have a severe load on the starter or the battery is weak. Since the Hyundai generally starts up fine, the load is probably not the issue.

After a 20 min drive, turning on the ignition long enough to roll down windows should not have drained the battery.

-Rod