For 20 miles of driving, that's about 4 hours of charging at 120 VAC. I would set your timer to charge every day to 80% with a stop time of around 5 am (depending on your work time). I have a 8 mile commute and charge to 80% from 1am to 5am Monday, Wednesday, and Friday. I try to keep the battery around 5-7 bars, but am not super worried about it.
Reddy
Reddy