At first the battery seemed to hold fine but I noticed that if I left the car unused for more than 3–4 days (very rare since I use it daily), I could not start it anymore… Oops! Fortunately I have jumper cables in the trunk! It’s actually the iPhone that drains all the power. A few weeks ago I left for 5 days and as an experiment, I unplugged the iPhone but left the Arduino plugged. I could start the car just fine when I came back. I’m thinking of adding some kind of circuit breaker that will cut off power to the iPhone if the car battery gets too low!