Aha, great to know thanks for the response.
One idea I can think of is to put the iPhone in airplane-mode at night to save battery or at least for most of the idle time, and when it gets disconnected from charge so it can last for those few days the car is parked. (I found AirplaneScheduler but I guess this sort of thing will need a jailbreak).
Anyway, one can always find things to improve on in such projects, but it’s definitely a success story.
Thanks for sharing once again!