How many times can I charge my iphone from my car (without ever turning it on) without my car battery dying?

I’m going camping soon, and I need to bring an iphone stocked with TV shows (complicated – please don’t judge). I’ll be gone for a week, and I know I can charge my phone from my car without turning it on. About how many times would it be safe to do this without killing the battery? How much power is needed to charge an iphone versus the power in an idle battery?

If all else fails, I can always just jump it right? (there will be lots of other cars around)