There’s a much simpler solution: put a spare battery (or two, if you’re paranoid) in the boot. You’ve got the car anyway, so weight isn’t an issue.
Relatedly: I do a lot of back-country solo walking, of the sort where I can easily go for a week or two without encountering any humans.
A few years ago, my Dad bought me a flint and steel, intended as a foolproof backup firestarter. It works quite well; starts fires easily.
But I never, ever carry it when I’m out walking. Why? Because the flint and steel weighs more than half a dozen disposable lighters.
What I actually carry are a few cheap lighters (to provide redundancy in case of failure), kept in a waterproof container, plus a few waterproof matches (for the unlikely possibility of me simultaneously dropping all of my lighters into a puddle or something).
