It used to be OK to ask a stranger in a bar “do you have a Nokia charger?” and borrow said charger for a while to juice your phone. These days, though, you’ll mark yourself out as a Low-Charge Loser, the kind of person who goes to bed without plugging in his iPhone. Worse, you’re probably carrying more than just a phone. Are you really going to ask a stranger for adapters to charge your iPad and Kindle too?
You are not. What you need is a beefy backup battery. And at this time of year, it should be waterproof, too.