Jumper cables? Nah....there is a newer better solution.

Are they really "new"?

They've been around for a while, but USB charging is new.

In fact, I found one in my dumpster last year.

Opened it up and it had a somewhat common 12v battery like you find in a computer UPS or an emergency lighting fixture.
I'm pretty sure that's the only thing wrong with it, and I have access to those type of batteries, but...

...the one that's in there has threaded posts that accept nuts, where as what I have has either the "flag" or "stak-on" connectors or a hollow square that takes a sliding post.