Can you give an example of one? It blows my mind that this is possible - I mean not the technology exactly, that makes sense, but just that something reasonably portable that I would carry around in case my phone dies can also have the level of utility of jumping my vehicle in a pinch.
The battery packs for jumping cars (at least the ones I've seen) are generally more of a size appropriate for putting in a trunk than a backpack or pocket.
I had one a while ago when road tripping in the US, served me a few times, it won't fit in your average jeans pocket but it easily fits in a jacket, you could fit 20+ in your glovebox
The future is pretty cool.