[stickfigure]

When battery life is a concern, I carry around a small flashlight and a (very) short USB cable. The flashlight can both receive charge (from the wall) and send charge (to the phone) through its USB port.

Lots of high-end flashlights do this. Mine isn't much larger than the 21700 cell it contains. If you want to go smaller you can get a 1850-based flashlight and still almost double the capacity of your phone.

Plus, it's a flashlight.

[JohnFen]

But it doesn't really address the use case well. It won't fully charge the phone and it takes time to do the charging.

[stickfigure]

I don't understand. Why won't it fully charge the phone? And you can use the phone while it's plugged in. In all respects it gives you ~2X the number of useful hours at the cost of having to carry around a very small item.