I kind of hate portable battery banks. While they are of course useful, the typical li-ion power supply starts to degrade after about 3 years and you end up having to toss it (responsibly dispose of it) to buy a whole new one.

I love these Victagen 18650 chargers though, which allow you to both charge the batteries, and then use them as a power supply.

From a quick search on Amazon, it looks like they have a newer version that probably won't fit in this case, but I figured I'd still post it for anyone who has the same one I do :)