We known that powerbank charging is possible for the Spark, which is great for portability. But one little hiccup is the battery has to be on the spark for this to work. This means you'd have to wait for an hour before you can use it again. If you can charge the battery externally, you can cut the waiting time down as you will be flying with a second battery while the first one charges. This shouldn't be hard to implement as we know the battery definitely charges...we just need a method of input..like a hub of some kind!

These have had some pretty good reviews. I missed on an amazon deal where they dropped the price to £65. I wont miss it again
Just plug your charging hub straight in and charge up all three in one go
**update**

Just found a bigger one for £120

an alternative is (5V 3A USB Battery Charger Converter for DJI Spark Drone). search for it on Amazon or ebay. it's very small and cheap but ridiculously slow. It'll probably take few hours to charge one battery using your powerbank.

I´m using that first one for charing 3 batteries at once with the wall-charger from the fly-more-package.
They can be charged at least one time.

So within 50 Minutes you have 3 fully charged batteries to fly again.