Yeah, I carry a Di2 charger and use either my dynamo USB output or a small cache battery and charge my internal Di2 battery on the go. There's no need for external Di2 batteries.
There is a mod you can do to build the charger in somewhat so you just need standard USB too.
Dynamo charging is something I'm planning as well at some point, I've got all the bits but haven't set the bike up to run it yet... and I have the external battery already so will stick with it...
Yeah, I carry a Di2 charger and use either my dynamo USB output or a small cache battery and charge my internal Di2 battery on the go. There's no need for external Di2 batteries.
There is a mod you can do to build the charger in somewhat so you just need standard USB too.