Just get a power bank for that, way more useful than carrying extra batteries that only fit your phone.

That's a fair point, and I'm looking into these now that laptops are coming out that can be powered off USB. For me, though, I like the ability to drain a battery to zero, then hot-swap to a 100% battery instantly, then charge all batteries when I get back to the hostel.

It has been getting difficult with TSA, though.