Oh, get a few of these[0]. They are pretty flimsy, but cheap (I've seen them for sale on eBay and various sites like aliexpress for ~20 euro-ish before. They sure hold a charge; I've had 9 hours of constant use before (of course, that was with a highly optimised Gentoo setup and mostly just coding without a graphical environment. I get 6 hours on average).
I don't know what expresscard I'm using, but it's an expresscard/54 that has 3 USB 3.0 ports. It's a generic Chinese card, and it works fine. Though, in order to use it to boot from USB you need install libreboot (there might be another way, I'm not sure).
[0] http://www.amazon.com/CBD-Hi-Capacity-Battery-ThinkPad-7800m...