It's not really a battery, just a small case for 4 AAA cells or rechargeables, with a convenient USB connector for output. The cable is a regular USB cable with a second plug for more power, for those external disks that require more than the standard 500 mW. You can use the set as pictured for one peripheral, or to power an unmodified USB hub for more.

I (and the guy who posted this picture, and some others around here) got these parts from a German mail-order store that specializes in gadgets and accessories, generally cheap, mostly tacky but sometimes useful and/or hard to find : http://www.pearl.de

There is also a French site (http://www.pearl.fr), but AFAIK nothing in English.