Sometimes you don't want to be tethered to an AC cord when using your DSLR, or the battery simply doesn't have an external supplemental option. The folks at DIY Photography demonstrate how you can connect two batteries to make your camera last longer.
This project requires some wiring and soldering work, as well as taking apart a lithium-ion battery, so you will need to take care (standard disclaimer: know what you're doing or things could explode). The instructions, however, are thorough. Basically, you hack a "sacrificial DSLR battery" and connect it to an external high-capacity DSLR battery.
This gives you the ability to do long time lapses or otherwise shoot for longer than you normally would.
How to Make a DSLR Battery Run 4 Times Longer [DIY Photography]