Battery extenders can be a photographer's best friend, especially when shooting a long event, doing time-lapse photography or long-exposure photography.
Since not every camera brand supports external batteries or can be prohibitively expensive, it's a good thing that the folks at diyphotography.net have put together a comprehensive list of instructions for building your own DIY battery extenders. According to the blog, this is no mere vanity project; if you follow the instructions correctly, the DIY battery extenders can stretch out your DSLR's battery life five times over. The instructions call for a sacrificial battery, and can actually be dangerous if you don't know what you're actually doing. Remember ladies and gentlemen, when you're dealing with soldering and lithium ion batteries, things can explode if they're not done right.
Get it right though, and you may never need to use a wall outlet for long term camera use ever again.