That feeling of smug you get with your sleek Macbook and its integrated battery may not be worth it anymore, as a security researcher has shown that Apple batteries are vulnerable to hacking. It's possible to brick the battery completely, or even worse, infect it with malicious code that not even a complete OS re-install can fix.
Laptop batteries are way smarter than they used to be. They're so smart, in fact, that they come with their own little processor with firmware on it that manages communications between the battery and computer to do things like control charging rates and send back information on how much juice is left. In the past, Apple has deployed hotfixes for malfunctioning batteries, and by analyzing those hotfixes, security researcher Charlie Miller was able to gain direct access to the battery firmware.
So what can you do with access to battery firmware? For one, you can instruct a battery to brick itself, neither discharging itself nor taking charge. Or, you can tell it to provide false information about charge levels to its computer. If you wanted to be really nasty, you could hide malware inside the battery firmware itself, meaning that a user's own battery could launch attacks against the rest of the computer, and no amount of virus scanning or re-installing things would help.
The good news is that it seems like it's not possible for a hacker to send instructions to the battery that will cause it to dramatically overheat and/or explode, but it sounds like patching the vulnerability will involve securing your battery against any future hotfixes from Apple, too. At least you can rest comfortably knowing that worst case, you'll just have to go out and buy a cheap new battery to replace the one that turned against you.
Oh wait, no you can't.