The 3DS definitely has some issues with battery life, so Japanese peripheral maker Cyber Gadget is helping to ease the pain with this Gachapin x Mukku Battery Adapter. Coming out (in Japan, of course) September 9th, this little battery (which you fill with 4 AAs) plugs into your DS Lite, DSi, DSi XL, or 3DS and provides some spare power for it! It also has a really cute picture of Gachapin riding a little bicycle to charge up a lightbulb-shaped Mukku. It's probably more practical if you use rechargeable batteries or aren't opposed to carrying a pack of AAs around with you, but hey, it's got Gachapin on it.
It's not available yet, so I'm not sure where to import it. It's likely you'll see it on PlayAsia or Rakuten soon though!