One AAA battery will hold you over for quite awhile with Nintendo's monstrous 3DS peripheral.
http://www.nintendoworldreport.com/news/28637
The Circle Pad Pro promises 480 hours of battery life with one AAA battery. The news comes directly from the device’s manual as the peripheral and Monster Hunter 3 (Tri) G were made available to a select few in Japan before the official release on December 10.
The Circle Pad Pro will be released in Europe coinciding with the release of Resident Evil: Revelations on January 27, 2012. Currently, there is no specific information for a North American release of the peripheral.
At first I figured the circle pad would just run off of the 3DS's battery itself, (guess not).It uses the IR port for communication. The 3DS wasn't designed with external electrical connections, so this was the best option.
What other option was there?Aside from wireless (which I'm not sure would have worked concurrently with Wi-Fi), they could have rigged up something using the microphone part of the headphone jack or even something using the SD card slot (though that would have been even more hackish than the IR port).