User blog comment:Plexioth- xD/Monster Hunter 3 Ultimate/@comment-74.12.4.199-20111205154756/@comment-3005793-20111206234507

That battery is not made by nintendo, or at least not the ones ive seen. Its made by Nyko, and doubles your battery life while also acting as a grip for the 3DS. Since it goes on the bottom of the 3ds, it wouldnt leave room for the circle pad extender. It also comes with another charging case.

http://www.nyko.com/products/product-detail/?name=Charge+Base+for+3DS

Theres also one from CTA, which seems to be pretty much like the stock charging dock with a battery in it.