If you want more power pick up one of these bad boys.
http://www.newtrent....r-p/imp1000.htm
With it you can recharge your 3ds roughly 9 times from over from being dead. The battery pack is small and lightweight about the size of a wallet, they also have a 3ds adapter for it, it's for the dsi but it works just the same. (sold seprate)
it's completely universal, if you have a device that can be charged via USB this thing will charge it. Works great for traveling when you need extra backup battery life. I originally bought it to recharge my blackberry in places where it wasn't convienent to do so.
Works better and looks better than any bulky ugly contraption that extends the 3ds battery. any 3ds non extended batteries will only result in giving you an extra hour or two of gameplay.
I am not endorsing the product or website, this is just the one I have and use. Beware shipping is pretty steep to Canada. It's better to buy two and split the shipping cost with a friend.