Nyko's Power Pak+, Hyperkin is planning to release a 3DS attachment this summer which promises to double the lifespan of the handheld. The battery, which attaches to the bottom of the device, also includes an improved grip and an LED indicator to show how much longer you can get your Nintendogs on before you need to recharge.
The Hyperkin PowerPlus battery attachment will ship sometime this summer for $19.99, the same price as Nyko's 3DS battery expander. You'll have to make your final choice based on form factor -- for instance, the Nyko battery seems to snap onto the back of the handheld, while the Hyperkin pack seems to give the 3DS a loving embrace. Only you can decide which one is right for you.