There's plenty to laugh about when it comes to the 3DS Slide Pad Expansion peripheral. Battery life isn't among them.
Someone who got the peripheral in advance of its December 10 release date, and was so anxious for dual analogue controls that he tore the box open, says that the manual promises 480 hours of play time off one AAA battery.
Good thing too! See the screw in the above pic? The battery goes into the compartment that's sealed shut by that screw. With 480 hours between changes, you probably won't need to pack a screwdriver with you.
The pic also show that the device comes with a strap.