Has been stated online that the battery life on the Nintendo 3DS is only about 4-5 hours but can be increased to over 16 hours with a replacement 4500mAh battery, this is 5000mAh so even more.
Nintendo 3DS, N3DS - Li-ion Ultimate Long Life Extended Battery with cover - 5000mAh
Amazon Customer Review:
5.0 out of 5 stars "An essential addition", 29 July 2011
"This review is from: Nintendo 3DS, N3DS - Li-ion Ultimate Long Life Extended Battery with cover - 5000mAh
One of the few flaws with the 3DS is its Battery Life, but this Battery fixes that problem wonderfully. Admittedly it makes the 3DS thicker & heavier, but that is a small price to pay to enable it to function for extended periods away from a power source, which then enables full use of its features.
Highly recommended. "