While the nook battery does last a long time, there may be times when you wish you had extra power. You may have not know that these type of mobile devices existed, but there are actually usb battery packs that can charge themselves and your device via USB. So when you are near your computer or a USB charger, you charge your nook and the USB battery and then when your nook runs out of battery, you have the extra USB batter pack to extend your nook farther. It’s great for long trips where you don’t always have access to a computer or an outlet to charge your nook with.