System-S Backup Battery Charger Extender for Asus Google Nexus 7 Insert four conventional 1,5V Mignon AA size batteries into the pack and have a portable power source Total running time with connected battery adapter will be extended for hours (depending on the used batteries up to 8 hours) Eliminates traveling with a ac adapter or charging cradle Charges your battery or provide backup power Lightweight , slim and compact size.