System-s Backup Battery Charger Extender with Micro USB for HTC HD7 7 Mozart Desire HD Z Wildfire Trophy HD Mini Legend HD2 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.