T-Mobile QCell MyTouch 4G Slide Black 3800mAh Extended Battery + Extended Battery Cover (NOT Compatible with MyTouch 3G / MyTouch 4G) 3800mAh Extended Battery! Best Li-Ion Battery in Market! Designed in Canada! Double Your Battery Life with this High Capacity Battery! Comes with Free Battery Cover! DYNAStop overcharge IC stopper built in! Voltage: 3.7V / Premium Quality and Best Material used!