The Fatcat Power Powerbar 4200 Mah Travel Charger (Red) is a spare external battery with3x the amount of power as a regular battery.The PowerBar 4200 uses ultra-thin lithium polymer technology, matched with advanced charge control circuits. This battery has a slim profile packed with tons of power, 4200 mAh,typically more than twice the battery capacity of your smart phone. The 4200 has over-charging, over-discharging, over-current, and short circuit protection.