Once this backup battery is fully charged it seems to last forever. It can charge my cell phone and tablet at the same time, 3 times and still have juice left on it. 3600 and 6000 mAh. Granted I don't start from a drained battery in either device but I go pretty low but this things carries a lot of power in a small device a little larger that my cell phone. I was going to buy another but I don't need another with this little powerhouse.