It seems like every mobile device, from phones and tablets to mp3 players, uses a rechargeable battery. Most of these devices are designed to both synchronize data and charge the battery when plugged into a computer's USB port.
This use of standard USB connections and USB power levels (5 VDC) has spawned battery packs designed to provide power through a USB connection. Rather than plugging the USB charge/sync cable into a computer, you can plug it into a battery pack and charge your device.
Like the batteries in your mobile devices, these battery packs are rated for their storage capacity, which are displayed as a number of milliamp-hours, abbreviated as "mAh". A milliamps hour is 1/1000th of a Amp Hour, so a 1000mAh = 1.0Ah
Most phones have batteries that range in size from about 1000mAh. The Apple iPhone 4S uses a 1430mAh battery while the Samsung Galaxy S3 uses a 2100mAh battery. Tablets and game consoles can have even larger batteries, with the new iPad topping more than 11000mAh!
Logic says that a 2100mAh battery pack should fully charge a Galaxy S3 battery, but when you try it you find that your Galaxy's battery is only charged to about 70-7 5%. So where did the rest of the power go? Is the battery pack really smaller than its rated capacity?
No, our theoretical 2100mAh battery pack does indeed have a 2100mAh battery inside. The difference is due to the voltage levels involved, heat, and even the led on the unit will take some of the actual charge that a battery back up can produce.
So as a general rule of thumb, you can figure that the charging capacity of any given battery pack is about 70% of the battery's rated capacity.