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 (5VDC) has led to the development of battery packs designed to provide power through a USB connection. Instead of plugging the USB charge/sync cable into a computer, you can plug it into a battery pack to charge your device.
Like the batteries in your mobile devices, these battery packs are rated for their storage capacity, which is displayed in milliamp-hours (mAh). A milliamp-hour is 1/1000th of an amp-hour, so 1000mAh = 1.0Ah.
Most modern smartphones have batteries that range in size from about 3000mAh to 5000mAh. For example, the Samsung Galaxy S21 uses a 4000mAh battery, while the iPhone 13 Pro Max uses a 4352mAh battery. Tablets and gaming consoles can have even larger batteries, with the iPad Pro 12.9-inch model featuring a battery capacity of over 1000mAh.
Logic suggests that a 5000mAh battery pack should fully charge a smartphone with a 5000mAh battery, but in practice, you might find that your phone's battery is only charged to about 70-75%. So where did the rest of the power go? Is the battery pack really smaller than its rated capacity?
No, the theoretical 5000mAh battery pack does indeed have a 5000mAh battery inside. The difference is due to factors such as voltage levels, heat, and even the LED on the unit, which can consume some of the actual charge that the battery pack can produce.
As a general rule of thumb, you can estimate that the charging capacity of any given battery pack is about 70% of the battery's rated rated capacity.