The most sophisticated chargers are used in critical applications (e.g. military or aviation batteries). These heavy-duty automatic “intelligent charging” systems can be programmed with complex charging cycles specified by the battery manufacturer. The best are universal (i.e. can charge all battery types), and include automatic capacity testing and analyzing functions.
Since the Universal Serial Bus specification provides five-volt power, it is possible to use a USB cable to connect a device to a power supply. Products based on this approach include chargers for cellular phones, portable digital audio players, and tablet computers. They may be fully compliant USB peripheral devices or uncontrolled, simple chargers.
A power or battery bank is a portable device that can supply energy and power from its built-in battery, typically through a USB port.
Power banks have various sizes and typically contain 18650 battery cells. The smallest power banks have a single cell. Moderately sized ones for mobile phones usually have few cells in a parallel circuit, and large ones additionally in two series.
Power banks are popular for charging smaller battery-powered devices with USB ports such as mobile phones and tablet computers and can be used as a power supply for various USB-powered accessories such as lights, small fans and external digital camera battery chargers. They usually recharge with a USB power supply. More recent power banks use USB-C and may feature an additional USB-B micro port for backwards compatibility.
The power bank includes a control circuit that both regulates charging of the battery and converts the battery voltage to 5.0 volts for the USB port. Power banks may be able to detect a connection and power on automatically. If the current load is under a model-specific threshold for a specific duration, a power bank may power down automatically.
Charging state is typically indicated through four LED lamps for each quartal, whereas some higher-end models feature an exact percentage display.
Some power banks are able to deliver power wirelessly, some are equipped with an LED flashlight for casual near-distance illumination when necessary, and some have a pass-through charging feature which allows providing power through their USB ports while being charged themselves simultaneously.
Some larger power banks have DC connector (or barrel connector) for higher power demands such as laptop computers.
Battery cases are small power banks attached to the rear side of a mobile phone like a case. Power may be delivered through the USB charging ports, or wirelessly.
Battery cases also exist in the form of a camera grip accessory, as was for the Nokia Lumia 1020.
For mobile phones with removable rear cover, extended batteries exist. These are larger internal batteries attached with a dedicated, more spacious rear cover replacing the default one. A disadvantage is incompatibility with other phone cases while attached.
