A power bank or battery bank is a portable device that stores energy in its battery. Power banks are made in various sizes and typically based on lithium ion batteries. A power bank contains battery cells and a voltage converter circuitry. The internal DC-DC converter manages battery charging and converts the battery stack's voltage to the desired output voltage. The advertised capacity on the product in many instances is based on the capacity of the internal cells, however the theoretical mAh available to output depends on the output voltage. The conversion circuit has some energy losses, so the actual output is less than theoretical.[1][2] The theoretical mAh of a 3.7 V battery power bank with 5 V output is 74% of the battery mAh rating. The RavPower RP-PB41 with advertised capacity of 26,800 mAh that was evaluated in the journal has a theoretical capacity is 19,832 mAh, although the delivered capacity was 15,682 mAh, 78% of theoretical value. Authors attributed the difference to internal resistance in battery and converter losses.[2] The circuit board can contain additional features such as over discharge protection, automatic shut off and charge level indication LEDs.[3][4] 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.[5]
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.[6] Some larger power banks have DC connectors (or barrel connectors) for higher power demands such as laptop computers.
Battery cases
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,[7] or wirelessly.[8] Battery cases also exist in the form of a camera grip accessory, as was for the Nokia Lumia 1020.[9] 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.[10]
Rental/exchange
In some parts of the world, there are kiosk based power bank rental or subscription services. Customers pay for the use of power bank for a specified period of time and return the depleted power bank to the kiosk.[11] In one case with a brand called FuelRod, it was sold at an elevated price at various amusement parks with the understanding that they get a perk of free exchange at participating locations.[12] FuelRod moved to discontinue the free exchange in 2019 and resulted in a class-action lawsuit reaching a settlement that early adopters would be grandfathered to free exchange privileges.[13]
Per US Federal Aviation Administration regulations, power banks in the United States are not allowed in checked-in luggage. Power banks up to 100 Wh are allowed as carry-on and those 101 Wh to 160 Wh are allowed with airline approval.[14]
