Power bank

Power bank

A power bank is a portable device consisting of a battery, a charger to interface battery with charging power source and an output interface to provide desired output voltage. 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. 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 of 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. The circuit board can contain additional features such as over discharge protection, automatic shut off and charge level indication LEDs. 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.