We have already seen that a power bank is composed of an internal battery of a certain capacity (mAh) and nominal voltage (V) which is usually 3.7 volts (V). However, when charging a device with a power bank we should know that the USB output port of any power bank works at a standard voltage of 5V.
A current of 1Amp or 1000mA will circulate through it as 5V is the standard USB output. The voltage is monitored with a voltmeter for a determined number of hours according to the power bank capacity. If the power bank battery lasts for the same number of hours as listed in the capacity, then it is the actual capacity.
Power banks use a USB-C port to charge other devices, these ports have a voltage of 5V and not 3.7V. So, when the 3.7V is converted to 5V the capacity of the power bank drops. To calculate the exact capacity of a power bank with a 5V output, you can use this formula: Capacity with 5V= 3.7V x (Advertised Capacity) / 5V
In practice, your phone will get less out of your power bank than 20,000mAh. In general, your power bank can transfer around two-thirds (66%) of its own battery power to your smartphone, and there are two main reasons for this. Reason 1: Power banks output at 3.7 volts, while due to USB technical standards, smartphone batteries charge at 5 volts.
We see that our power bank stores 38.5 watt-hours (Wh) of energy that we can use to recharge the battery or power other devices. During the process of charging a device, the electronic circuit of a power bank raises the nominal voltage of the battery (e.g., 3.7V) to the voltage of the USB output port (5V standard voltage).
The batteries in smartphones and the batteries in power banks typically run at 3.7 volts. However, USB ports and charging circuits operate at 5 volts. Performing the same calculation on other power banks gives an answer between 3.6 and 3.8 volts.