This blog is about the Power Output of batteries.
Batteries output power when they are connected to a circuit. A battery that is not connected to a circuit provides no current and therefore outputs no power. However, once you have connected your battery to a circuit, you can determine power output by measuring the voltage drop across the load of the circuit. If you are familiar with the equations that relate power to voltage, current, and resistance, you will be able to navigate between these concepts easily.
Technically the standard USB port on your battery pack will fit any standard USB cable. However, the amount of power it can provide may vary.
The ports on most power banks are in parallel and so you get full current on either one only if there is one device connected. Whether the power is split evenly or not when you hook up two devices depends on how much power each draws.
If you want to know more about the Power Output of batteries, you can click here.