I am powering my Arduino from standard 5V powerbank, and I want to measure its battery level by checking its voltage and sending it to arduino. I already made a circuit for it. Only thing I want to know is how is the voltage related to remaining charge in this case. For example, for 12 V battery it works like this: 12.73 V at 100% charge, 12.62 on 90% charge, 12.10 on 50% etc. How does this relationship work for 5V powerbank?