5V Powerbank - relationship between voltage and charge level

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?

Thanks

A power bank contains a boost converter that keeps output voltage at a steady 5volt until the battery is dead. No way of knowing battery voltage unless you attach a wire to the actual battery and bring it outside the case. Leo..

Which is a useful mod for such a powerbank, just make use to use a series resistor to prevent accidentally shorting the LiPo battery and starting a fire, and don't solder onto the cell directly(!)

OK. When I disassemble it, how can I measure battery level then? Its about 4.8 V now, powerbank LED shows 75% charge. What is the relationship between charge and voltage?

What is the relationship between charge and voltage?

The problem is that you are assuming that there is a relationship. Battery technologies differ and how they change while losing charge depends on chemistry. Voltage is a result of the chemistry... but many modern batteries will give you an almost flat discharge voltage until nearly depleted. just checking Voltage, is therefore, not an ideal 'linear' indicator.

But you can approximate. Google 18650 discharge curve to get the numbers you want. one example |500x250

NuclearShaman: OK. When I disassemble it, how can I measure battery level then? Its about 4.8 V now, powerbank LED shows 75% charge. What is the relationship between charge and voltage?

Did you even [u]read[/u] the first reply? If you're measuring the USB output, there isn't a relationship.

NuclearShaman:
OK. When I disassemble it, how can I measure battery level then? Its about 4.8 V now, powerbank LED shows 75% charge. What is the relationship between charge and voltage?

Measure the voltage of the battery directly (via the positive and negative poles of the battery inside, not the 5V output). You should see a voltage up to 4.2V when the battery inside is fully charged. As @vinceherman showed with his image example, the voltage vs remaining capacity is very much related to the current draw or load of the cell around the time of measurement.