jackrae:
Your battery bank probably has a converter unit contained within it to convert the battery voltage up to 5 volts. The 5 volt output may have its negative terminal directly coupled to the battery negative terminal (I used -ve to denote negative). Alternatively it may have the 5volt positive terminal directly connected to the battery positive terminal. (I used +ve to denote positive) A third alternative (unlikely) is that neither 5 volt terminal is directly connected to a battery terminal; ie the converter is an isolated one.If the negative terminals are common then your measurement will be easy as the micro measures positive voltages and you will be measuring a voltage ranged from 0 to +4.2 volts. If however the positive terminals are commoned then you won't be able to directly measure the battery voltage as it's actual value will be negative relative to the micro ground reference.
It's a common +ve situation, if I measure the positive terminal of the tester with positive terminal of the circuit I get 4.12V (fully charged)
Jiggy-Ninja:
Certainly not! Batteries have a fairly characteristic discharge curve that depends on their chemistry. Most of them will maintain a relatively constant voltage for most of their capacity, and then the voltage basically falls off a cliff towards the end. It is definitely not linear. Here's a generic discharge curve for rechargeable lithium batteries:
The legend with "C" numbers refers to different discharge currents. C ratings are used to scale the currents to the battery's capacity. 1C is the current necessary to dischage the battery's rated capacity in one hour. 2C is twice that current, 18C is 18x the 1C current, etc. So for a 2300 mAh battery, a current of 1C is 2300 mA.As you can see from the graph, for most of the discharge levels most of the battery's energy is gone by the time the terminal voltage hits 3.5V
The different chemistries will also have their own unique characteristics. Lithium batteries are the delicate little snowflakes of the battery world; overcharging or overdischarging can make them burst into flame. The Note 7 was recalled (twice!) because of battery issues. Any responsible use of a lithium battery will have some kind of protection circuit (either built into the battery or in the circuit) that will prevent the battery from being damaged.
Oh, I see I thought battery discharging cycle was linear, thanks for the info.. Now I've got somewhere at home (I just have to search) another power bank that has 2 cells but it's not working properly it discharges really fast (I don't know if it's a battery issue or a converter one). It was used to charge an iPhone no more than 5 times I think. I wanted to know if I could have those batteries connected in series to the battery of this power bank in order to increase it's capacity? Or do I need to measure something in order to see if they are compatible, before attempting on doing this.