All cells and batteries, being electrochemical sources of electrical potential, can only maintain that electrical potential for as long as the chemicals that create that potential exist inside the battery. As soon as the cell or battery is connected into the device it is designed to power, current immediately begins to flow though the device, and as a result, the chemicals inside the cell or battery which create that electrical potential are consumed. As the chemicals are consumed, the electrical potential reduces.
This is why the voltage of a battery changes during discharge. Unfortunately, with lithium based batteries there is no direct correlation between voltage and state-of-charge. This is because lithium batteries have different internal resistances depending on the state-of-charge, and whether or not the battery is being discharged or resting. The only way to accurately inform a user of a battery’s state of charge is to count the coulombs leaving or entering the battery or to measure the cell’s impedance and compare it against characterized data until the cell reaches top of charge or end of discharge conditions.
