What is the Voltage of Li-ion cell?

Peoples ask me, what is the Voltage of your Li-ion cell? Honestly, this can not be answer strict always if they want the precise answer. Unless they only need some idea.

Voltage per cell is depending on your application, basically most Li-ion operate in the Voltage range 4.2V down to 3V. For light load application, the manufacturer will label it as 3.7V per cell, for heavier load application, manufacturer will label it as 3.6V per cell, because the chemical response is not fast enough and generated heat, according to the law of conservation of energy, power lose as heat will result in less potential generated.

Due to chemical different, some cell are still great to charge up to 4.35V and some can discharge down to 2.75V and yet still have acceptably stable molecular structure.

The resistance of the cell is low, typical in few decade of mili-Ohm range, contribute by the conductor use to connect the electrode plate and the thermal cut off switch or fuses inside the cell. Unlike normal dry cell, the resistance of the Li-ion cell maintain constant along the entire life of the cell. But the impedance of the cell do will change. It even change at different potential level, that is why impedance tracker bq20Z90 have to trace the impedance of the cell along the discharging Voltage.

Every charge and discharge cycle will destroy some group of the molecular structure of the electrode use, and the damage is irreversible, in chemistry we call this process as oxidation even though there may not have oxigen take place. For example Ferum (iron) will become Ferum Oxide if contact with water. Ferum Oxide molecule do not have strong grid among each others.

More damage in electrode result in rising of impedance in the cell. Therefore, the potential generated by the cell will reduce, resulting in a lower average Voltage.

3.6V, 3.7V or even 3.8V is just a reference. It unable to tell us the true story doesn't mean the labeled Voltage lie to us. This is why all the measuring equipment come in.

If the battery pack manufacturer do not label their product by nominal specification but real approximation, we can actually judge the performance of the battery pack by look at the Voltage labeled. Example if it label 10.8V(3.6x3), it usually last less than 2 hours. If it label 11.1V(3.7x3), it usually will last more than 2hours. Again, this may not be the problem of their battery, but it reflect that your notebook consume more power if the battery label 10.8V.