Α Li-on cell works at about 4, 2V, but in the applications where are used these cells such as EV, portable electronics, laptops, Power Banks etc. a much higher voltage than the nominal voltage is required.
That’s why the Battery Pack designers combine more than one cell in a series to form a higher voltage value battery. When the batteries are connected in series, the voltage increases. For example, when 4 cells Li-on 4.2 V are connected in series, the output voltage of the resulting battery will be 16.8 V.
Connecting multiple cells in a row is like putting a lot of horses on a chariot. Only if all the horses run at the same speed the chariot will be driven with maximum efficiency. Of the four horses if a horse runs slowly, then the other three must also reduce their speed, thus reducing efficiency and if a horse runs faster, it will eventually damage itself by pulling the load of the other three horses. Likewise, when four cells are connected in series, the voltage values of all elements should be equal for the operation of the battery pack at peak performance. The method of maintaining equality of trends of all elements is called Cell Balancing.