What is the state of charge of electric vehicle battery?

The state of charge (SOC) is defined not only for an electric vehicle's (EV) battery but for any battery. It is the term used for indicating the amount of energy available in the battery at a point in time. SOC is expressed as a percentage and ranges between 0% and 100%.
In the case of EV, SOC gives the information to the user about how long the electric vehicle can be driven before it needs recharging (or replacement). It works in a similar way as the fuel gauge of a traditional internal combustion engine vehicle (ICEV).
For example, if the SOC is 100%, the EV can be driven for a distance indicated by the range of that particular EV specified by the manufacturer. On the other hand, if the SOC is 20%, then the battery should be charged. Generally, the ideal value of the minimum allowed SOC specified by the battery manufacturer is much higher than 20% while the maximum value of SOC is maintained below 100% for efficient working and long life of the battery.
In the case of an individual cell of the battery pack, the SOC is the ratio of the capacity of the cell at any point in time to the rated capacity. The capacity of the cell decreases with aging and the maximum capacity of an old cell can be much lower than the rated capacity of the bew cell.