The state of charge (SOC) of a lithium battery can only be known by using a shunt-style battery monitor.

These battery monitors at a basic level measure the current to and from the battery bank to calculate the battery's SOC.

Solar controllers and other devices that display SOC use the battery voltage to determine SOC.

The issue with using voltage to determine SOC with a lithium battery is that a lithium battery state of charge is not a linear relationship to battery voltage.