1. A calculation method causing a calculation apparatus configured to calculate a voltage of a battery to implement a computation capability, the calculation method comprising: referring to a detection value database configured to store a charge amount of a battery and a voltage of the battery, the voltage being generated when the battery is charged, while associating the charge amount of the battery with the voltage of the battery, and a function information database configured to store a function representing a relationship between the voltage and a charge amount of each of a plurality of active materials included in the battery; and performing a regression calculation on the voltage of the battery that is stored in the detection value database, with an amount of the active materials of the function stored in the function information database being set as a variable.