A learning algorithm for controlling a battery voltage, which reduces an error of the battery voltage by using a power generation voltage of an alternator while a vehicle is driven includes steps of: calculating, by a processor, a first error voltage indicating a difference between the power generation voltage of the alternator and the battery voltage after the vehicle has been assembled; calculating, by the processor, a second error voltage indicated by updating the difference between the power generation voltage of the alternator and the battery voltage according to an output amount of the alternator and a driving time; and calculating, by the processor, a line-to-line voltage between the alternator and the battery by adding the first error voltage and the second error voltage. This learning algorithm is advantageous since there is no side effect in voltage control such as open-loop control and there is a low voltage control error.