Apparatus for Charging Battery and Method Thereof

It discloses a battery charging device and method. Battery charging device detects the state of charge of the sub for sensing the state of charge of the battery sub-battery sensor unit, the main battery is connected in parallel with the sub-battery, and receives the state of charge of the sub-battery from the sub battery sensor unit, the sub- based on main battery sensor and the calculated percentage values ​​for computing a ratio value of the resulting sum of the charge and the charge of the main battery of the sub-battery for the resulting sum of the total capacity and the total capacity of the main battery in battery battery generating a charge command, according to the generated battery-charging instruction, and a ECU which controls the charging of the sub battery and the main battery.