A method includes: estimating a first estimated time required for the battery to be charged from the state of charge estimation initial value to an upper limit of the state of charge estimation section; estimating a second estimated time required for the battery to be changed in temperature from the temperature estimation initial value to an upper limit of the temperature estimation section; determining a new state of charge estimation initial value, a new state of charge estimation section, and a new temperature estimation initial value and a new temperature estimation section based on a smaller estimated time between the first estimated time and the second estimated time, until an upper limit of an state of charge estimation section reaches a target state of charge and a smaller estimated time is a first estimated time; and accumulating every determined smaller estimated times to obtain an estimated remaining charging time.