A refresh charging method for an assembled battery constituted from a plurality of lead-acid storage batteries, configured to completely charge the assembled battery, the plurality of lead-acid storage batteries being used in a lower charged state than a fully-charged state when not undergoing refresh charging, the method comprising the steps of: performing a constant current charging mode of charging the assembled battery with a constant current until a voltage of the assembled battery reaches a predetermined set voltage; switching the charging mode from the constant current charging mode to a constant voltage charging mode; performing the constant voltage charging mode operable to charge the assembled battery with a constant voltage when it is detected that the voltage of the assembled battery has reached the predetermined voltage; characterized in stopping charging, in the constant voltage charging mode, when timer counting for a maximum timer period T m is completed or when it is detected that a charging current reaches a lower limit current value for the charging current lower than a predetermined threshold value So, wherein the timer counting for the maximum timer period T m is started after the charging current reaches the predetermined threshold value S 0; the maximum timer period T m is predetermined based on testing as a time period which can be estimated to be required to completely charge the assembled battery by continuing to charge the assembled battery even after the charging current has reached the predetermined threshold value S 0 on an assumption that each lead-acid storage battery in the assembled battery does not reach an end of life thereof; and the lower limit value for the charging current is predetermined based on testing as a current level at which it can be estimated that the lead-acid storage batteries are in a completely charged state, wherein in the constant voltage charging mode, n intermediate threshold values S 1 to S n which decrease stepwisely are set between the predetermined threshold value S 0 and the lower limit current value, n being an integer equal to or larger than one, and n additional timer periods T 1 to T n are set, counting of the n additional timer periods T 1 to T n being respectively started when the charging current reaches the n intermediate threshold values S 1 to S n; the n additional timer periods T 1 to T n are set to satisfy a relationship of T m > T 1 >... > T n as timer periods respectively estimated to be required for the lead-acid storage batteries to come into the completely-charged state when the charging is continued even after the charging current has reached the respective intermediate threshold values; and the charging is stopped when the counting of one of the maximum timer period T m and the n additional timer periods T 1 to T n is completed before the lower-limit current value is detected. performing a constant current charging mode of charging the assembled battery with a constant current until a voltage of the assembled battery reaches a predetermined set voltage; switching the charging mode from the constant current charging mode to a constant voltage charging mode; performing the constant voltage charging mode operable to charge the assembled battery with a constant voltage when it is detected that the voltage of the assembled battery has reached the predetermined voltage; characterized in stopping charging, in the constant voltage charging mode, when timer counting for a maximum timer period T m is completed or when it is detected that a charging current reaches a lower limit current value for the charging current lower than a predetermined threshold value So, wherein the timer counting for the maximum timer period T m is started after the charging current reaches the predetermined threshold value S 0; the maximum timer period T m is predetermined based on testing as a time period which can be estimated to be required to completely charge the assembled battery by continuing to charge the assembled battery even after the charging current has reached the predetermined threshold value S 0 on an assumption that each lead-acid storage battery in the assembled battery does not reach an end of life thereof; and the lower limit value for the charging current is predetermined based on testing as a current level at which it can be estimated that the lead-acid storage batteries are in a completely charged state, wherein in the constant voltage charging mode, n intermediate threshold values S 1 to S n which decrease stepwisely are set between the predetermined threshold value S 0 and the lower limit current value, n being an integer equal to or larger than one, and n additional timer periods T 1 to T n are set, counting of the n additional timer periods T 1 to T n being respectively started when the charging current reaches the n intermediate threshold values S 1 to S n; the n additional timer periods T 1 to T n are set to satisfy a relationship of T m > T 1 >... > T n as timer periods respectively estimated to be required for the lead-acid storage batteries to come into the completely-charged state when the charging is continued even after the charging current has reached the respective intermediate threshold values; and the charging is stopped when the counting of one of the maximum timer period T m and the n additional timer periods T 1 to T n is completed before the lower-limit current value is detected.