Secondary-battery charging system and method and battery pack

A charging system includes battery pack and charger. During the charge of secondary battery, charge control unit controls charger so that the charger performs constant-current charge at a first charge current, and, when at least one of the following conditions is satisfied, switches the first charge current to a second charge current lower than the first charge current and continues the constant-current charge. The conditions include the condition that the SOC of secondary battery arrives at a threshold SOC value and the condition that the inter-terminal voltage of secondary battery arrives at a threshold inter-terminal voltage. In response to the degree of degradation of secondary battery, the charge control unit drops at least one of the threshold SOC value and the threshold inter-terminal voltage.