Battery pack having linear voltage profile, and SOC algorithm applying to the battery pack

A battery pack (100) is provide that comprises a rechargeable battery module (110), and a battery management system (120) for controlling charging/discharging of the battery module (110). The battery module (110) comprises a substantially linear charging and discharging voltage-time profile within at least part of the charging and discharging cycle of the battery module (110). The battery management system (120) is arranged to calculate a state of charge of the battery module (110) by using the linear charging/discharging characteristics of the battery module (110).