a charger for charging a rechargeable battery . the charger comprising a circuit for producing a charging current having a variable level and a control circuit for controlling the charging current . during a first charging period the current is maintained at a level to rapidly charge the battery at a rate in amperes which is greater than the capacity of the battery in ampere - hours . the controller includes means for interrupting the charging current for a predetermined interruption interval . during the interruption interval , the charging current is maintained at a non - zero value . the controller also includes means for determining a resistance free voltage for the battery . during a subsequent charging period , the charging current is controlled in response to the value of the resistance free voltage .