A portable electronic smoking system comprising rechargeable primary and secondary devices, the secondary device being an electrically heated tobacco device, said secondary device comprising a heater configured to heat an aerosol-forming substrate, the primary device comprising a housing, said secondary device being configured to be received within the housing of the primary device during a recharging cycle, the primary device having a first battery and the secondary device having a second battery, wherein the capacity of the first battery is between five and forty times the capacity of the second battery, and wherein the primary and secondary devices are configured to recharge the second battery from the first battery wherein the primary device comprises: a pair of output terminals for connection to the second battery; a DC power source; a voltage regulator connected between the DC power source and to the output terminals for controlling a charging voltage; and a microprocessor coupled to the voltage regulator and to the output terminals, wherein the primary device and second battery are configured to be coupled together and to form a charging circuit, and wherein the microprocessor is configured to: control the voltage regulator to supply a first charging voltage; and only after the first charging voltage has reached a predetermined maximum charging voltage level of the second battery, determine an internal resistance of the charging circuit by measuring the current in the charging circuit at the first charging voltage and at a second charging voltage, wherein the second charging voltage is lower than the first charging voltage; and limit the first charging voltage supplied by the voltage regulator to a maximum first charging voltage based on the determined internal resistance and the predetermined maximum charging voltage level of the second battery. a pair of output terminals for connection to the second battery; a DC power source; a voltage regulator connected between the DC power source and to the output terminals for controlling a charging voltage; and a microprocessor coupled to the voltage regulator and to the output terminals, wherein the primary device and second battery are configured to be coupled together and to form a charging circuit, and wherein the microprocessor is configured to: control the voltage regulator to supply a first charging voltage; and only after the first charging voltage has reached a predetermined maximum charging voltage level of the second battery, determine an internal resistance of the charging circuit by measuring the current in the charging circuit at the first charging voltage and at a second charging voltage, wherein the second charging voltage is lower than the first charging voltage; and limit the first charging voltage supplied by the voltage regulator to a maximum first charging voltage based on the determined internal resistance and the predetermined maximum charging voltage level of the second battery. control the voltage regulator to supply a first charging voltage; and only after the first charging voltage has reached a predetermined maximum charging voltage level of the second battery, determine an internal resistance of the charging circuit by measuring the current in the charging circuit at the first charging voltage and at a second charging voltage, wherein the second charging voltage is lower than the first charging voltage; and limit the first charging voltage supplied by the voltage regulator to a maximum first charging voltage based on the determined internal resistance and the predetermined maximum charging voltage level of the second battery.