NOVELTY - The method involves determining primary impedance value for two primary frequencies and determining impedance value for secondary frequencies. The impedance curve is determined based on the determined impedance values, and is represented in coordinate system having an x ??axis and a y-axis. The threshold value is determined, which corresponds to a given x value. The value for the surfaces is calculated, which results from the impedance curve, the x-axis and the threshold value. The control signal is generated depending on the value for surface.    USE - Method for determining state of rechargeable battery such as lithium ion battery and for checking contact resistances in battery system used in motor car. Can also be used in lead-acid battery, nickel-cadmium battery, nickel-hydrogen battery, nickel-metal hydride battery, lithium polymer battery, lithium metal battery, lithium-manganese battery, lithium iron phosphate battery, lithium titanate battery, nickel-iron storage battery, sodium nickel chloride battery, high-temperature battery, silver-zinc accumulator, silicone accumulator, vanadium redox battery, or zinc-bromine battery.    ADVANTAGE - Since the value for the surfaces is calculated, which results from the impedance curve, the x-axis and the threshold value and the control signal is generated depending on the value for surface, the state of rechargeable battery can be determined and the contact resistances in battery system can be checked. The utility period of the lithium ion battery can be increased.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for device for determining state of rechargeable battery and for checking contact resistances in battery system.    DESCRIPTION OF DRAWING(S) - The drawings show the equivalent circuit diagram for lithium ion secondary battery and the apparatus for determining the remaining life of the battery.    Lithium ion battery (20)    Control device (28)    Shunt resistor (30)    Voltage dividing resistor (32,34)    Signal device (42)