NOVELTY - The method involves detecting an operational data of a traction battery. A communication unit is adapted to transmit the parameter of the battery to the mobile working machine. A battery control device is adapted to adjust the output voltage and output power of the battery at the power terminals with start-up on a threshold level below the rated voltage and power rating. The output voltage and output power are switched (S5) to full value of the rated voltage and power rating, in a case of receiving (S4) a communication request over communication unit or in case of detecting fault.    USE - Method for controlling unloading operation of traction battery such as lithium ion battery, nickel cadmium battery and nickel metal hydride battery for electrical propelled mobile working machine such as industrial truck e.g. forklift (all claimed). Can also be used for controlling unloading operation of lead acid battery.    ADVANTAGE - The reliable communication of the communication unit of the traction battery can be achieved. The traction battery can be used even without using the communication unit. The loading operation of the traction battery can be achieved, even if the traction battery is shutdown and the mobile working machine is not controlled by the operator. The power consumption of the battery can be reduced. The vehicle can automatically indicate the operator to charge the traction battery, by reducing the acceleration and terminal velocity of the vehicle.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for traction battery for a mobile working machine.    DESCRIPTION OF DRAWING(S) - The drawing shows a flow diagram illustrating the process of controlling unloading operation of traction battery.    Step for adjusting output voltage at power terminals of traction battery to specific value (S2)    Step for determining whether communication request is received from vehicle tax system of mobile working machine (S3)    Step for receiving communication request over communication unit (S4)    Step for switching output voltage and output power to full value of rated voltage and power rating (S5)    Step for checking logical operational sequence for mobile working machine (S6)