NOVELTY - The device has gas-permeable pressure compensating opening (6) formed in gas-tight housing (5) and connectable to degassing duct (9). A pressure compensation device (10) connected to degassing duct until occurrence of pressure difference of predeterminable amount has gas-permeable pressure compensation opening (13) whose cross-section is lesser than that of gas-permeable vent (12). The response pressure of one bidirectionally or two unidirectionally acting safety valve (15) of pressure compensation opening is less than cracking pressure of safety valve (14) of gas-permeable vent.    USE - Device for balancing difference of pressure between pressure within gastight housing of battery cell such as lithium ion battery cell of battery system and pressure in environment of housing mounted in motor car.    ADVANTAGE - The bidirectional pressure balancing in normal operation of the accumulator and unidirectional pressure balancing are made with degassing of battery cell of battery. The sudden pressure developing due to degassing of battery cell over the safety valve arranged at gas-permeable vent is degraded without exposure of components of battery to the pressure for long time. Hence desired pressure balancing in the normal operation of the battery is performed, without opening of the safety valve arranged at the gas-permeable vent. The pressure compensation device is compact and is easy to install at less installation space. Thus pressure difference balancing device is manufactured under a relative small expenditure for material and supplies, and has less weight. The handling and assembly of pressure difference balancing device are simplified.    DESCRIPTION OF DRAWING(S) - The drawings show the perspective view of the battery system arranged in vehicle floor of motor car and cross-sectional view of the device for balancing pressure difference.    Gas-tight housing (5)    Pressure compensating opening (6)    Degassing duct (9)    Pressure compensation device (10)    Gas-permeable vent (12)    Pressure compensation opening (13)    Safety valves (14,15)