The invention relates to a method for determining the state of charge (SOC) of a chargeable battery pack (5) comprising at least one battery cell (5a, 5b, 5c), said method comprising the steps of: providing an initial state of charge value (SOC1) for said battery pack (5); estimating a battery cell voltage value (Vest) using a cell model (7b) based on input values representing at least a measured voltage (Vmeas) of said battery pack (5); comparing said estimated battery cell voltage value (Vest) with said measured battery cell voltage value (Vmeas) for said at least one battery cell (5a, 5b, 5c); and determining an estimated state of charge value (SOCest) to update the state of charge (SOC) from the initial state of charge value (SOC1), based on said comparing step and by using an observer module (7a). According to the invention, the observer module (7a) comprises an influence factor, said influence factor representing errors in the state of charge (SOC) based on the parameterization of said cell model (7b). The invention also relates to an arrangement for determining the state of charge (SOC) of a battery pack (5).