A method for operating a traction energy storage system of a vehicle having a plurality of energy stores which are able to be connected in parallel, in which method a subset of the energy stores are connected in parallel and at least one energy store which is to be added is not connected in parallel with the parallel-connected subset, comprising: determining an internal resistance of the parallel-connected subset of the energy stores, an open-circuit voltage of the parallel-connected subset of the energy stores and an open-circuit voltage of the at least one energy store which is to be added; controlling a current through the parallel-connected subset of the energy stores, which current is dimensioned such that the open-circuit voltage of the parallel-connected subset of the energy stores and a voltage drop effected by the current and the internal resistance produce a voltage of the parallel-connected subset of the energy stores which differs from the open-circuit voltage of the at least one energy store which is to be added by less than a predetermined switching voltage; connecting the at least one energy store which is to be added in parallel with the parallel-connected subset of the energy stores while the voltage of the parallel-connected subset of the energy stores differs from the open-circuit voltage of the at least one energy store which is to be added by less than the predetermined switching voltage; and determining an operating mode of the vehicle, wherein, in the case of an operating mode which is primarily drawing energy from the traction energy storage system, the open-circuit voltage of the parallel-connected subset is greater than the open-circuit voltage of the at least one energy store which is to be added and/or, in the case of an operating mode which is primarily supplying energy to the traction energy storage system, the open-circuit voltage of the parallel-connected subset is less than the open-circuit voltage of the at least one energy store which is to be added, characterized in that the current is controlled by changing an electrical power which is taken up and/or output by an auxiliary load connected to the parallel-connected subset of the energy stores. determining an internal resistance of the parallel-connected subset of the energy stores, an open-circuit voltage of the parallel-connected subset of the energy stores and an open-circuit voltage of the at least one energy store which is to be added; controlling a current through the parallel-connected subset of the energy stores, which current is dimensioned such that the open-circuit voltage of the parallel-connected subset of the energy stores and a voltage drop effected by the current and the internal resistance produce a voltage of the parallel-connected subset of the energy stores which differs from the open-circuit voltage of the at least one energy store which is to be added by less than a predetermined switching voltage; connecting the at least one energy store which is to be added in parallel with the parallel-connected subset of the energy stores while the voltage of the parallel-connected subset of the energy stores differs from the open-circuit voltage of the at least one energy store which is to be added by less than the predetermined switching voltage; and determining an operating mode of the vehicle, wherein, in the case of an operating mode which is primarily drawing energy from the traction energy storage system, the open-circuit voltage of the parallel-connected subset is greater than the open-circuit voltage of the at least one energy store which is to be added and/or, in the case of an operating mode which is primarily supplying energy to the traction energy storage system, the open-circuit voltage of the parallel-connected subset is less than the open-circuit voltage of the at least one energy store which is to be added, characterized in that the current is controlled by changing an electrical power which is taken up and/or output by an auxiliary load connected to the parallel-connected subset of the energy stores.