A method for operating a computer system to handle switch over between several accounts at account selection in a real time charging system, wherein at a charging rating of a session, a first account is selected from a set of accounts, to be used and rated with its charging interval; the cost of a first charging interval is calculated for the first account; it is determined if the cost for the first charging interval is covered by the balance of the first account; if the cost for the first charging interval is not covered, a second account is selected to be used from the set of accounts; the cost for a next charging interval is determined; it is determined if the cost for the next charging interval minus the residue money left on the first account is covered by the balance of the second account; and if so, continue with the next charging interval, wherein the residue money left on the first account is used before the selected second account is used.