An embodiment method of diagnosing a power source arrangement includes a plurality of n power sources connected in series between output terminals, wherein n≧2. At least two different groups of power sources are selected from the power source arrangement. A voltage of each of the at least two different groups is measured between the output terminals. During the measurement of the voltage of one group, the power sources of the power source arrangement that do not belong to the one group are bypassed. The at least two measured voltages obtained through measuring the voltage of each of the at least two different groups or at least two voltages that are dependent on these at least two measured voltages are compared.