The diagnostic method applies to a standalone system including a generator, a power regulator and a power storage element. The method includes comparison of the effective charging power or current of the power storage element respectively with a predefined power or current threshold. If the effective charging power or current is lower than said threshold, the power storage element is disconnected. An abnormal behavior is then detected either by comparing the effective charging power with the smaller of the values representative of the theoretical charging power of the power storage element and of the maximum power able to be delivered by the generator and comparing the effective and theoretical charging voltages, or by comparing the effective and theoretical charging currents.