Present article describes the method of operative diagnostics of available capacity of the lead-acid accumulator (battery) at 10 or 20-h discharge mode. This method is developed on the basis of mathematical model of process of the discharge of the lead-acid accumulator. This allows calculating the available capacity of the accumulator (battery) on the basis of measurement of function of a response to the set short-term test signal without necessity of carrying out the control discharge of the battery. Calculation of available capacity is carried out on the basis of the parameter which is responsible for discharge capacity of the accumulator (battery), a total electric charge of an active surface of a limiting electrode.