Discharge circuit and voltage testing circuit are setup at an input end of power source. The output end of the voltage testing circuit is connected to the control end of the timing circuit. Output end of the timing circuit is connected to the display circuit. The voltage testing circuit includes referenced voltage regulator circuit, sampling circuit for source voltage, and voltage comparator. Output ends of referenced voltage regulator circuit and sampling circuit for source voltage are connected to two input ends of voltage comparator respectively. Voltmeter is connected between two input ends. After accumulator is discharged in short time, voltage of accumulator is dropped to voltage smaller than voltage references, and then the voltage comparator is inverted, the timing circuit is locked, display circuit displays capacity of accumulator in simulation. Advantages are: local test and quick testing.