A multimeter includes a main body, two probes extending from the main body, a battery unit arranged in the main body, and a charging system arranged in the main body and configured for charging the battery. The charging system includes a microcontroller with an external input voltage sampling circuit, a battery voltage sampling circuit and a voltage regulator circuit each electrically connected the microcontroller. The microcontroller compares sampled signals from the external input voltage sampling circuit and the battery voltage sampling circuit, and controls the voltage regulator circuit to regulate the external input voltage to be applicable to the battery based on the comparison.