A method for estimating a SOC of a battery electrically coupled to at least one of a load or a power source includes detecting, by a voltage sensor, voltages of the battery. The method further includes determining, by a processor, an average voltage of the battery by averaging the detected voltages of the battery over a predetermined period of time. The method further includes determining, by the processor, a present operating state of the battery based on at least one of the detected voltages of the battery. The method further includes determining, by the processor, a present SOC of the battery based on the present operating state of the battery and the average voltage of the battery. The method further includes transmitting, by the processor, the present SOC of the battery to an output device for outputting the present SOC of the battery.