A battery system comprising a voltage sensing circuit and a battery, the voltage sensing circuit comprising: a first sub-sensing circuit including a light emitting device electrically connected in parallel to the battery and a diode string, including at least one diode, electrically connected in series to the light emitting device such that the voltage of the battery is applied in a forward direction to the diode string; and a second sub-sensing circuit including a light receiving device optically coupled to the light emitting device, and electrically isolated from the first sub-sensing circuit, wherein the light emitting device is configured to generate an optical signal in response to a voltage across the light emitting device, wherein light intensity of the optical signal has unique correspondence relationship with the forward voltage of the light emitting device, the second sub-sensing circuit is configured to output a voltage sensing signal indicating a level of voltage across the battery in response to the optical signal, when the voltage across the battery is equal to a first reference voltage indicating an overvoltage state of the battery, a second reference voltage which is lower than the first reference voltage is applied across the light emitting device, and the voltage sensing circuit being configured such that the second reference voltage is lower than a threshold voltage of the light emitting device, wherein threshold voltage of the light emitting device indicates a forward voltage drop of the light emitting device when a predetermined level of electric current in forward direction flows through the light emitting device, and wherein a resistance of the light emitting device when the second reference voltage is applied across the light emitting device is larger than a resistance of the light emitting device when the threshold voltage is applied across the light emitting device, wherein the second sub-sensing circuit further includes: a resistor electrically connected in series to the light receiving device between a voltage source (V CC) and a ground, wherein the light receiving device changes in its resistance in response to the light intensity; and an analog-digital converter configured to generate the voltage sensing signal from a voltage across the resistor. a first sub-sensing circuit including a light emitting device electrically connected in parallel to the battery and a diode string, including at least one diode, electrically connected in series to the light emitting device such that the voltage of the battery is applied in a forward direction to the diode string; and a second sub-sensing circuit including a light receiving device optically coupled to the light emitting device, and electrically isolated from the first sub-sensing circuit, wherein the light emitting device is configured to generate an optical signal in response to a voltage across the light emitting device, wherein light intensity of the optical signal has unique correspondence relationship with the forward voltage of the light emitting device, the second sub-sensing circuit is configured to output a voltage sensing signal indicating a level of voltage across the battery in response to the optical signal, when the voltage across the battery is equal to a first reference voltage indicating an overvoltage state of the battery, a second reference voltage which is lower than the first reference voltage is applied across the light emitting device, and the voltage sensing circuit being configured such that the second reference voltage is lower than a threshold voltage of the light emitting device, wherein threshold voltage of the light emitting device indicates a forward voltage drop of the light emitting device when a predetermined level of electric current in forward direction flows through the light emitting device, and wherein a resistance of the light emitting device when the second reference voltage is applied across the light emitting device is larger than a resistance of the light emitting device when the threshold voltage is applied across the light emitting device, wherein the second sub-sensing circuit further includes: a resistor electrically connected in series to the light receiving device between a voltage source (V CC) and a ground, wherein the light receiving device changes in its resistance in response to the light intensity; and an analog-digital converter configured to generate the voltage sensing signal from a voltage across the resistor. a resistor electrically connected in series to the light receiving device between a voltage source (V CC) and a ground, wherein the light receiving device changes in its resistance in response to the light intensity; and an analog-digital converter configured to generate the voltage sensing signal from a voltage across the resistor.