A battery that includes: an output terminal configured to be electrically connected to an engine control unit and an on-vehicle device that are located outside the battery; wherein the battery further includes the following components which are located inside the battery: an electric storage device comprising a plurality of battery cells connected in series; a monitoring apparatus including a detector and a controller, wherein the detector is configured to measure a voltage value corresponding to an amount of charge of the electric storage device; and a relay connected to the electric storage device and the output terminal of the battery; wherein the controller is further configured to: determine whether the voltage value measured by the detector is equal to or lower than an opening threshold voltage value, wherein the opening threshold voltage value is larger than an engine activation low-threshold voltage value by a predetermined voltage value, and wherein the engine activation low-threshold voltage value is a lowest voltage level of the amount of charge at which the engine is able to be started; and execute an opening process to switch a state of the relay from a closed state to an open state if the measured voltage value is equal to or lower than the opening threshold voltage value. an output terminal configured to be electrically connected to an engine control unit and an on-vehicle device that are located outside the battery; wherein the battery further includes the following components which are located inside the battery: an electric storage device comprising a plurality of battery cells connected in series; a monitoring apparatus including a detector and a controller, wherein the detector is configured to measure a voltage value corresponding to an amount of charge of the electric storage device; and a relay connected to the electric storage device and the output terminal of the battery; wherein the controller is further configured to: determine whether the voltage value measured by the detector is equal to or lower than an opening threshold voltage value, wherein the opening threshold voltage value is larger than an engine activation low-threshold voltage value by a predetermined voltage value, and wherein the engine activation low-threshold voltage value is a lowest voltage level of the amount of charge at which the engine is able to be started; and execute an opening process to switch a state of the relay from a closed state to an open state if the measured voltage value is equal to or lower than the opening threshold voltage value. an electric storage device comprising a plurality of battery cells connected in series; a monitoring apparatus including a detector and a controller, wherein the detector is configured to measure a voltage value corresponding to an amount of charge of the electric storage device; and a relay connected to the electric storage device and the output terminal of the battery; wherein the controller is further configured to: determine whether the voltage value measured by the detector is equal to or lower than an opening threshold voltage value, wherein the opening threshold voltage value is larger than an engine activation low-threshold voltage value by a predetermined voltage value, and wherein the engine activation low-threshold voltage value is a lowest voltage level of the amount of charge at which the engine is able to be started; and execute an opening process to switch a state of the relay from a closed state to an open state if the measured voltage value is equal to or lower than the opening threshold voltage value. determine whether the voltage value measured by the detector is equal to or lower than an opening threshold voltage value, wherein the opening threshold voltage value is larger than an engine activation low-threshold voltage value by a predetermined voltage value, and wherein the engine activation low-threshold voltage value is a lowest voltage level of the amount of charge at which the engine is able to be started; and execute an opening process to switch a state of the relay from a closed state to an open state if the measured voltage value is equal to or lower than the opening threshold voltage value.