A charging system comprising: • an external apparatus including ∘ an operation device configured to be manipulated for battery charging, and ∘ a processor configured to process billing authentication on the basis of the manipulation of the operation device; and • a charger including ∘ a power converter configured to convert power into power at a predetermined voltage, ∘ a feeder configured to supply the converted power to a battery, ∘ a charging starter configured to be manipulated to instruct start of charging, ∘ a controller configured to control charging of the battery, and ∘ an interface interposed between the controller and the external apparatus, characterized in that • the controller is adapted to wait for a cyclic signal to be inputted from the external apparatus via the interface, • in case the controller receives the cyclic signal, the controller is adapted to select a first mode, which is a slave mode in which the charger operates in accordance with instructions from the external apparatus and a in case the controller does not receive the cyclic signal within a predetermined time, the controller is adapted to select a second mode in which the charger operates on a stand-alone basis, and • the controller is adapted to start to control the charging of the battery in the first mode under the condition that a second signal is inputted from the external apparatus via the interface and to start to control the charging of the battery in the second mode independent from the second signal. • an external apparatus including ∘ an operation device configured to be manipulated for battery charging, and ∘ a processor configured to process billing authentication on the basis of the manipulation of the operation device; and ∘ an operation device configured to be manipulated for battery charging, and ∘ a processor configured to process billing authentication on the basis of the manipulation of the operation device; and • a charger including ∘ a power converter configured to convert power into power at a predetermined voltage, ∘ a feeder configured to supply the converted power to a battery, ∘ a charging starter configured to be manipulated to instruct start of charging, ∘ a controller configured to control charging of the battery, and ∘ an interface interposed between the controller and the external apparatus, characterized in that ∘ a power converter configured to convert power into power at a predetermined voltage, ∘ a feeder configured to supply the converted power to a battery, ∘ a charging starter configured to be manipulated to instruct start of charging, ∘ a controller configured to control charging of the battery, and ∘ an interface interposed between the controller and the external apparatus, • the controller is adapted to wait for a cyclic signal to be inputted from the external apparatus via the interface, • in case the controller receives the cyclic signal, the controller is adapted to select a first mode, which is a slave mode in which the charger operates in accordance with instructions from the external apparatus and a in case the controller does not receive the cyclic signal within a predetermined time, the controller is adapted to select a second mode in which the charger operates on a stand-alone basis, and • the controller is adapted to start to control the charging of the battery in the first mode under the condition that a second signal is inputted from the external apparatus via the interface and to start to control the charging of the battery in the second mode independent from the second signal.