An external battery is disclosed. In one aspect, the external battery includes a battery and an input stage configured to receive external power and output a control signal. The external battery also includes a charger configured to convert the external power into a plurality of charge currents based at least in part on the resistance of at least one resistor detected at a control terminal of the charger, wherein the control terminal is electrically connected to the resistor, wherein the charge currents have different amplitudes, wherein the charger is further configured to provide the converted charge currents to the battery. The external battery further includes a controller configured to control the magnitude of the resistance of the resistor based at least in part on i) the control signal and ii) at least one switch.