Abstract: A charging device can include an input interface for receiving electrical power from a power source and an output interface for outputting electrical power to a mobile electronic device. The charging device can include a supplemental battery. A bypass electrical pathway can couple the input interface to the output interface to pass electrical charge from the power supply through the charging device to the mobile electronic device. A charging electrical pathway can couple the input interface to the supplemental battery. A discharge electrical pathway can couple the supplemental battery to the output interface. The bypass electrical pathway can include a voltage modifier configured to modify the voltage output by the output interface. The charging device can be configured to empirically determine the power capacity of the power supply. The charging device can monitor temperatures and/or battery health information, which can be used to reduce current or disable the charging device.