In one aspect, an apparatus for charging a device includes a charger and a controller. The charger includes a capacitance and has a charger input and a charger output. The charger input receives an AC input voltage waveform, and the charger output outputs an output voltage waveform and an output current waveform. The controller determines whether an amplitude of the output voltage waveform is within a voltage range. In response to determining that the amplitude of the output voltage waveform is within the voltage range, the controller directs an amplitude of the output current waveform to be substantially proportional to an amplitude of the AC input voltage waveform. In response to determining that the amplitude of the output voltage waveform is not within the voltage range, the controller increases the capacitance of the charger to adjust the amplitude of the output voltage waveform to be within the voltage range.