In some implementations, a device includes first terminals to couple with a first battery arranged to supply power to the device, including a positive terminal and a negative terminal. The device includes a first winding and a second winding that are arranged to be inductively coupled. The device includes a transistor. The first winding and the second winding are coupled to the positive terminal with opposite polarity. The device includes a light emitting diode (LED) and second electrical contacts to couple with a second battery to be charged by the device. The device is arranged to couple the LED and the second battery in series with each other, with the LED and second battery being coupled in parallel with the second winding. The LED is arranged to be reverse biased with respect to the first battery and the second battery.