I made a joule thief and am charging a 3.7 volt NiCad battery from a 1.5 volt AA battery using a secondary coil on the joule ringer.
This is a standard joule thief circuit on the front side. Once I got the circuit working and had the LED light glowing, I removed the light. This is just an indicator of the circuit working and actually draws power from the battery.
I have 14 winds on the primary coil and 28 winds on the secondary. Two wires are wound together as the primary and two on the secondary.
I am only using one secondary coil at this time though. The output of the secondary coil goes through a diode to the NiCad to be charged. The battery was resting at less than 0.3 volts when I removed it from the device it was in.
Currently it is above 0.4 volts and rising. The digital volt meter somehow interferes with the output of the joule thief and lowers the battery voltage when it is connected. I am not sure if this is some sort of surface charge or real charge but the volt meter certainly has a negative affect on the circuit.
I plan to find a smaller analog volt meter and try that out.
Stay tuned for more projects and experiments in alternate energy and electronics.