Step 1: Make the Charger
My joule thief uses twisted network wire passed through a small ferrite core. I use 6 turns of wire. You can find a core from a burned out compact fluorescent bulb. You can see how others have wound the coil and built the Joule Thief, since so many have done so. Just add a diode and LED in series with the charging battery. The LED is useful as a charge indicator.
A high-speed schottky diode would be the most efficient. The 1N4005 was handy at the time and works.
Step 2: Open Circuit Voltage
More than enough voltage to charge a rechargeable battery.
Step 3: Charge Current
Step 4: Charger in action
Step 6: Plotting the results
Steps 7 and 8 show the results using Excel.
Step 7: First 8 hours
Bottom blue trace is the donor cell. Notice how the voltage is slowly dropping off. The life force of the donor cell is slowly slipping away.
The Joule thief output jumped between two voltages and is a bit steppy. Nothing is perfect.