Build Your Own Solar USB Charger

Have you ever had your cell phone or mp3 player die and you're not close to a computer? In many cases, today's cell phones and other portable electronics use a standard miniUSB or microUSB jack. That means you can use a standard computer's USB cable to charge your phone or mp3 player. This post discusses the components of a typical solar USB Charger.

A USB jack provides +5V DC. The phone or mp3 player's internal circuitry uses this +5VDC to charge the battery. What does this mean for us? Simply that a solar panel USB charger only needs to provide +5VDC - the circuitry needed to charge the device's internal battery handles the charging. That's nice because a charger performs all sorts of tasks like

  • Slowly charging the battery when it is completely discharged until it gets to a point where it can begin fast charging.

  • Fast charging the battery while monitoring the vital signs of the battery like the temperature, voltage, and limiting the charging current.

  • Performing the final top-off charge of the battery - this is usually a low current.

  • Turning off the charger once the battery is fully charged.

  • Performing any maintenance charge. Over time, the battery will naturally deplete called self-discharge (Lithium Ion batteries discharge at a lower rate than NiMH or Alkaline).

  • This article details how to build your own solar power USB charger - a DIY cell phone solar charger.

    Build Your Own Solar Powered USB Charger

    In the diagram above, you're attaching a series of smaller solar panels together to generate a voltage of 7V or more. The string must supply at least 7V because the drop across the voltage regulator (voltage from IN to OUT) is 2V for a standard 7805 regulator. You can choose another voltage regulator that doesn't have as much drop but they are harder to find (not stocked in Radio Shack or Frys) and they're more expensive.

    You'll need to experiment with a capacitor between the OUT terminal and GND as well. Typically a 10uF (microFarad) capacitor is fine. You will also want a capacitor between the IN terminal and GND. Use 10uF for this as well. This is needed to stabilize the +5V output.

    Another option is to buy one of these Solar Panel USB chargers. The ones shown in the following links include a battery so that it is more portable. Contact me if you're interested in the schematic for something like this. I haven't tried the 2nd or 3rd items - the 2nd item gets mixed reviews.

    Chad Decker