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Portable Solar USB Charger

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  • Portable Solar USB Charger

    Portable USB chargers are incredibly useful for adventures in the great outdoors, festivals, traveling, or if you are out-and-about all day. Adding in a solar panel provides an additional source of mobile power useable nearly everywhere.
    This project can be built for ~ $20, even if you don't have a soldering iron!
    Step 1: Materials

    -- Solar Panel;

    To effectively charge the battery, the solar panel needs a voltage output equal to or greater than 9V. I recommend going with low power solar panels (e.g. less than 6 W) so that you can use the trickle charge effect to avoid damaging your battery (e.g. one 1.5W, 9V panel). In general, it is recommended to disconnect the solar panel when the battery is fully charged.

    -- 1N914 or similar diode

    This protects the solar panel by allowing current to flow only from the panel to the batteries (aka prevents discharge from the batteries onto the solar panel). If you choose to use a similar diode, be sure it works w/ the given solar panel specs (voltage/current output).

    -- USB car charger

    -- Rechargeable 9 V battery*

    Use two if you want to charge an Apple product.

    *Why a 9 V battery?
    USB car chargers expect 12 VDC from the car, but will accept between 6 VDC and 14.5 VDC. Using a single 9V battery is the easiest way to get a sufficient input voltage for this USB circuit in order to get an output of 5 VDC.

    -- Battery holder for 9V (or use alligator clips)

    -- Project container (e.g. tupperwear, altoids tin, cookie tin, etc.). Be creative!
    Step 2: Tools

    -- Wire strippers

    Scissors also work. To strip the wire, make cuts on both sides and pull off insulation with your fingers.

    -- Electrical Tape

    -- 5-minute epoxy, or other similar adhesive (gorilla glue probably works)

    -- Soldering iron

    Alternative methods for making electrical connections: twist wires together and coat in epoxy. Other connections can be MacGyvered together; take apart old electronics for connectors and wires, use paperclips, and be creative with conductive objects like pennies.

    -- Multimeter, if available.

    Massively helpful for testing electrical connections and checking if the circuit works as expected.

    Step 3: A Lil' Bit About USB

    As shown in the photo, USB chargers have 4 pins. All USB chargers output 5 Volts (V) DC on the USB Vcc pin. However, the amount of output current depends on the type of USB charger.There are three main types: a standard downstream port (500 mA), a charging downstream port (1500 mA), and a dedicated charging port (900 mA).

    Apple USB is a bit trickier (unsurprisingly..); one of the data pins is set to 2.7 VDC. So, if you finish your portable USB charger and you want to charge an iPhone or iPod, you need to increase the voltage. This can be done by using a bigger battery or two 9V batteries connected together in series.

    Step 4: Build it! Pt. 1

    Note: if you are using the epoxy method for connecting wires, wait until after you've tested the whole system to coat with epoxy; epoxy is rather permanent and once it has set there is little you can do to fix a broken connection besides curse at it (won't really help, but might make you feel better!).

    1. Strip wire on end of solar panel (remove colored insulation to expose the metal).

    No leads on the panel and there's no soldering iron?! It's all good! Get creative.

    Here's one way: tape two wires onto the metal pads on the back of the panel w/ electrical tape (colors don't really matter, but convention is red = positive, black = negative). Test it with a multimeter, or by connecting the leads to the USB car charger to make the "charging" LED light turn on. Coat in epoxy, let dry & you're done!

    2. Connect diode to positive end of solar panel lead. If possible, solder the two ends together:

    Otherwise, twist wires & coat in epoxy (at the end).

    Important: install the diode so that the side w/ the silver band is connected to the battery, like in the photo above.

    Step 5: Built it! Pt. 2.

    3. Connect diode to positive (red) side of battery holder. Connect negative (black) solar panel lead to negative battery holder lead.
    Leave one side so that it can be easily disconnected and connected (aka a simple switch).
    4. The front metal part of the USB car charger is the positive terminal. One of the metal side tabs is the negative terminal. Determine which side of the USB car charger is the negative (or ground) side by using one of the following methods:
    -- Open up the charger; see which metal tab is connected to a wire.
    -- Use the panel to turn on the charger. Connect the positive battery/solar panel lead to the front metal lead. Touch the negative battery/solar panel lead to each side. The side that causes the "on" light to light up is the negative side.
    Step 6: Build it! Pt. 3

    5. Connect the negative battery/solar panel lead to the negative tab on the USB car charger. Connect the positive battery/solar panel lead to the front metal lead on the USB car charger.
    There are a few ways to do this, depending on your available tools and materials. The easiest way is to use alligator clips (and coat them in epoxy when it's all done & tested).
    6. Test it!
    Connect a USB device (like the Raspberry Pi!!) and make sure it lights up.
    If it works, epoxy all the electrical connections, put it into a container and take it with you on an adventure!

    Step 7: Go 'Splorin!

  • #2
    Hi Cayla - interesting project.

    But, are you related to the author "jenfoxbot" on the instructables website where this project with pics seems to be lifted straight out of?

    Might want to give credit where credit is due...


    • #3
      Get lost.
      MSEE, PE


      • #4
        Cayla is fixin to release a Spam bomb.


        • #5
          It's a fun little benchtop hack, but probably should have stayed on the original site.

          These days, usb solar charging is such a commodity item (like an Anker batt and folding solar panel with regulation), this project is akin to trying to make your own lead-acid battery at home in your bathtub.


          • #6
            Hard to tell if your panel is broken or your battery. Since these are both commodity items, just replace the device that is broken. It isn't worth it these days to cobble junk together.

            Battery pack tip: Since the cells inside these banks are only really rated for 300-500 full cycles, if you make your bank too small and you cycle from full charge to nearly empty, you'll only last a year or so. Not to mention having solar insolation anxiety.

            The better idea is to get a very large battery bank. That way, you can run in a PSOC or partial state of charge all the time and improve the cycle life. Ie, you'll never have enough time in a day to fully charge from the commodity panels, and the bank is so large that you never fully discharge it. So it improves your autonomy, and you just use and replace energy as convenient. Unless of course your gear is a total power hog.

            Moral for those whose only solar experience is with this stuff: get the largest battery bank you can afford.