Introduction: Solar USB Charger

About: Making and sharing are my two biggest passions! In total I've published hundreds of tutorials about everything from microcontrollers to knitting. I'm a New York City motorcyclist and unrepentant dog mom. My wo…

Let's make something super useful— your own solar powered USB backup battery! After some simple soldering, you'll be ready to charge your phone and other portable electronics on the go while camping or during the next power outage. What follows is a basic recipe which you can follow exactly, or switch out the solar panel and battery size to match your desired capacity, charge speed, and budget.

This project is part of my free Solar Class, where you can learn more ways to harness the sun's energy through engraving and solar panels.

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Step 1: What You'll Need

Parts (Adafruit shopping list):


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First soldering project? No problem! This is a great project for beginners, and you'll get a variety of types of solder practice while building it. You can learn how in the soldering lesson of Randy's free Instructables Electronics Class, then come back here to assemble your solar charger.

Step 2: Circuit Diagram

The solar charger circuit board comes with a USB port, DC jack for the solar panel, and two JST ports already attached to the board. The battery comes with a JST plug and will attach to the JST port labeled BATT. The solar charger comes with a JST pigtail cable which will connect to the LOAD port and be soldered directly to the PowerBoost input terminals.

The power switch (at the top of the diagram above) should be attached to the PowerBoost pins labeled EN and GND. Flipping it will turn on and off the PowerBoost. This switch does not have to carry the circuit's current load, so choose almost any on/off switch you like. I chose an illuminated on/off pushbutton, which also needs to be connected to the PowerBoost's 5V and GND pins, with a 220ohm resistor in series. The illuminated portion of the switch is optional, but it is a nice indicator that the device is ready to charge your USB devices.

Step 3: Test Fit Components

You'll want to pick an enclosure that fits all your components snugly, without too much squishing. I had an extra Moo business card box that fits the length of the battery and the height of the solar charger perfectly, and even has a little extra space left over for business cards still.

It closes with magnets embedded in the layers of cardboard and paper. If you can't find a stiff paper/cardboard box, you can choose an enclosure made from wood, plastic, or metal, however these harder materials will require different tools for creating port openings, such as a drill with a step bit.

In addition to physically fitting inside, you must also plan out where to create the openings so that your device is useable. I chose to put the illuminated power button next to the USB port, since the light indicates it's ready to charge. This area of the box is recessed, making the button less likely to get accidentally triggered while the device is in my bag. Opposite the button and USB A port (PowerBoost) are the solar panel DC port and USB mini B port (solar charger).

Step 4: Solder Capacitor to Solar Charger

The solar charger board comes with most of the components soldered to the board already, with the exception of the large filtering capacitor. Look for the large circle on the circuit board, with holes matching the capacitor's lead spacing.

The capacitor's polarity is important! The negative side of the capacitor is labeled with a white stripe and minus symbol, and the negative lead is typically shorter. The positive side of the capacitor is not labeled, and the leg is typically longer.

Line up the positive lead to the hole marked +, and the negative lead to the hole marked -.

If your enclosure doesn't have enough space to fit the height of this large capacitor, you may bend it over slightly before soldering, or use wires (and heat shrink tubing) to move it to another part of your enclosure. According to the official assembly instructions, you should be careful to avoid contact with the hot chip in the center of the board.

To learn how to solder, check out the soldering lesson in Randy's free Instructables Electronics Class.

Step 5: Assemble PowerBoost

Install the USB port to the PowerBoost circuit board, and be sure it is seated completely and evenly before soldering the terminals to the board on the underside.

The large clip joints connecting big areas of metal will require longer heating and more solder other solder joints. Allow to cool in your third hand tool for several minutes before attempting to handle the USB port, as it will get very hot.

Although the PowerBoost comes with two different connectors for its input power and ground terminals, we're going to leave those off and solder the JST pigtail wire directly to the circuit board.

Heat up and tin the ends of the wires and the pads marked + and -. Reheat the pad and wire as you bring them together: red to + and black to -.

Step 6: Mark & Cut Port Openings

Now that your circuit board elements have taken their final shapes, it's time to mark and cut openings for the ports in your enclosure.

Arrange the components inside your enclosure as you did during test-fit, and trace around the ports and power button using a pencil or marker.

If you're using double-stick foam tape instead of screws to secure your components, adjust the markings to accommodate for the width of the tape.

Carefully cut the openings with a sharp craft knife. If you're not using a paper box (for example plastic, metal, or wood), you may need a drill with a step bit, rotary tool with cutoff wheel accessory, small hacksaw, or other cutting tools appropriate for the material. If you're using a metal enclosure, line the inside with adhesive vinyl, thick tape, or other insulating material, to prevent short circuits.

Step 7: Connect Power Switch

My switch has a threaded plastic ring that will secure it to the enclosure, so I removed that first.

Tin and solder wires onto the leads of your power switch, and use heat shrink tubing to insulate the connections.

Since my switch has an internal LED, I'm also wiring up a 220ohm resistor to one of the leads (doesn't matter which), then a wire onto the opposite resistor lead. The LED is optional— you can leave it off or use any on/off switch you like (such as a toggle, slide switch, or tactile on/off button).

This on/off switch must be installed from the exterior of the enclosure, and therefore must be installed before we can solder the other ends of the wires to anything.

Insert the switch with wires through the opening in the enclosure, and thread the nut back onto the switch and tighten it against the inside of the enclosure.

After the switch is in place, you can solder its wires to the various pins on the PowerBoost board as described in the circuit diagram. The LED is connected to 5V (LED +) and GND (LED -), and the switch leads are connected to EN and GND. To attach each wire, trim it a little longer than you think you'll need, then strip off a bit of the insulation. Twist the wire strands together and lightly tin the wire so the strands stay together. Insert the tinned end into the hole on the circuit board, and apply heat and solder to connect. Trim the remaining wire end with flush snips, but be careful not to let bits of loose wire get stuck inside your enclosure.

Step 8: Secure Components Within Enclosure

Before securing the boards, let's test out the circuit! Plug your PowerBoost's JST pigtail cable into the LOAD port on the solar charger, and your battery into the BATT port.

Toggle your power switch, and the PowerBoost's onboard LED should light up, as well as your power switch LED if you have one. If yours doesn't, toggle the power back off, disconnect the battery, and double check your wiring against the circuit diagram, as well as the integrity of your solder joints. Post a photo in the comments if you still can't get it to work after these troubleshooting steps.

Further check that plugging in a USB cable to the solar charger triggers the battery to start charging, as indicated by the amber CHRG LED on the circuit board, as well as the red DCIN LED when power is connected. Verify that the battery keeps charging even when you toggle off the PowerBoost's switch. Rechargeable batteries usually ship charged, but if you're using a battery from a previous project or unknown origin, you may need to let it charge for a while before use. When it's finished charging, the green LED on the solar charging board will light up.

It's better to find any wiring mistakes or cold solder joints now, before attaching everything inside the enclosure. After you're sure the circuit is working properly, use screws or double stick foam tape to secure the circuit boards to the enclosure. I used double stick foam tape to hold the battery in place, too.

Step 9: Power Up!

Close up your enclosure and take it outside on a sunny day! Plug in your solar panel with a DC barrel jack adapter. The panel will charge up the battery and power the LOAD port at the same time, if it is getting enough direct sunlight.

You can put it into charge-only mode by powering down the PowerBoost. Later when your phone's battery is getting low, you can plug in and power it up.

Consider mounting your solar panel on your backpack to charge the battery while you're outside, or find a sunny spot outside a window at home. Be careful if you decide to mount a solar panel to the roof of your car (and consult a professional if you need help to mount it safely).

Step 10: Make It Your Own

You may wish to extend or change the connector on your solar panel's wire. After all the soldering you already did to get this far, splicing the cable is no big deal. If you have the extra time, go for it! There's a step-by-step guide in the Solar Panels lesson.

You can expand the capacity of this charger by using a bigger battery, and speed up its charge rate with a bigger 6V solar panel.

If your battery is above 1000mAh and you're using a big panel, you can increase the max charge rate of the board by soldering a 2.2K resistor across PROG, as detailed in the official product guide.

If you want to charge your battery unattended, it's smart to install the solar charger's optional thermistor. First clip off the surface mount resistor inside the marking labeled THERM.

Trim the probe wires to an appropriate length to reach your battery inside your enclosure, then strip, tin, and solder the wires to the holes marked THERM on the solar charger board.

Use tape to connect the probe to the surface of your battery. This prevents the device from charging while the battery is too hot or too cold.

I'd love to see your finished USB charger in the comments! Are you taking it to the beach? Adding it to your hurricane kit? Tell us about your version and enclosure. Thanks for following along!

This project is part of my free Solar Class, where you can find another backyard project and several lessons on working with solar panels. Check it out and enroll so you can post photos of your builds!