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!

Once your first version works, make upgrades and modifications as necessary!

Just for fun: more info about solar!

The panel in this tutorial is a 1.5W, 9V panel from RadioShack. It outputs ~ 166 mA of current. To avoid over-charging your battery, disconnect the panel when the battery is charged. If you have a panel larger than 6 W, a charge controller to protect the batteries is a helpful addition. Here's a list of various charge controllers, find the one that fits your panel.

Solar panels have a relatively low energy efficiency rating, typically around 12-15%. Research is continually improving solar efficiency, and a lab in Germany set the world record for solar cell efficiency at 44.7%.

In 2012, average costs of solar per watt were between $1 - $2, with some as low as $0.70 per watt. Although this does not include the cost of additional equipment (e.g. batteries, transformer for AC applications, mounting system, etc.), it is beginning to seriously compete with fossil fuels. Yay, solar!!