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 portable power useable (nearly) everywhere. 
The whole project can be built for ~ $20, even if you don't have a soldering iron!

Parts

• 1.5W Solar Panel 9V 
• Suggested to use a low-power solar panel, typically if you are not using a charge controller.  
• Note that the solar panel voltage output MUST be higher than the battery output for it to actually charge.
• 1N914 Diode 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 a similar diode, be sure it works w/ the given solar panel specs (voltage/current output).
• USB car charger
• Rechargeable 9 V battery**
• 9V Battery Case (or use alligator clips)
• Project container (e.g. tupperwear, altoids tin, cookie tin, etc.). Be creative!

Tools

• Wire Strippers
• Scissors also work. To strip the wire, make cuts on both sides and pull off insulation w/ your fingers.
• Electrical Tape
• 5-minute epoxy, or other similar adhesive (gorilla glue probably works)
• Soldering iron.
• Other 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 w/ conductive objects like pennies.
• Multimeter, if available. Massively helpful for testing electrical connections and checking if the circuit works as expected. 

Background Info

advenira.com

Solar panels are awesome for many reasons: 
1. Renewable energy technology, woo!
2. Handy in remote locations (like Burning Man..).
3. Lifetime of 25 - 30 years.
etc.*

solar-wind.co.uk

Solar panels, or photovoltaic (PV) panels, output direct current (DC). Digital devices, like cellphones or iPods, run on DC. This means our charging circuit is fairly simple. As in the photo on the left, we need a panel, a battery, and our device, or load. Charge controllers regulate current flow primarily to protect the battery. We can avoid using one in our USB charger, but they are ideal for larger systems.

The solar charging system works w/out the batteries. The batteries are there so you can use the system whenever you need it.

 
A lil' bit about USB
As shown in the photo to the right, 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 (aka use a bigger battery.. or two 9V batteries connected together in series.


Build Process

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

The silver band is on the right, away from the panel.

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 and black = negative). Test it w/ 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. Super important: install the diode so that the side w/ the silver band is connected to the battery, like in the photo to the right.



 





3. Connect diode to positive (red) side of battery holder. Connect negative (black) solar panel lead to negative battery holder lead. 




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. 
Here are a couple easy ways:
-- 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.

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 w/ you on an adventure! 
Once your first version works, make upgrades and modifications as necessary! Google is super helpful.

*More info about solar!
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!!

**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.