Low Cost Portable USB Solar Charger

So, you want a solar USB charger? Well, you've come to the right place for a pretty cheap one :)

I created this charger as a workshop on DIY electronics for a sustainability club. The chargers cost a little under $9 each (materials bought in bulk) and and can be made in under an hour. The picture above does not do it a complete justice, because once you make it, you immediately fall in love with it.

The concept of this charger is simple, create a solar USB charger that works both outdoors and indoors for maximum utility. It works outdoors based off of the sun :) and indoors off its internal rechargeable battery pack.

The design requirements were simple: be able to fit a functional solar USB charger in a user's pocket at a net cost under $10/charger.

For reference, look at the attached circuit diagram.

The first step involves USB standards. USB 2.0 runs off of 5 V at 0.5 A max. So we are talking about a maximum 2.5 W of power required. A solar cell able to produce this power would be large and over budget, so right off the bat I knew that was outside of the design criterion.

Many cheap solar cells produce under 5 V, even if they are rated at 5 V. So this meant a DC-DC step up converter was necessary. I looked around for ways to inexpensively make these, and could not find a good method. So I decided to buy a step up DC-DC converter with the USB port built into it. I will call this module a 'USB Boost Module' from here on out.

Solar cells are more or less specialized photodiodes. However, they are not ideal diodes, and can behave like a resistor when not in sunlight. This could cause the solar charger to reverse charge your USB device when in your pocket. Nobody wants a solar charger to become a 'discharger,' so I decided a blocking diode was necessary. After researching diodes and their specifications, I decided a 1N914 diode was sufficient.

Cheap solar cells produce a very small amount of current. This current might be small enough that, even in sunlight, the average cell phone might not even register charging. So, to increase the current output, I decided to have the battery pack deliver the USB power. The solar cell would exist only to recharge the battery pack.

As stated above, the charger should be able to run off of battery power. Since the charger will be used multiple times, the batteries need to be rechargeable. To recharge the batteries, somehow they need to be placed in parallel with the solar cell. I decided to place them directly in parallel, because this means anytime the solar cell produces a voltage it recharges the batteries.

To enable the user to be able to turn on/off the charger, a tap switch was placed in series with the USB boost module. All that is required of the switch is for it to be single pole single throw (SPST).

Altogether, here are the four possible states:

• Sunlight and USB charging:
  • Battery pack provides power to USB boost module
  • Solar cell recharges battery pack
    • Solar cell also slightly contributes to charging
• No sunlight and USB charging:
  • Battery pack provides power to USB boost module
  • Blocking diode prevents solar cell from sinking power and discharging battery pack
• Sunlight and not USB charging:
  • Solar cell recharges battery pack
• No sunlight and not USB charging:
  • Blocking diode prevents solar cell from sinking power and discharging battery pack

If you are going to make this charger, it's time to buy some parts. As I was leading a workshop for about 12 people, I bought enough components to make 16 chargers. The prices listed here will change depending on how cheap you can find these components and how much shipping time you have. For some components, I waited 2 months :/

I have attached my BOM spreadsheet to this step, you can look at it for the details

• Batteries and Battery Holder
  
  • Unit cost = $2.19
  • 3 AA batteries can provide 3.6 V (gets stepped up to 5 V in USB boost module) rated at 3 Ah (series configuration)
    • This is 10.8 Wh, which is plenty of power
• USB Boost Module
  • Unit cost = $1.59
  • Takes anything between 2.5 - 6 V and converts it to 5V @ 92% efficiency and 1 A max current
    • Efficiency increases with voltage
• Solar Panel
  • I chose to buy a finished small 'solar panel' for its durability compared with a brittle solar cell
    • I put 'panel' because for most people calling it a solar panel implies a large size and cost
  • Just a standard 5 V solar cell inside the 'panel'
• Wire
  • I chose speaker wire because it contains both red and black wire in one purchase
  • 20 gauge is sufficient for the power flowing through the system
  • Make sure the wire is stranded for flexibility
    • Solid core wire tends to fatigue quickly, especially in a workshop for first timers
• Diode
  • 1N914, nothing much to say here
• Switch
  • The diode in the switch will not be used as there is no proper 'ground' in this portable charger
  • This switch was cheap, all that's really necessary is a SPST switch

You will need multiple tools to make this charger.

• Soldering iron, flux, and solder wire
  
  • Preferably helping hands as well
• Wire Strippers
• Electrical Tape
• OPTIONAL: Hot glue gun
  
  • Can make covering solder jobs easier
• OPTIONAL: Multimeter
  
  • To test solder connections

This is simple and should take only a few minutes. If you do not know how to do any of these steps, view this instructable.

1. Cut 9" of speaker wire and then cut that into thirds
2. Split the speaker wire red and black wires apart
3. Strip all wire ends
   
   1. I like to strip about 3/8"
   2. Red wires will be used in "positive" lines, black wire will be used in "negative" lines
4. Tin all wire ends

Solar Panel

1. Flip your solar panel over to the back end
2. Solder red wire lead to "+" tab of solar panel
   
   1. Try to face the wires towards the interior of the cell to minimize footprint
   2. Try to solder quickly, because we do not want to apply excess heat to the solar panel for extended periods of time
3. Solder black wire lead to "-" tab of solar panel
4. Cover solder jobs in electrical tape or hot glue

Boost Module

1. Solder red wire to "+" tab of boost module
   
   1. Be careful! It's easy to solder wire to other components on the board as the tab is close to multiple surface mount components
   2. It helps to solder to the bottom of the module where there are no components so this risk is not present
2. Solder black wire to "-" tab of boost module
3. Cover solder jobs in electrical tape or hot glue

If you are not sure which direction your diode goes, read this article: Diode Polarity.

1. Solder diode to "+" lead from solar panel
   1. The black line end (cathode) should be on the opposite end of the panel's "+" lead
   2. Diode test this connection for the right direction to be sure
2. Solder a black lead to the other end of the diode
3. Cover exposed solder joints in electrical tape

Helping hands make this step much easier. The above image shows a battery pack with only two battery slots, ignore this, I just used this one during prototyping.

1. Strip/tin wire ends on both red and black wires from battery pack
   1. Do not put the batteries into the battery pack yet
2. Twist all three black wires together to the best of your abilities
3. Solder them together into one big negative node
   1. Make sure the joint is strong
      1. If it looks ugly, don't worry it will be covered in tape or glue
4. Slather on the hot glue or wrap in electrical tape

Form Positive Node:

1. Twist red lead from battery pack around the red lead coming off of the diode
2. Solder them together, forming the positive node
   1. Again helping hands is useful here.

The tap switch has three leads. One of which is ground for the LED (mine was colored gold), do not use this one. The other two leads will correspond to the switch. You will be soldering the positive node to one of these, and the red lead from the USB boost module to the other. The switch's pins you solder the wires to correspond to the closed position. I chose to depress the side of the toggle switch with the LED to close the switch.

1. Solder the positive node to the outermost lead of the switch
2. Solder the red lead coming of the USB boost module to the last lead (on my tap switch it was located on the middle)
3. Cover the connections with electrical tape or hot glue

To finish the charger

1. Remove the plastic covering on the solar panel
2. Toggle the switch to the 'open' position
3. Insert the AA batteries
   1. They should come pre-charged enough to test your charger's operation

To test your charger:

1. Toggle the switch to the 'closed' position
2. There is an indicator LED on the USB boost module, check that it has turned on
   
   1. It's small and dim, so you might need to cup your hand over it to see the LED's glow
3. If it turned on, it's creating 5 V at the USB port. If the LED is off, check all soldering jobs and that the batteries are producing their rated voltage.
4. If the LED is on, place the charger in the sun for a few hours to guarantee the rechargeable batteries have some charge
5. Plug in a USB cord and connect the charger with a peripheral of your choice
   1. I chose my cell phone - it worked!
6. Test your charger. if it fails, re-read this instructable and error check everything. Lastly, check that the individual components are still functioning

Advantages:

• Small footprint
• Inexpensive and requires little tools
• Small time investment

Disadvantages:

• Does not hold much charge and charges devices slowly
• Does not have a case: solder joints can be easily broken

Areas for Improvement:

• As someone in the comments pointed out, the 5 V solar panel is charging the battery bank of 3.6 V. This over-voltage charging will decrease the lifetime of the batteries and could cause them to burst (not good!)
  • Could be solved by using some sort of voltage regulation or bringing the battery bank's voltage to 5 V (which would also be more efficient through the USB boost module)
  • Could also be solved using a charge controller, like a real solar panel!
• Use a more complicated switch to enable the battery bank to be disconnected from the solar panel
  • This way the battery lifetime can be extended
  • This can alternately be done with a SPST switch between the solar panel and the battery bank
• If the budget was higher, purchase higher quality components
• As stated above, design a case for it
  • The case can be as simple as an Altoids can

I am always looking for recommendations, please post your comments if you see any gaps in the electronics!