Portable Solar USB Charger

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Introduction: Portable Solar USB Charger

About: Dabbled in dark matter, settled into engineering with a blend of inventing and teaching, always trying to solve problems + learn new things!

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!!

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    106 Comments

    will it damage the 9v battery? should i use the rechargeable ones? thanks.

    1 reply

    If you are using a low-power solar panel and you disconnect the panel when it is not charging it shouldn't damage the battery (look up trickle charging). Yes, you will have to use a rechargeable 9V if you want to use the solar panel -- you could build the same project WITHOUT a solar panel and just a regular ol' 9V, but where's the fun in that!

    Quick question, if i do two 9V batteries in series, would i need two 9V solar panels also? Or would one solar panel still work.

    1 reply

    Good question! Voltage adds in series, so your effective battery voltage would be 18V. To charge the batteries, you'll need a panel (or 2 panels in series) that put out more than 18V. You can think about it like a roller coaster: the roller coaster has to start higher (i.e. more energy) than the subsequent hills or it won't have enough energy to get over them. Likewise, the solar panel needs more energy output than the batteries to push electrical energy into the batteries and not the other way around (which would damage the panel). Hope that helps!

    Hi, thanks for your awnswer!

    I would like to ask another two questions, thanks!

    Firstly, how do you connect the two 9 volt batteries to the solar panel? I'm thinking about connecting the two 9 volt battery holders in series and connecting it to the solar panel, but the diode I ordered hasn't arrived yet and I'm scared about testing it :P

    Secondly, I couldn't find a 9 volt solar panel, so I connected two 6 volt solar panels in series for a total of 12 volts. Do I need a regulator for this or is 12 volts fine?

    I really like this instruct able, it's very detailed. Thanks for making this!

    2 replies

    Absolutely! Here are my responses:

    1. The solar panel voltage needs to be higher than the battery voltage for the panel to charge the batteries, so two 9V won't charge with a 12 V panel -- you'd need another 6V panel connected in series w/ the first two. Alternatively, you could connect the 9V in parallel to get a higher battery capacity, which would charge your phone faster.

    Without the diode, you should be fine testing the two panels (@ 12 V) with one 9V. Once you get the diode, you can put that between the panel and the battery so that the current can only flow from the panel to the battery. Then try testing it with two 9V and see what happens! I'm a big fan of experimentation -- try different things to see what works (best). At this low voltage/current you probably won't break anything.. although that's not a 100% guarantee, lol. But hey, if something does break it's an excellent learning experience!

    2. Yes! Totally good. Shouldn't need a voltage regulator, the USB car charger will do that for ya.

    Thanks! Very happy to hear that you found it helpful!

    Absolutely! Here are my responses:

    1. The solar panel voltage needs to be higher than the battery voltage for the panel to charge the batteries, so two 9V won't charge with a 12 V panel -- you'd need another 6V panel connected in series w/ the first two. Alternatively, you could connect the 9V in parallel to get a higher battery capacity, which would charge your phone faster.

    Without the diode, you should be fine testing the two panels (@ 12 V) with one 9V. Once you get the diode, you can put that between the panel and the battery so that the current can only flow from the panel to the battery. Then try testing it with two 9V and see what happens! I'm a big fan of experimentation -- try different things to see what works (best). At this low voltage/current you probably won't break anything.. although that's not a 100% guarantee, lol. But hey, if something does break it's an excellent learning experience!

    2. Yes! Totally good. Shouldn't need a voltage regulator, the USB car charger will do that for ya.

    Thanks! Very happy to hear that you found it helpful!

    I'm planning to do this for a school project! Just one question: How long does it take for the 2 9 volt batteries take to charge from zero power? Thanks for sharing this project!

    1 reply

    That's super awesome you're building this for a school project!
    I haven't precisely measured charge time, mostly b/c this design works on a trickle charge, which means it uses a small current to keep the batteries charged. I'd expect it would take a few hours to fully charge a 9V battery (which would be ~ 7V when "dead"). Hope that helps!

    Hello. What do you think? Can I leave out the battery and repleace the car charger with an 5v usb step up module?

    3 replies

    Technically you don't have to use the battery, but solar panels only output power in direct sunlight so you will only be able to use the charger during the day. I'd definitely recommend including a battery so you can use the charger whenever you need it.

    Re: 5V USB step-up module, I would assume yes, but without knowing the exact specifications of the module I can't say for certain. Definitely worth a try, just be sure to check the voltage/current output before you plug your phone to avoid damaging it.

    Thank you for your answer, it helped a lot. :)

    Absolutely, glad to be of help!

    Yup! As long as it's rated to handle the circuit current and voltage.

    thanks, and I'll try it without a battery :)
    just a solar panel , voltage regulator ,USB female and USB cable.

    It'll work without a battery, but you'll only be able to use it when the sun is shining directly on the panel. I highly recommend a battery so that you can store the energy generated by the panel to charge your USB device at night or when the sun isn't shining.

    may I replace the diode with another one

    how are you getting enough voltage to charge a 9v battery or even power that 12v car charger with a 6v solar panel. You would need 2 in series to make 12v ....