Introduction: 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 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!!

Comments

author
Jason_S (author)2017-08-27

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

author
jenfoxbot (author)Jason_S2017-08-28

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!

author
NoahQ1 (author)2017-07-09

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.

author
jenfoxbot (author)NoahQ12017-07-10

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!

author
Lazerbolt (author)2016-06-05

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!

author
jenfoxbot (author)Lazerbolt2016-06-07

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!

author
jenfoxbot (author)Lazerbolt2016-06-07

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!

author
Lazerbolt (author)2016-05-24

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!

author
jenfoxbot (author)Lazerbolt2016-05-25

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!

author
ZoltánK12 (author)2016-05-20

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

author
jenfoxbot (author)ZoltánK122016-05-20

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.

author
ZoltánK12 (author)jenfoxbot2016-05-22

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

author
jenfoxbot (author)ZoltánK122016-05-23

Absolutely, glad to be of help!

author
سميرا (author)2016-04-25

may I replace the diode with another one

author
jenfoxbot (author)سميرا2016-04-25

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

author
سميرا (author)jenfoxbot2016-04-25

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

author
jenfoxbot (author)سميرا2016-04-25

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.

author
سميرا (author)jenfoxbot2016-04-26

off course

author
سميرا (author)2016-04-25

may I replace the diode with another one

author
sspence (author)2015-11-10

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 ....

author
jenfoxbot (author)sspence2015-11-10

This design uses a 'trickle charge' effect; the solar panel provides a
small, constant current to the battery to effectively keep it fully
charged. Hope that helps!

author
sspence (author)jenfoxbot2015-11-10

No, it doesn't. You can't trickle charge a 9v battery with a 6v source. The battery will never exceed the voltage of the solar cell. It will always be discharged. It takes a higher than battery voltage to push a charge into it.

author
jenfoxbot (author)sspence2015-11-15

Ah, now I see what you're saying. That is correct, to charge the battery the supply needs to have an equal or higher potential than the battery. Double checked the panel that I used for the final version and it's a 1.5W, 9V panel. Fixed that and added a note in the materials section about supply voltage. Thanks for catching that issue!

author
schiller12345 (author)2015-06-04

How can you tell when the battery is finished charging?

author
jenfoxbot (author)schiller123452015-06-04

This design uses a 'trickle charge' effect; the solar panel provides a small, constant current to the battery to effectively keep it fully charged. So you shouldn't ever need to disconnect the panel or the battery.

If you have a larger solar panel (aka higher power), you can use a multimeter to check the battery voltage and/or estimate how long it will take based on the panel's voltage/current ratings and the capacity of the battery.

author
Slapshot135 (author)2015-05-18

Could I replace the diode with an NPN Transistor. I've been testing it out with a multimeter and a 9v battery and I'm pretty sure it should work, but I don't want to try it and then find out that I can't.

author
jenfoxbot (author)Slapshot1352015-05-20

Hmm.. interesting question! The function of the diode is to protect the PV panel from battery discharge by only allowing current flow in one direction (from the panel to the battery). I'm not an expert w/ transistors, but I wouldn't necessary *replace* the diode w/ the NPN transistor. However, my guess is that you could just add the NPN transistor in series w/ the diode as an overall current booster for the circuit. As long as the NPN transistor current output is w/in the car charger specs it should be totally fine (and I would expect that most car chargers could easily handle a substantial increase in current). If you do try it, would love to know what happens!

author
Slapshot135 (author)jenfoxbot2015-05-21

Ok, so I added the transistor along with the diode, but it only gave out about 4 or so volts to the car charger. Also, with the normal setup that you had in the tutorial, It didn't output enough to charge my ipad, which I would like it to do. But I was wondering, if I doubled up on solar panels and batteries, along with a transistor, which I've estimated would reduce the battery voltage to about 12 volts, would that work, or would it be too much voltage?

author
jenfoxbot (author)Slapshot1352015-05-22

Ahh that's interesting! Are you using a 9V battery or just a PV panel w/ the transistor?

IPads (and other Apple products) are picky, so you'll need to provide
sufficiently high current input to get out enough current to charge
Apple products.

Try adding 2 9V batteries in series (i.e. connect the negative terminal of the 1st battery to the positive terminal of the 2nd, and use the remaining open battery leads to power the car charger). If you want higher capacity (aka you can draw more current and the batteries will last longer), try 4 - 6 AA batteries in series (each AA battery has a nominal voltage of 1.2 - 1.5V, so 4 AAs will give you ~ 6V and 6 will give you ~8V).

The car charger can handle between 7V - 15V, and based on some quick research, it looks like most car fuses are rated for ~ 15A, although people recommend not drawing more than 2 - 3 A from the car charger. All that is to say, the car charger is pretty robust and can handle fairly high current, so you should be fine as long as you're playing w/ low power batteries and PV panels.

author
Slapshot135 (author)jenfoxbot2015-05-22

I think I've got this worked out now, thank you. I added two 6v panels in series with a single 9v battery along with some accessories I added. It now has a switch that will activate/deactivate the battery, which, when full, does charge my ipod a little, however its better to power it with solar which can get up to 12v. I also have a switch that will turn the usb port on and off so that I can recharge the battery more efficiently. And finally, I added a couple of terminals so that I can easily check the voltage going into the usb port, or, add something other than the usb. I might publish this in my own project, I'll link this project if I do, Nice work on this, It really helped me a lot for what I was doing.

author
jenfoxbot (author)Slapshot1352015-05-22

Heck yes, that's so awesome!! Very cool design modifications. Would love to see the tutorial if/when you get it written up. It sounds like a perfect project to link to in this tutorial as an example of how to modify this basic design.

Thank you for the feedback, that is exactly why I chose to focus on simple, open-source designs :D

author
Slapshot135 (author)jenfoxbot2015-05-21

Thank you, I am waiting for a solar panel to arrive, but I'll let you know what happens when I get it. One more thing though, I'm pretty new to this kind of stuff, so this is going to probably sound pretty stupid, but does this circuit charge the battery at the same time that it is powering the car charger?

author
jenfoxbot (author)Slapshot1352015-05-22

Great question! The PV (photovoltaic) panel "trickle charges" the battery, and the battery is what actually powers the car charger. Trickle charge just means that, when connected, the PV panel is continually and slowly charging the battery to keep the battery at full capacity. So, yes! The circuit does charge the battery while powering the car charger.

author
dpizetta (author)2014-09-19

There are some cheap charging circuits that could improve your system to not damage your battery. Nice work :)

author
jenfoxbot (author)dpizetta2014-09-19

Yea, more robust ways to build a USB charger do exist. This was the cheapest and easiest method I conceived. Improvements and modifications are definitely encouraged!

author
PattyCA (author)jenfoxbot2015-05-06

Hi Jen, what kind of a charging circuit would I use if I wanted to prevent overcharging the battery. Then I could leave it and wouldn't have to worry. Thanks, Patty

author
jenfoxbot (author)PattyCA2015-05-06

Hi, Patty,

This design relies on trickle charging; as long as you have a low power solar panel (~ 1W) the battery will not get overcharged. Essentially, the panel just keeps the battery at full capacity, so no need to add or change the design unless you want to.

author
PattyCA (author)jenfoxbot2015-05-07

Thank you. It's so nice of you to help like this. My goal is to have a solar system to run some lights in my living room, charge my iPhone and perhaps some small appliances. I want to understand solar, so I can supplement my electric bill and use clean energy. Also, I live on the coast south of Boston and we have lots of "no electricity for weeks" type storms. What size panel would I need for a system like this. Would a 10 watt panel be to large if I use a couple of 12V batteries? Again, I appreciate you answering my questions. Patty

author
jenfoxbot (author)PattyCA2015-05-20

Hi Patty,

Of course, happy to help promote renewable energy technology! The first thing about PV (photovoltaic) panels is that they output DC current, which is what digital devices (e.g. computers, cellphones, cameras, iPods, etc.) run on. Anything powered by a battery is digital. However, our home electrical outlets output AC current. This means that if you want to power an AC load (anything that plugs into the wall), you need to convert the DC output of the PV panel into AC. This is done w/ a power inverter.

It sounds like you want a fairly large system, on the order of kilowatts. This type of system will likely not be cheap, but will definitely pay for itself in the long run. I highly recommend you speak w/ a solar installer because you'll be dealing w/ high voltage, which is very, very dangerous. The solar installer will be familiar w/ power inverters and will also include a battery bank to store the energy generated from the PV panels during the day, which you will be able to use during peak load time or when the power is out.

If you want to start small and avoid the solar installer, I recommend getting a medium-sized PV panel (~ 20 W or so), a multimeter, and a 12V rechargeable battery. Then, play around with the basic components until you get a basic understanding of how each of them work (just remember to always be very, very careful about touching any exposed wire leads w/ your hands). Once you get a feel for these components, you can add in other components, like a power inverter (note that the PV panel will charge the batteries, and the batteries will be the input to the power inverter). I also recommend doing a bit of background research, here's a good intro article.

I hope this helps!

All the best,

Jen

author
PattyCA (author)jenfoxbot2015-05-22

Thanks Jen,

That's exactly what I want to do. I appreciate you taking the time to help me. I'm learning and my goal is to understand what I need to know to set up a small solar system to run a few lights and perhaps a small TV for emergency use. Also, if I understand enough, perhaps a system to supplement my home electricity bill.

author
jenfoxbot (author)PattyCA2015-05-22

Absolutely! It's folks like you taking small steps to actually use renewable energy technology that will actually make a difference in how our society generates & consumes energy.

Re: energy rebates, give your local DWP (Department of Water & Power) utility a call. They might be able to refer you to licensed solar installers in your city, or at the very least can give you a general overview of what you need to do to receive rebates on your bill. (Aside: Most states have a rebate system; basically the utility will pay you back for the extra energy that your home generates up until your bill is $0.) Definitely worth looking into!

author
nodyrhode (author)2015-01-03

hey guys,you can just add a small voltmeter so that you don't over charge the battery

author
jenfoxbot (author)nodyrhode2015-01-04

Yup, that definitely works!

author
PattyCA (author)jenfoxbot2015-04-30

Hi Jen, Thanks for this project. I'm really new to this stuff and it's helping me learn. Where would I put the small voltmeter in the diagram. I'd like to know when the battery/batteries are done charging. Thanks, Patty

author
jenfoxbot (author)PattyCA2015-05-04

Thank you, v. glad to hear that! To measure the voltage across the battery, place the red voltmeter probe on the positive battery terminal, and the black voltmeter probe on the negative battery terminal.

author
Md Shuvro (author)2015-04-09

will try it!!

author
jenfoxbot (author)Md Shuvro2015-04-11

Awesome :D

author
corrinasthename (author)2015-04-09

Hi! Thank you so much for being so detailed with this Instructable! Since I'm not familiar with Electrical Engineering stuff a lot of the other tutorials I found were difficult to understand. Before finding this tutorial, I found another which instructed me to purchase a "5V Step-Up/Inverter USB Charging Circuit" and for good measure I also bought a "DIY 5V Boost PCB Module for Mobile Power - Blue" and "FX-608-PCBA DIY 1.2" LCD Dual-USB Output 5V Boost PCB Module w/ LED for Mobile Power - Silver + Blue". Would I be able to substitute one/any of these parts for the USB car charger? Thanks!

author

Thank you! Glad you found the tutorial helpful :)

The car USB charger converts the battery voltage into the standard USB voltage (5VDC). Check out the technical specs of those components you bought to see if they have the same basic function. You want one that can take a battery input (AAs output 1.2VDC, so 2 AAs output 2.4VDC), and that will output 5VDC. Connect the solar panel + diode in between the batteries. Use google to find tech specs of the components, it's also a great resource for basic questions on the functionality of specific parts.

author
The Prickly Potato (author)2015-03-14

What if I leave it outside in the sun and forget about it will the battery explode from overcharging?

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