Introduction: Solarize Your Backpack and Power All Your Gizmos

Picture of Solarize Your Backpack and Power All Your Gizmos

In this instructable I'll show you how to build a detachable solar panel and battery charger for your backpack. This can power or charge all your gadgets (cell phone, mp3 player...) while on the road. HAMs can use it to power small QRP transmitters and receivers on a field day etc.
I was inspired by the excellent instructable "Rain or shine solar charger" by Blondietheblond, but wanted to avoid sewing, since I don't have a machine. So this is the result.

Step 1: Gather the Stuff You'll Need:

Picture of Gather the Stuff You'll Need:

0) a backpack (duh):

Most backpacks have enough possibilities to attach stuff. Mine provided these ribbons on the back. No idea what they are supposed to be used for.

1) for the solar panel:

- 4 Encapsulated 2V/200mA solar panels (Velleman SOL4).
- Self adhesive pads for cable ties (3M Scotchlok Ab02).
- Nylon cable ties to fit the adhesive pads above.
- Velcro adjustable cable binders (Tesa 55236-00000, 12mmx20cm).
- Heat shrink tubing.
- Electric wire (capable of carying 0.5A is more than enough).
- Connector to go to the battery box.

2) for the charger/battery box:

Parts list for this is less critical, so improvise. There's an explanation with important details ahead.

- Small plastic project box.
- 4 AA-size NiMH batteries (GP 2700 series is ok, see further).
- Battery holder .
- Two 2-pin panel-mount connectors of some sort.
- Components (see further).

...and some tools:

- soldering iron.
- cutters.
- pliers.
- sharp knife.
- drill for making holes in the project box.

Step 2: Assemble the Solar Panel:

Picture of Assemble the Solar Panel:

- Attach two adhesive Scotchlok pads per encapsulated solar panel on the back.
- Use nylon cable ties of correct size to lace the panels together. The "head" of the ties goes in the middle of the pads. This fixes the position of the small solar panels.
- Connect the panels in series (red-to-black etc) and solder. Insulate with heat shrink tubing.
- At the end you will have a black and red cable ( minus and plus). Solder a 0.5m cable to this and attach a connector plug to go to the battery box, as in the picture.

Tip: I placed 4 panels in a row. You can also place them in a rectangle or other shape that fits your backpack, the important part is that the small panels have a more or less fixed position relative to each other, so they don't slide over each other.

Step 3: Attach the Panel to Your Backpack:

Picture of Attach the Panel to Your Backpack:

Now comes the "detachable" part.
Attach the panel using the velcro cable binders and whatever nooses you have on your backpack.
To get the power cable to the inside, I used the opening that's there for headphone cables.

Step 4: The Charger/battery Box:

Some theory about NiMH batteries:

The solar panels used here are rated 2V/200mA in full sunlight. I used 4 in series to that gives me 8V/200mA or 1.6W. Now, I want to use this to charge 4 NiMH AA batteries rated capacity C=2600mAh.
How do we know the batteries will be full?
A decent NiMH charger checks the temperature and also voltage drop at the end of the charging. However, to be able to check for the small drop at the end of charging, the charging current must be something like C/2 (the capacity, divided by two, without the "hour").
In full sunlight I measured the short circuit current of the panel to be 270mA, so about C/10. This is the short circuit current, so at higher voltages (the battery voltage) the panel will charge the batteries at less current than C/10. Constantly charging batteries at low currents (compared to the capacity/hour) is called "trickle charging".
Now, it used to be in the past that NiMH batteries did not handle trickle charging well, if above C/20 or even C/50. However modern NiMH can be safely (for the battery I mean) trickle charged at C/10.
Add to that the fact that the sun won't be up all the time, and we conclude that our charger can be very simple: one diode.
The diode makes sure that the batteries can't discharge back in the solar panel, once the sun is down.

What about the load?

NiMH can take quite a load (up to 2C),but there is one thing that they don't like and that's deep-discharge.
Deep discharge, meaning drawing current from the batteries when their voltage is below a certain point (0.9V ... 1.0V) will shorten their life time considerably.
Obviously, there are two things you can do to prevent deep discharge:
1) use a switch to disconnect the load when the batteries are low.
2) use some electronic circuit to disconnect the load, once the voltage is low.

I used the second method in my battery box to prevent overdischarging the batteries: when the voltage is above 1.2V / cell (4.8V for the pack) the output is connected. Then, when the voltage drops below about 0.9V/cell or 3.6V for the pack, the output is disconnected, until the solar panel charges the batteries sufficiently again. In this way you do not need to wurry about overdischarging.

I attached the data sheet for the batteries used here, which shows the charge and discharge characteristics.

Step 5: The Charger/battery Box (2):

Picture of The Charger/battery Box (2):

Now build the battery box.

Drill holes for the connectors, one coming from the solar panel and one to go to your gizmos.
I used the same type of connector for both, in hindsight it's better to use different types.
Prepare box for battery holder if necessary.
The batteries go in a battery holder with a slide door, for which a rectangular hole in the side of the box had to be cut.
Glue the circuit diagram in the lid of the box, in case you'll ever have to open it again.
Glue the circuit board in the box with hot melt glue (or use any other method you like).
Label the box "in" for solar panel and "out" for loads.

The circuit is built on a piece of perfboard (note the MOSFETs I used are SMD and placed on the underside of the board). Nothing is crictical in this circuit, for all components you can use replacements:

- The diode can be any Schottky diode capable of something like 30V/ 0.5A.
- The p channel mosfet needs a low on voltage (Vsg<3.9V at currents you like to use it), it should also have low Rdson (<0.2 ohm or so) you can parallel them to increase current handling capability.
- The resistor from base to emitter of the BC547 is 62kohm, I forgot to label it.

Once the circuit is ready, connect a load (say 100 ohm).
Then using a power supply, go up slowly starting from zero volts. Adjust the trimmer so the circuit trips at 4.8V. Then go down again, the circuit should disconnect the load somewhere between 4.0V en 3.6V NOT LOWER. To adjust the lower threshold, you can also replace the 470kohm resistor with a trimmer if needed.

Step 6: Put the Box in Your Backpack and You're Done!

Picture of Put the Box in Your Backpack and You're Done!

Some more ideas/hints/tips:

- The solar panels aren't critical, just make sure V> Vdiode + 1.2V*number of batteries and Ishort circuit


jtbennett0119 (author)2014-11-29

I want to do this on my school backpack, so it must be durable. I am looking at the flexible solar panels from Silicon Solar. It looks like it would take more panels to reach your requirement of 800 mA. Is 800 necessary? Is there a better option for durable solar cells that are fairly accident proof for everyday use?

The_Black_Hole (author)2010-11-11

Ok I finally got the bank to build one of these, now i have a few noob questions for you here,
1. does this setup have a plugin for charging like via usb or anything? and if not could i add plugins and how easy would that be?
2. what would i have to to to make something like this that could charge a laptop?

basically i want to make something like this where i can carry my laptop, phone, mp3 player, and my psp and charge them. idk if its possible....HELP!!

Most likely you would not be able to charge a laptop because of the amount of power a laptop takes (over 50 watts). If you wanted USB plugs, you would just buy a 5v voltage regulator from somewhere like radioshack and a usb jack and wire it up to the battery. Most things like cell phones and mp3's can be charged by this, but it takes time.

lothar1511 (author)2011-07-10

Now thats a good idea!!!!!! haha thats my next project

mahesh.ganpate (author)2011-03-05

nice aswoom tech..

gabe009 (author)2010-12-09

Would look better if you incorporate the sun panles into the design of the backpack.

bikeboy (author)2010-04-06

I'm building solar charger circuit for GPS assisted trek in Spanish Pyrenees.
Made a "Mintyboost" circuit to keep pack of 3 1.2V AA's at 5V even when "drained" (To charge PDA and GPS receiver). Mintyboost IC steps up as low as 0.7V to 5V, so your circuit will be good addition if indeed deep discharging (0.7V) is not good for battery lifetime (although GP datasheet only mentions "prolonged charging time" after deep discharge).
If I understand your circuit, the lower transistor "opens" the NDS332P, which has a higher current capacity (?). But how much current would there be "lost" through the lower transistor? (after all, after Spain, i'll be back in cloudy Belgium... :)

Kajnjaps (author)bikeboy2010-04-08

Not much, more or less the battery voltage divided by 100k, since no (static) current flows through the gate of the mosfet.

lucidn (author)2008-08-12

Do you happen to have a larger version of the charge box schematic? It's a bit tough to understand.

Kajnjaps (author)lucidn2008-08-13

I'm sorry, I don't. It basically is a just schmitt trigger with two transistors. Let me know if you have a specific question I can help with.

lucidn (author)Kajnjaps2008-08-13

Here's a few questions How are the ND5332Ps hooked up? Does the gound refer to the ground of the battery, or the ground of the solar panels? And I'm a bit confused about how the panels are hooked up. And help would be much appreciated, as I'm designing a PCB for this.

Kajnjaps (author)lucidn2008-08-14

Hi Lucidn, the NDS332P (Fairchild Semiconductor) are in parallel, meaning gate to gate, source to source and drain to drain. You can always do this with mosfets to increase the current capability (note: don't do this with bipolar transistors, as they have a negative resistance temperature coefficient). The solar panels are in series so you end up with two wires, positive (red) and a negative (black) . The black wire is the ground for the system,so for batteries, the circuit and the solar panels. If you are going through the trouble of designing a pcb for this, it might be a good idea to include a variable resistor in series with the 1Mohm.

lucidn (author)Kajnjaps2008-08-14

Thanks, and I'll send you a jpg of the PCB once I'm done it. It shouldn't take me long.

Kajnjaps (author)lucidn2008-08-15

What program do you use to design the PCB? One more thing: you might want to use other mosfets as they will work well too, as long as they have sufficiently low turn on voltage. bye, K.

lucidn (author)Kajnjaps2008-08-15

Well after looking for a CAD program that's both free and for mac, I remembered I still had Appleworks, from before I upgraded to OSX. So I've been using that, designing the PCB and then shrinking it down to the right size.

lukaj2003 (author)lucidn2009-07-27

Yenka is a good program and its free if you just want it for personal use. It's really an educational tool, but the electronics features allow you to design, test and create PCBs ready for manufacture for circuits. You can get it at It has other things such as chemistry and physics and such, but the electronics stuff is what you might be interested in.

Kajnjaps (author)lukaj20032009-07-28

Thanks, I'll check it out!

lucidn (author)lucidn2008-08-24

Here's PCB, actual size. Unfortunately there's no guide to go with it, but it's not hard to figure out.

Sandisk1duo (author)lucidn2008-11-26

add names of components!

ScubaSteve (author)2009-07-19

what about Lithium-Ion (Li-ON) batteries?

Kajnjaps (author)ScubaSteve2009-07-20

much more difficult to charge/discharge safely...

grundisimo (author)2009-06-10

you should get a patent on this.

Kajnjaps (author)grundisimo2009-06-11

I don't think it is patentable; there is no inventiveness in it.
Most patents are just a waste of money anyway.
This text my eyes about patents.
highly recommended reading.

Kajnjaps (author)2009-06-06

not at all, go ahead.

McPie (author)2009-04-19

This is awesome! This is exactly what I was looking for!

Kajnjaps (author)McPie2009-04-22

Thanks, have fun!

ElectricMan1 (author)2009-03-29

You should have used flexible solar cells from Silicon Solar. They are so strong they can even be walked on! Great for your kind of project.

vtec2k7 (author)2009-02-05

What is the transistor you are using to the left of the 1Mohm resistor? I cant read your handwriting. I am returning to the semi-conductor scene after being out of the game since college 6 years ago. Is there anyway you can explain the parts needed exactly for this so i could build it. I need to get back into this and I think this project is the one for me. Sorry i know its asking alot because i know you have alot on your plate but it would be very much appreciated. Thank you sooooo much Dan M.

Kajnjaps (author)vtec2k72009-02-06

it's a BC547(B), common garden variety small signal transistor, you can replace it with almost anything.

vtec2k7 (author)Kajnjaps2009-02-06

the one that is labeled 4748? Its circled in red in the picture. I just cant read the writing above it. I just want to use the same exact parts as you so i know it will work. Also where is a good place to buy all this stuff? I found the solar panels for cheap but all these other components are going to be hard to find local because in New York they dont have these kinds of stores anymore. Only circuit city's and they suck.

edfel01 (author)vtec2k72009-03-18

Is there a radio shack?

Kajnjaps (author)vtec2k72009-02-06

sorry for the handwriting... Thats a 1N4148 diode, also quite common. There's a gazillion other types of diodes that should do the same. Funny, you're from THE New York and there's no electronics shop nearby. I live in some backward little town in some Euro country, that's probably smaller than most of you're landfills, and at least we have one shop where I can get parts.What is this world coming to... :-)

vtec2k7 (author)Kajnjaps2009-02-06

yeh this city has severely gone the way of the "Big Business". None of the big stores carry components anymore. Its purely a "if it doesnt work right just swap it out for a new board" mentality. So sad. Great instructable by the way. I have been checking out the site for so long and nothing except the bread making has been in my price range or great interest. This hit me right where i like it though so ill let you know how it goes.

vtec2k7 (author)vtec2k72009-02-06

also the Cap on the top left, is that a Polarized Cap or a trimmer?

Kajnjaps (author)vtec2k72009-02-06

100nF unpolarized, doesn't really matter, you could use polarized 1uF or something else, only there for "good practice".

edfel01 (author)2009-03-18

this is a really cool instructable

BrettAlive (author)2009-03-08

I am going to attempt to make this setup on my daypack/bike pack for charging my MP3 player, and integrate another Instructable for on-board lighting for night hiking or bicycling. I found the cells for 8 dollars American each. Good show mate. I like the project. -Brett

Kajnjaps (author)BrettAlive2009-03-09

Thanks! Post some pictures when you're done.

DIYDragon (author)2008-12-17

I need to get into soldering so I can start doing cool stuff like this.. neat instructable. Favorited. : )

Kajnjaps (author)DIYDragon2008-12-17

Thanks, one day, I should write an Instructable on getting started with electronics. I guess many people would like to learn, but get scared off by the 'difficult' theory. Also nowadays there is such an overwhelming amount of information available that newcomers don't know where to start. My advise: take one step at a time. Don't run off to build a time-vortex-black hole machine before you can , for instance, calculate the correct resistor to use for a LED.

romedeiros1970 (author)Kajnjaps2009-01-07

Do you have an instructible for a time-vortex-black hole machine? I have been looking for one for some time... I would appreciate the beginner's guide too.

I think you're wrong. If you really are as determined in finding a time-vortex-black hole machine as you seem to be, how do you know you won't find one in the future? If so, you would not need to worry about it right now, because you've got a time-vortex-black hole machine in your future

Simple: If I am going to have a time-vortex-black hole machine in the future, I am going to go back in time and warn me about certain girls in my current past. If that had happened, I would not have met those she-devils, so I know what my future does not hold. Sadly, that also means that I will search and wait, but no Instructible will come to the rescue...

Kajnjaps (author)romedeiros19702009-01-08

yes, a beginners electronics Instructable would be a good idea, just to point people in the right direction. Unfortunately I don't have time to write one. Anyone else?

Rossiroller (author)Kajnjaps2008-12-18

Do it! That would be insanely helpful!

duckythescientist (author)2009-02-04

Question... In step 6, Why does the current need to be less than C/10 for the solar panels?

as long as the current is less than about C/10, there is no chance of overcharging the batteries. In that way, a simple diode charger is safe.

Ok, It makes more sense now, thanks!

fultron89 (author)2009-02-03

How much do solar panels like this run for, and where's the best place to get them?

Kajnjaps (author)fultron892009-02-03

If I remember correctly, they were between 5 and 10 euros a panel, but it all depends on quantity, what country you're in etc. I found them at a local electronics shop. surf around, you might find similar parts cheaper by mail order.

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