Small solar charger (backpack style)

I am in the process of building a very small solar charger using 3 ebay 6 volt panels (approx. 135 x 110 mm each). The theoretical output is in the area of 6 volts and 900 or so ma.

This is not something I need but if it works as planned I’ll likely make a few to give away for gifts. I already have a grid tied system (3.2 kw of panels) and a smaller system of about 245 watts of 12 volt panels that runs a 12 volt to 120 v inverter to charge all of our electronics and run our computer system.

So far I’ve wired up the 3 - 6 volt panels in parallel and in full sun have been able to easily power a small Micro USB TP4056 charger to charge 18650 batteries, one at a time. I’m awaiting a few different pieces including female usb sockets and a ml 102 charger (I gave away the one I had). I suspect that in order to power cell phones and maybe tablets the ml102 will have to be used as a power supply for consistent usb voltage (5 volts).

A few “discoveries”:

- hot melt glue softens in the sun enough that it won’t hold the panels to a temporary cardboard backing and won’t hold wiring in place either.

  • the small usb chargers tolerate a wide range of voltage.

I’ll post a few photos a bit later. In the meantime if anyone knows of a charger/power supply based upon AA nimh batteries (has to be able to accept a charge and at the same time provide usb power) I’d appreciate knowing about it.


I always wanted to make a solar 18650 charger, It would be nice to see the pictures.

One photo for now showing my temporary cardboard mount along with the very unfinished wiring. The panels are wired in parallel with silicon wire (22 gauge I believe) and two schottky diodes are wired in series in the red (positive) wire to prevent the panels from drawing power from the charger and to reduce the voltage a bit. I've been experimenting with the diodes to see how many I need to keep the voltage near 5 volts. One is all that's needed to prevent power going back to the panels.

The gray wire (one end on the right of the photo and the other near the charger) is a usb lead with the regular male end cut off and a mini usb (?) end to plug into the charger. The red wire inside the gray one is wired to red from the panel and the black is wired to the black on the panel. I've bent the other wires to one side as at this point I'm not worried about trying to charge phones apple products etc. This will be replaced with a short wire from the panels with a female usb socket, once they arrive.

The charger shown above is based on the little usb boards - double sided taped to the aluminum heat sink and beside it is a small 2 wire voltmeter. Very simple and inexpensive and works well.

I have a sanyo usb charger for nimh aa's and aaa's. It should be a good companion to the panels but may need the ml104 to "buffer" the power and voltage from the panels. Not sure if it can take much variation in voltage and don't really want to find "the magic smoke" as it is a really handy travel charger. They aren't very easy to find now although I have seen a similar panasonic charger.


Nice…adafruit has a really cool charge controller that can charge a Li Ion battery and charge other devices all in one

Well maybe not a backpack…but the bag doesn’t matter…

Good luck…give us pics when completed…very cool project!

Hi John

I've been messing about with four similar little panels 5V, 400mA I bought from IO about 18 months ago, as well as several other solar charging projects. You certainly seem to have pretty good grasp of this stuff. Anyway - some observations or tips you may (or may not) find useful:

  • Most Li-Ion USB chargers will function even in low light and with days of passing cloud, though some of them may not restart if the cell is over 4.05-4.1V.

  • "Smart" NiMH chargers seem to need a good, reliable power supply. The termination detection circuitry cannot function correctly with an intermittent supply. These are dumb chargers but work well and continue to supply charge to the cell even on days of very low light. They are not fast but handy to get a bit of charge into a AA on a cloudy day. The charge rate of 160mA is low enough that they wont damage a good quality AA even if left overcharging for some time.

  • Your Sanyo USB charger (no longer made) is a very good little unit. I have several. Note that it will not charge sucessfully unless you have consistent, strong light. The indicator light will flicker unevenly and from all of my experimenting and searching of threads here and at CPF I've concluded it doesn't put any worthwhile charge into a AA in this state.
    You are correct in needing a "buffer" between the solar panel and the Sanyo charger. I typically use one of my USB battery boxes or a ML-102 with a laptop pull inside.
    The trouble with this is the amount of "stuff" that you end up having lying out in the sun to shift about, though I guess you could always make the ML-102 a permanent fixture on any finished backpack-type design.

  • As to finding a device to charge and power from 4AA, I will be very interested in this search myself. The only one I know of so far is the Goal Zero Guide 10+. I bought one with the Nomad 7 solar folder last year from an online deals site. At the RRP they are way overpriced IMO but I have to say I have used this a lot and it works VERY well. Apple devices are compatible - it will even put charge into my iPad 4, though obviously not at the full rate. Certainly one to read up on.

  • If you been there yet, do a seach for Cottonpickers at CPFMarketplace. David makes a variety of solar folders and matching USB chargers along very similar lines to what you are playing with. Even if you don't want to purchase any of his products, there is a wealth of information in his threads. He is a very helpful guy and has a loyal customer base.

  • One of the bits of info I did pick up from Cottonpickers is that you could simply wire a 4AA battery box with USB output (available in many electronics stores) direct to your panels with a diode to prevent reverse/discharge when it gets dark.
    In this arrangement there is no need for any charging "smarts" - just wire the cells in the series pack direct to the panel. The current will drop as the overall voltage rises, to the point where you won't cook your cells at such a low charge rate. The downside is that your AAs will probably never reach absolutely full charge and this won't be a quick charge solution. On the other hand, this will give you your "buffer" to run the Sanyo USB charger. There would be no Apple charging in this case unless you hack it with an aftermarket adapter board like you have in your post.

  • Don't go too thin on the wires running from the panels (especially to the USB port). Something a little beefier will have lower resistance which is important when dealing with such low outputs. Also, for a backpack style arrangement, the thin wires inside most USB leads are easily damaged once the outer protective casing is stripped away.

  • Lastly, I have found (using a multimeter and charger doctor) that my panels drop in output significantly when the temperature rises. If you are getting them hot enough to melt hot glue, then you are losing output. Perhahps think if there is any way to provide some ventilation in your final design. Something similar to raising them off the cardboard a little or cutting a few holes in the backing. Try to place them where you can get at least a little airflow.

Good luck. I'll be following this thread with interest.

Warhawk - thanks for the links. I do have an older (version 2) mintyboost (adafruit) but it will only supply usb power, not provide a charge to the aa batteries. Will look at your links in a bit.

Gadabout - thanks for the input. One thing I will mention is that I’ve been through a lot of information provided by CottonPicker’s about his chargers. As you say he is very generous with information and it is appreciated.

Anyone thinking about a small charger may be better off buying on of the Cottonpicker’s chargers rather than making one. His prices are very reasonable and they are well tested. He also provides lots of options and chargers to go with it.

The small aa charger from fasttech looks good. I was not aware of it and the price is right.

The wire I used was the most flexible on hand. Will likely change it out if I can find something a little beefier. You are certainly correct regarding heat. My big system will produce the best power on a cold sunny day - much better than on a hot sunny day. Unfortunately all the solar panels I’ve seen are a dark colour so heat is a big factor.

I’m not yet set on how to mount/back the panels. I’m not necessarily building this for backpacking - the two people I have in mind to provide them too are likely to use them near their home or whilst camping out of a truck and camper or a trailer. Weight is unlikely to be a problem although making it simple and easy to put together is a priority.

Thanks again for the ideas and thoughts,


I didn’t name this very well in the first post - see comments above regarding it’s intended use. Whatever the outcome I will try to keep it compact and light so it may still be applicable for backpacking but perhaps not optimized for that use.


Great solar charging info and tips in this thread, thanks for starting.

I would sure look to 0.032” AL plate or some 1/8” ABS as a backer, or even a combination thereof to create an enclosure. Lots of AL shapes and bar to be had on ebay, Amazon, or the hobby store.

No long term usage yet, but my solar rechargeable flashlite project seems to work fine with 100% silicone window and door caulk, for securing the mini solar panels to an AL backplate.


I’m actually considering a loop of paracord or webbing secured to each panel with some kind of glue and then a male/female connector to join each one. That way a small divided fabric bag could be used to hold the setup when not in use but it would be easy to put it together and use it - the paracord or whatever would help to secure it while in the sun.

Folding three panels together seems to stress the wiring and I’m beginning to think that going with two slightly larger panels would have been a better idea. Can’t recall if I saw any 6 volt panels putting out about 500 ma each but they are likely available.

Not sure if the thin aluminum would “heat sink” the panels enough to matter (if that’s what you were suggesting). Perhaps having them with open backs for ventilation would be better?

Thanks for the suggestion about silicon caulking - I have some so will give it a try. I have lots of bow making epoxy that I can try but it will soften with high heat - using heat to cure it will help in this regard. I believe Cottonpickers uses crazy glue so that’s another option. Unfortunately there are no borders on the panels so it isn’t easy to use a mechanical fastening system.



I only mentioned the AL and ABS because they are light weight materials, and you suggested that you hadn’t fixed on a backing solution yet. The panels that I used were not fully encased to the elements, and needed to be sealed up to something on the back side. I don’t think the AL backing would make much difference as far as shedding heat.

If you go bifold then something akin to a fly box could hold everything (batteries, electronics, and solar panels).

EDIT: Some mini solar panels

I like where you are going with this. Please keep coming back and updating. I have one of the cottonpicker rigs. It works well. I had to make some small tweaks for my intended use. You can tell his are home made. But it works like a champ. Especially if you like to travel & camp.
I too have some larger home panels that I play with. Not grid tied gear. Just run radios and such.

But, keep us posted & curious to see what others can bring to the thread.

Might be a misscommunication…the mintyboost takes the Li Ion voltage and steps it up to 5vdc, using a NiMH USB charger you can then charge NiMH, the charge controller charges a Li Ion battery, so while the minyboost makes 5vdc output the solar charge controller ensures the Li Ion battery in system stays charged, its the regulated stable 5vdc output that is used to charge other things

Big list

I guess the adafruit thing could be considered a “large capacity” storage device (depending on size of Li Ion battery) for solar energy…charge up the battery inside it, then use the 5vdc output for whatever you want, charge the AA/AAA NiMH, a cell phone, whatever you need 5vdc

I just re-read the OP, you want the ability to boost the NiMH up to a useable 5vdc…if you can get 3xof those AA in series it will produce 4.5vdc~ that will run the mintyboost

But you want the solar charger to charge those NiMH rather than a Li Ion correct?

Doing a search found a guy doing that NiMh battery charger project on CPF, link to his blog, link to ultrasmartcharger

Nickelflipper - I checked today and the silicon caulk is good for high temperatures (one tube I have says 205C (401F)) so that should be suitable. The flybox idea is interesting. A bit of searching may also reveal a chocolate box or small bisquet tin that would work. I'll likely make up a divided bag to protect the panels from getting scratched.

Warhawk - I think I understand now. The minty boost "solar kit" powers a lilon battery through a lilon charger and the output from the battery goes to the minty boost which brings it up to a steady 5 volt usb power. I could take apart my minty boost (remove the aa battery boxes) and use it. One of my small usb lilon chargers might be suitable instead of the adafruit version.

One reason i asked about an nimh power box was that the recipients of these kits are unlikely to know anything about lilon batteries and safety. If I could bypass the lilon battery and to go nimh directly it would be safer. However it does seem that lilon chargers are better able to cope with the varying voltage and amperage of a solar panel(s).

I do wish to keep the costs down in making this set up and also keep it simple so that little or no fabrication is required. In that regard using an ml102 along with an unprotected 18650 is inexpensive and simple. One can always use a usb powered charger plugged into the ml102 to charge a pair of nimh aa's, or charge a phone, tablet (possibly), mp3, etc.

I read through the thread on cpf about the ultrasmart charger - it was expressed there that nimh charging is more intolerant of voltage and amperage changes than that for lilon. I think that was the message however I'm not really electronically inclined so bear with me if I've got it wrong.

Still awaiting some parts so may not post more till I get them in.

Thanks for all the replies. John.

One thing I forgot. The backs of the panels are well sealed so backing isn’t required for protection. A bit of silicon on the solder points should be all that’s required. Will get a photo of the back as time permits.


Portable Folding Solar Panels from Fasttech with USB socket.

5 watt $ 25.22

7 watt $ 33.77

10 watt $ 47.31

15 watt $ 68.93

Just out of curiosity, are the solar panels you are using flexible?

Xv-750 - thanks for the info on fasttech’s solar panels. First time I’ve seen them. Likely not worth making one’s own with those prices. Anyway onward with what I have on hand.

Yellowhorse - the ones I have are not flexible. At the end of all this I will post links to the different parts and pieces that I’ve used.

See next post for progress.


I received my ml-102 charger/power box in the mail yesterday. Still awaiting the female usb sockets but I've worked around that by hacking the end off a usb extension cable.

I decided to make this system as light and versatile as possible. That meant no backing plate - another advantage of this is that it should keep the panels a bit cooler.

In order to provide mounting points for the panels and make them as useful as possible I decided to make webbing loops for each of them. You can see them below - the pieces of webbing have a loop sewn into each end. Disregard the blue elastic pieces - they were to be used for tucking in the connectors but it didn't work out.

I decided to use connectors for each panel as it makes it easy to add further panels and to store them. These were connectors I obtained locally - there may be better ones available elsewhere. They come apart fairly easily so should also protect the wiring from being ripped off the panels. The connection on the left is the female usb socket - hidden under shrink rap is one diode.

A view of the front:

The loops in each end of the webbing allow the use of paracord or a small clamp to hold the panels in place. The webbing adds very little weight and yet gives a lot of different mounting options. The loops also function in another fashion as shown below - they are a place to put the connectors when storing the panels:

A quick case for storing/packing the panels. I wanted something to protect the fronts from scratches and this was quick and easy.

A view with it all wrapped up:

The weight of the above package is a bit over 250 gms. I don't have an accurate scale for this range so that is an estimate based upon a kitchen scale.

This is almost finished except for:

- need to install another female usb outlet (and diode) on the 3rd panel. Thus in good sunlight one can be used to charge the ml-102 and the other to trickle charge an aa or aaa nimh or another 18650.

- am awaiting a usb doctor and plan to use it to see how the system performs.

- wiring needs to be glued down in a few spots to protect the solder joints. I used GE premium shower silicon to glue the straps down to the back of the panels and am unsure if it will work well over time.

- make another small case for the ml-102, usb extensions/wiring and a few feet of paracord. And one bigger case to hold the panels and the ml-102 package.


Here are a few sources/prices for the completed charger:

2W 6V 330mA Mini Solar Panel Module from Greenpoweronline seller ebay - $7.59 us each.
(Note that this seller carries other panel sizes - 2 of the 3.5 watt panels may be of interest - likely best to stay with 6 or 5.5 volt output).


- Type A Female USB 4 Pin Plug Socket Connector&Black Plastic Cover - $1.59 for 5 (ebay)

- 2.1mm x 5.5mm Female DC Power Plug Socket - $2.19 us for 10 (ebay)

- 5.5X2.1MM Male Electrical socket outlet DC-005 DC - $2.33 us for 10 (ebay)

- 1N5406 Diodes Rectifier 3A 600V - $1.59 us for 20 (ebay)

- silicon wire (I’d suggest a foot each of black and red 18 guage) - about $2.00 on ebay.

- 1 inch webbing - about 4 feet - check ebay, or local outdoor stores or salvage from thrift shop bag(s).

- material for bag(s) - check material in thrift stores or use what you have on hand.

  • buckle for 1 in. webbing or use some velcro or paracord to close bag.

- ml-102 lilon charger - fasttech or Intl-outdoor - about $9 us.

  • USB Mini Emergency Charger for Ni-MH AA AAA - ebay - under $2 us.


Just saw this on Fasttech

SKU 1523806

Takes and charges 4 AA via Mini-USB input, outputs up to 500mA via Std USB port. Specs seem to indicate charging of 2AA in series so you'd be best to use matched cells.

With current limitation I am guessing that this probably won't work with Apple products.

Looks like it might be usefully incorporated into some of these small solar projects. Does anyone know anything about this product yet?