High quality 2W 6V solar panel by Voltaic Systems or other panel you wish to use
Adafruit Solar lithium charge controller
5000mAh 3.7V lithium polymer battery
DC to DC voltage regulator, 5V 1A from eBay
3.7V single cell lithium protection board from eBay
small plastic rocker switch
several JST 2pin connectors or similar
5mm yellow LED
100 ohm resistor
1N5817 or similar diode
3mm lock nuts
Barrel plug that fits the adafruit charger board
USB type A male plug
For the case, I designed and fabricated it using my Ultimaker 3D printer. I think the case is quite compact and the whole thing looks store bought (neglecting the standard switches and LEDs).
-It lacks a full charge indicator. I was too lazy to add one.
-No thermistor to monitor battery temperature. I am relying on the bubble wrap to insulate the battery from heat. You can add a thermistor to adafruit's solar charger board to better protect the battery from blowing up.
-The eBay parts are probably unreliable. the over discharge protection from the lithium protection board is most likely insufficient to keep the battery healthy if you drain it completely. The DC step up regulator is also running quite hot and therefore inefficient.
- The real output of the solar panel is about 1.4W at best, as demonstrated by Voltaic Systems themselves in one of their blog posts. On a sunny day, ideally, the battery will charge from 0-100% in 13 hours. On a cloudy day.., the battery will charge from 0-100% in... never. Imagine how long it would take an altoids solar charger(commonly posted in instructables) using a 1W panel to fill a decently sized battery(~2000mAh) completely.
Its not really practical to rely on this gadget to charge in the sun, but it was a great little project to test my skills at integrating the different parts together. Do notice that I have taken into account the importance of mounting the hardware properly within the case. Whenever I had the option to use fasteners, i use them.. if not, hot glue is the best alternative. I have also designed walls within the case to isolate the parts to prevent shorting.
In case you haven't noticed, the solar panel screws into the wall of the case, and the wall of the case is fastened to the base which holds the electronics.
Next up, the wiring.
Step 2: Overview of Wiring
Overview: USB, DC, & Solar Lipoly Charger
You can read more about the charger, its capabilities, limitations there. They also have really good pics of assembling the solar charger, compared to mine.
My simple schematic of the charger is shown above. The terminal positions shown in the schematic are not representative of the positions on the actual components. There will be detailed pictures of the connections in the next few steps of this instructable.
Self-explanatory, right? Panel feeds power into charger board, charger board regulates power into battery. Charger board sends battery power / panel power into Boost converter via a switch.
A standard 5mm round LED with a 100 ohm resistor is connected to the charger board "Charging" indicator port. The port sinks current when charging, so i connected the +ve of the LED to the panel, and the -ve to the port. I used an amber LED as my charging indicator; you can use any colour you want.
The switch between the boost converter and the charger board is to prevent quiescent current being drawn by the boost converter. Call me paranoid or what, but I don't want my lithium polymer cell to get discharged to unsafe levels. Especially if i leave my charger at the corner of my room for months.
Step 3: Solar Panel
Notice that I pasted a piece of foam-foil insulating layer behind the panel. I got these foam-foils from a dollar store. Not an expert on heat transfer (I returned most of my knowledge to my professor), but I believe it will help a little with insulating the circuit behind the panel from heat. Solar panels can get pretty hot.
I assume you have the skill to connect wires and protect them. Its not exactly clear in the close-up of the connector, but you may notice a bulge in the red heat shrink. There is a 1N5817 diode within it. Supposedly it is to prevent backflow of current from the board to the panel. But I thought the charger chip should be smart enough to do that..
Did I forget to mention what connector I used? It is a cheapy "JST" 2 pin connector that you can buy from eBay. I use them for many projects; they are super useful.
Step 4: Protection Board for Lithium Battery
The lithium polymer battery i used in this instructable is meant for RC hobby. It is able to provide 20C of discharge current, which means 20 * 5000mAh = 100A of current!! If you are using RC batteries, do take note that they don't have protection boards, fuses, whatever. Never short them! If you are clumsy and tend to fumble about with electronics, I recommend you to use low discharge lithium batteries. I think you can get them off eBay; they are usually wired with a little connector. The seller might mention the existence of the protection board.
With the protection board, the cell is supposed to be short circuit-proof (don't take my word for it) from the protection board output terminals onwards. Notice the kapton tape covering the battery terminals and the protection board? That is to prevent accidental shorting of the terminals/contacts.
Also, the protection board is supposed to prevent over discharge of the cell by cutting it off at about 2.8V. Still, 2.8V is not a healthy voltage for lithium cells, I recommend you to monitor how many times you will use it to charge your cell phone. A full 3.7V 5000mAh cell has about 18.5Wh of energy; if you use a large smartphone, you should be able to fully charge your phone fully 0% and still have a little more left in the lithium battery (after accounting for losses).
The board has "common" +ve terminals; the +ve of the battery and the +ve output are both connected to this terminal. Look at the photos for more detail. Connections on other protection boards may be different.
Step 5: Solar Charger Board
Official documentation can be found here and here.
Of course, if you are using some other charger board, you will have to figure out which ports are for what.
Step 6: Boost Regulator and Switch
Next up: How I designed the case.
Step 7: Done!
I tried to reduce the number of components I needed to make this charger, especially the case. The 2W panel had a very tough aluminium composite backing, eliminating the need for me to design and print a lid for the case. The panel also had mounting bolts on each of the four corners, allowing me to secure the panel to the rest of the case.
There are only two parts to the case: the walls(where the ports and switches are located) and the bottom. By measuring the thickness of the thickest electronic component, I determined the minimum height of the wall. Internally, I made little walls surrounding the major electronic components and hot glued them in place.
The solar charger board is screwed onto "stand-offs". The charger board has mounting holes for that. The lithium polymer cell is fit snugly between walls to keep it in place. There is also some bubble wrap on top of the cell to insulate and absorb shock.
Unfortunately I didn't design this charger to be splashproof or waterproof at all. Maybe if you were to make something like this, you could use rubber gaskets or silicone or whatever to seal it.
Its a little abrupt, but I have reached the end of this instructable. Do leave your comments or suggestions! Thanks for reading!