Step 1: Solar cells
[UPDATE NOTE: as I said you can choose between different sizes and prices of solar panels, and if you want a charger which charges the battery in few hours in direct sunlight, or that charges in shadow too even though slower, you have to spend more and having a bigger surface and weight]
Step 2: Charger modules
In this instructable you see two similar PCB, one has two leds to show charging process (red led) and full charge (green led), other one has no leds and is a bit cheaper. Both are meant to charge a single cell, although you can in theory connect two cells in parallel, but I don't suggest it because that will reduce their life. They also both provide +5V output.
These battery charger modules usually have three pairs of pads for connections, for all three couples of wires beware to respect the polarity (don't follow my example, try to use black and red wires):
- DC input (also marked as "charge", "charging port", "P+ and P-", "3.7V+ and 3.7V-") where you have to connect your solar cell(s), this usually accepts from 3.7 to 6V or more.
- 5V DC output (also marked as "increasing output" or "V+ and V-") where you will connect your device, through an USB or another power female plug, as you see in picture this voltage is very precise.
- Battery connections (usually marked as "B+ and B-"), here connect your battery (or better your battery holder), you can also try to connect two or more batteries in parallel, but I don't suggest it.
Step 3: Batteries
Step 4: Soft-power
Step 5: Prototypes
Step 6: The hub
This is an USB hub with four USB 2.0 (you don't need 3.0 for this project, actually 1.0 is good enough) ports, each one with switch and blue led, all this for only 4$. That’s perfect! I see that there is also a +5V DC-IN socket to receive an external power source. That would be fantastic, we neither need to open it! But I want to put my tiny pcb inside it, so let’s open it to see if there is enough space.
Step 7: Some desoldering and salvage
Before cutting the cable look where the black wire is soldered, that is the ground (-), and it’s connected with the ground pin of the DC-IN socket, we need to know where to solder the wires from the charger.
Step 8: Avoiding power dissipation
Beside I've discovered it's better interrupting the signal traces that come out from the black IC (the plastic drop) and arrive at second and third USB pins. I had some difficulty to power some device before doing that.
You can also decide to replace blue leds with similar red ones (which use less current) or to remove them completely to save energy from the battery, after all this solar psu has to be as much efficient as possible.
Step 9: Wire them all
Step 10: Attach solar panels
Step 11: Accommodate the pcb
Step 12: Complete with battery (holder)
Maybe that pcb will reset when you take off or disconnect the battery. In that case you have to soft-power it again, notice that DC input wires are accessible from the narrow slot between solar cells and container, try to keep an easy accessibility to them, otherwise you have to open the case. Also remember to mark the wires so to know which one is positive and which is negative. Yes I know, much better to use red and black wires... :-P
Step 13: Enjoy the sun
Now enjoy and be sure to not enter a too dark wood ;-)
Step 14: [UPDATE]
I also glued pivots between panels and case, so that the solar charger is foldable.
I've still to add the battery socket, but unfortunately shipping time is very long.