How long does your cellphone's battery keep the charge? I guess not for long time... and your GPS? Do you have a spare battery to not lose yourself in the woods? A solution is to buy a solar charger, they're cheap nowadays, but not many of them allow you to power your device during the charging process. Indeed this is an effective solution in many cases, and saves you from using a specific battery. Therefore, what are we waiting? Let's build it!
Step 1: Solar cells
You can buy different types of solar cells: very cheap 2V tiny or huge and more expensive 5V 700 mAh panels. Also halfway choices are good. You need to obtain about 4-5V output voltage from them (a voltage up to 6V doesn't hurt), so if you choose the tiny cells you can connect two of them in series, with sun light they will provide 5V, although the amperage will remain very low. Scissors are there only as size comparison, don't cut the bigger solar cell to obtain many little ones!!
[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
As charger circuit you also have multiple choices. You can find circuits to charge one or more cells, with or without charge indicators (leds), and with many shapes and dimensions. Some modules only charge the battery, but others also provide +5V output voltage from the battery (usually 3.7V), pay attention to buy one this last type.
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):
Step 3: Batteries
As battery I suggest an 18650 Li-ion cell. These are very handy, powerful, cheap and common. Also buy a clip holder. Anyway you can chose to buy a flat Li-ion 3.7V battery for cellphones, although those are not powerful as 18650 cells.
Step 4: Soft-power
To start the circuit you have to soft-power it. That means to provide +5V to DC input with battery connected. Then you can disconnect +5V and check that the output provides +5V. To soft-power the device the solar cell can be already connected or not (to the same DC input pads), it doesn't matter.
Step 5: Prototypes
Here I tested my two prototypes. The bigger solar cell is able to charge the battery with no direct sun light, day light is enough, even in a cloudy day. With little cells you need exposing them directly at the sun. My GPS is here connected to the solar PSU and it has no internal batteries, this is pretty useful when you otherwise have buy expensive specific Li-ion cells for a device equipped with USB plug (as a modern phone or a digital camera).
Step 6: The hub
Now letís find a proper, easy, and cheap of course, way to connect devices to the charger. As you already saw I used an USB female socket to connect an USB cable to the bigger solar cell, but that was a prototype, now I want something more elegant and maybe with more than a single connection.
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$. Thatís 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 letís open it to see if there is enough space.
Step 7: Some desoldering and salvage
It probably will fit, so we can begin adapting our bus. Letís cut out the USB cable and unsolder the DC-IN socket and the power blue led, which otherwise would slowly discharge our battery. We now also have obtained a few free useful components ;-)
Before cutting the cable look where the black wire is soldered, that is the ground (-), and itís 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
Then cut the trace which bring electricity to the hub's IC, since it consumes power and we don't want that. As you see in the picture, the big trace I cut is the only positive power supply to the black plastic drop which contains the hub's brain, interrupting it you'll avoid any power waste.
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
I then made two couples of holes in the plastic container to let wires passing through. Use soft narrow short wires, because they have to stay inside with everything else, mine are a bit too long. After inserting wires in the holes solder each wireís end to the right pad.
Step 10: Attach solar panels
Using double side tape, attach the pair of solar cells to the plastic surface on the side of the USB bus.
Step 11: Accommodate the pcb
Now search a place where to put the pcbÖ I thought it was simpler, I had to change three different positions. Closing the cover the pcb pushed against the components. Finally I found the best position, between switches and USB sockets. A double face tape avoid the pcb touches the metal of the sockets. Before closing remember to soft-power the device as I explained before.
Step 12: Complete with battery (holder)
Closing the cover we keep the battery wires out. Now a 18650 clip holder is neededÖ mine is travelling from China to Italy together with his nine mates. So you will find here an update in a month or a bit more. For now letís use a rubber band (from a bicycle tire) to keep wires connected and letís attach battery with a piece of double face tape.
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
Blue LEDs appear violetÖ maybe Iím color-blind and my camera has the right senseÖ anyway, to see if the solar charger works, measure the voltage of the battery with your multimeter, expose the solar cells to the sun light, wait a few hours, then check the voltage again, it should reach about 4.1V to be full charged. For more skeptical persons I added an image showing the GPS powered and three more devices being charged (two databanks and a GPS tracker).
Now enjoy and be sure to not enter a too dark wood ;-)
Step 14: [UPDATE]
After the right concern of many readers about the very low power (about 150 mA if I'm not wrong) of these tiny solar panels, I decided to improve my solar psu doubling the solar cells. Indeed now two more cells are connected in parallel with the first two. Output is still 4-5V but currend is double than before, so charging time (referring to the Li-ion battery under the solar cells) is half than before.
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.