Ah, solar power. The tingly feeling of getting free energy and saving the environment.
My HD2, the HTC phone/device I own at the moment, has its own reserve of four batteries to switch in case I need to make an intense use without a mains plug handy, yet I wasn’t satisfied as I didn’t have a renewable source of energy for charging it if those batteries died altogether, namely a decent solar panel; let’s face it, the solar chargers you see on eBay simply suck, they are built around teeny tiny solar cells capable of maybe 40mA @ 5V, and actually have an internal battery which does all the job, and that you’re supposed to charge at your mains at home before going out, because the builtin panel is going to take a couple of days in full sunlight to fill that battery, let alone being able to charge both backup battery, and phone battery, while you’re using your phone, which is actually the ideal usage scenario, as you’re going to need the backup battery when there is no sun, during the night.
My first attempt at building a solar charger was by using a 1W (5V@200mA) solar panel, that was exactly the same size of my HD2, but wasn’t enough in my opinion, since movie playback takes more than 200mA for itself, and there is nothing left to charge the phone. So I have that one set aside (and for sale, if you’re interested), but then I discovered this solar panel that outputs 700mA@5V, obviously bigger, but that is what I needed to playback movies, and at the same time charge both the phone battery and the charger internal backup battery.
So this page is a chronicle of my experience, and a DIY tutorial about building such contraption with the smallest expense possible.
First of all, the shopping list:
- Digital multimeter to check the project as it develops, to avoid discovering any mistakes when it’s too late
- Soldering iron
- Hot melt/hot glue gun, you wll be using at least a full stick of glue
- Solar panel (obviously), the bigger the better. You need a 5V model (higher voltage models are fine only if you add a voltage regulation circuit which I am not going to cover in here), and buy the one with the biggest current rating available, my 700mA one is pretty impressive in itself but you may be able to find better. You can go with lower output, but then it makes no sense to add a backup battery because it won’t be powerful enough to charge everything
- “Naked” female USB-A port, you can easily take it out off an old, not working motherboard, or hub
- Electrical wires, preferably in the standard colours red and black, but as you can see I used white and green since I only had those spare, as long as you remember which colour is which polarity (in my case, white is positive and green is negative)
- Two Schottky diodes (to stop the backwards flow of current to the solar panel and from the solar panel to the battery; you don’t need to buy them, just desolder off an old motherboard, an old phone charger, whatever
- A LiIon battery, my choice fell on a 18650 “Ultrafire” 3000mAh battery, but anything 3.7V with decent capacity is ok, really
- A protected charging circuit (a circuit that takes a variable voltage input and outputs static 4.2V, and has “flowback” protection, I got it by disassembling an OEM desktop charger for my old Nokia batteries); in alternative, just buy a “protected ultrafire”, costs more but delivers both things in less the hassle; in this case you will need just one diode.
Here is the step by step photoguide:
UPDATE (9/29/11): several visitors asked me how to change the procedure in order to build a vanilla solar chager with no backup battery; easily done: take the circuitry scheme above, ideally remove the part regarding the battery, the charging control PCB, and the cables coming and going to these, and you’re set.
UPDATE (11/4/11): mpigio in the comments pointed out a serious flaw in the electrical scheme: the protection circuit was useless, since the battery was directly in parallel with the solar panel; I just added a diode coming out of the battery, so this is finally safe.
On a side note, as other pointed out in the comments, instead of digging for a recharging protection, you may just buy a protected 18650, that is a battery that has its own integrated protection circuit.
UPDATE (11/9/11): adding here something I needed to clarify in the comments more than once. Obviously, with no sun whatsoever, it’s the backup battery that gives the output voltage: this will not be 5V like a normal USB port, but at max 4.2V, and down to 3.7V or even less. Now, most devices should still consider this as a valid charging voltage (my HD2 did) even if for technical reasons they won’t charge up to 100%, but someone in the comments reported about it not being enough for a GPS navigator, which didn’t consider enough even the 4.8V that four 1.2V AA rechargeable batteries in series gave; in this cases it’s needed a voltage regulator (of the “boost” or “step-up” kind) that can take lower, variable voltages and always outputs 5V, so you can connect it to the USB port. This component should be safe to be the last and only one connected to the USB port, so that both solar panel and backup battery converge into it, and only a regulated, constant 5V output is given from the port.
With this setup you can charge anything which has a USB charging cable, be it iphone, ipod, ipad, mp3 player, every htc phone… you can even attach a USB hub to it if the panel is powerful enough, and charge more than one device at the same time!
Since the desktop charger had its own charging plug, I can use a standard Nokia charger to charge the backup battery.