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Solar powered cell phone charger case hack Answered

After finding solar chargers and DYI solar videos I had an idea...

I'm getting an HTC Evo soon. This phone has a lot of screen real estate. It's going to require a decent sized hip case. Why can't I slap a couple solar panels on the outside of the case with a small USB cable plugged into the phone while it's resting on my belt? I want to take advantage of the size of this thing and have my own hacked solar case.

So here's my idea. I want to grab a couple panels out of some all-weather garden lights. Mount two solar panels to the outside of a good case when I find the right one. Then add a 5V regulator wired to a mini-USB plug. When I put my phone in it's pouch I could just plug the end of the short wire into the phone to charge it, or at least help it keep a charge while I'm out and about without having to worry about carrying around a solar kit to pull out and pay attention to.

The reason I need advice. I don't know much of anything about solar panels other than what I've heard in other DYI tutorials. I'm not an electrician so I don't know much about the power requirements of the phone outside of standard USB voltage.

What would I need for amperage requirements? Is a 5V regulator enough to ensure no damage to the phone? The phone will only pull as much amperage as it needs as far as I understand it. Will the phone safely stop charging when it's full or will I blow up my battery?

I know they make cases for iPhones like this for $80+. I think I can build one for ~$20 save for the actual case itself.

Any help would be appreciated. Thanks

A little bit of clarification on my project. I've looked at a lot of other Instructable projects as well as ones all over Google. I've got a lot of good info from them, and I know I *can* do this, but because I'm trying to charge such an expensive device I want to make sure I have every little detail worked out. I've read warnings about issues with part of a solar cell being in the shade (wasn't detailed on why). With my scope of electrical understanding, I would feel comfortable following these guides if I was doing the exact same project with the same amount of panels, but I'm going to have to change things up a little bit mainly due to space and weight constraints since this is going to be mounted on my hip.

The biggest requirement here is that the panels produce enough power to charge a USB device without having to trickle charge a separate battery first. I won't have the space on my cell phone pouch for an extra mounted battery. The entirety of this device should be no more than the pouch itself, one or two solar panels and a few wires. The main concern here is getting the amperage I need to charge the phone faster than it drains with only one or two panels. 

I'm not looking for step by step instruction here, just a bit of advice from someone who knows a thing or two about PV to give me some tips on solar panel size and power. Also any safety precautions I need to take to keep from causing electrical damage to my phone.


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Dear HittingSmoke,

It's not as easy as you sound. People around the world have been doing that for their gadgets, but most of them hit the walls. Why?
1) Low efficiency of solar cells - at 18% under strong sunlight, what about room light? 1-3% is already premium. Don't count wattage, even voltage and current are both not high enough to charge the phone. What would a normal person respond to that matter? Sunbath the phone, really? After a few times of sunbathing the phone, we found that it will harm the phone permanently with such scorching temperatures under the sun, it proves that you need to separate the solar cells from the phone to be charged individually.
2) Now voltage not enough, the next thing is to get a DC/DC boost converter. A lot of people will happily buy one from the net, and found that using the voltage from solar cells even under strong sunlight, the output part of the converter can't get to 5v, why? People start to realise the oscillator-powered converter needs at least 0.8v and maybe 100mA to sustain power-up. And the small little area of solar cells connected which give that 0.8 x 0.1 = 80mW of power won't start-up the boost converter, failure.
3) They survey the web for lower power converters, yeah we saw 0.5v ones, 0.3v ones, but they are either of 8 pins or not marketed yet, With 8-pins or 6-pins, you need electronic skills to assemble into workable units.
4) Ok, now we can see the lowest ready-to-use converter is at 0.5v (lower voltages means minimal start-up), buy it. With one panel of solar cells, it still can't power it up. Try better cells like monocrystalline or polycrystalline cells but they really need thick sturdy base and design because the cells are extremely fragile like glass. One cell of the size of the phone is yet to enough, last try: Get two and make it foldable. It works, so everytime you lay your phone on the table, you open up the folded cells to bath two solar cells in light.

Comments on converter: go for high efficiency converter that operates at 90% and up. Less power is wasted then.

Circuit: The most efficient way to charge a battery is directly connect a solar cell to the battery, but since the cell voltages is too low, you need a boost converter to boost the voltage of solar cell to charge that battery (5v usb in, although you can try to charge directly to the battery at 3.7v). When the solar cell is not operating, a backflow current from the battery to 'the converter and the solar cells' can drain your phone battery. You need a small diode then.

Solar cells: Get quality solar cells and connect them in series to obtain higher voltage, but this will also increase the circuit resistance. As long as under room light you can minimally power up your converter using series-connected cells, then further extension is to connect your cells in parallel to increase current.

To make your charger charge fast, the best way is to invest in more solar cells. So everytime you charge your phone, you open up more solar plates. Haha, you will only probably use it only when the phone battery is depleting, not convenient though. Since you need to open up the folded flaps everytime you need to charge, why not you separate the solar charger from the phone, bring the charger always, and use it when needed?

If you think you don't need to be so extreme about extracting energy from your environment, you can think of upgrade your phone battery to a bigger capacity ones, or you can do it manually by attaching 2, 3, 4, 5 batteries on top of each other and connect their leads in parallel. By that way, you double, triple the capacity of your battery packs.

If you do an I'bles search for "solar USB" the first four hits are Instructables that tell you exactly what to do. Just take what they show you, and adapt the layout to the case you want to use.

The USB spec is 5V 500 mA, which is easily achieved with PV. I'm not even sure you need to worry about a regulator.  The phone will stop charging when the battery is full.  USB doesn't deal with that; it's up to the phone.

Thanks. I did a very thorough search of Instructables, Google (which turned up some Metacafe WikiHow's) and YouTube. There was a LOT of results but none of them described in detail enough to venture off on my own project, only to follow their directions specifically. This is an expensive device. I'd rather ask detailed questions and get detailed answers than play a guessing game.

What I'm doing is extremely sensitive to space taken up by the solar panels and their durability. I also need these to produce enough power to charge the device directly instead of charging batteries. All of the DYI's here with small enough panels seem to suggest you can't charge directly with anything but something fairly big.

One I saw on Metacafe was designed to charge directly with four panels, but I could only use a maximum of half that attached to my case.

Does anyone who has experience with different solar panel types have any tips for where to look for the right size panels I'd need or is my original plan of chopping up garden lights doable?

Thanks for all the additional details!  You might consider pasting some of this text into the original comment.  It'll help others who read your topic to understand where you've already gone, and what level of information you really want.

In order for me to answer your actual questions, I'll need to think a little :-)  There are users on the site who have a lot of expertise with PV.  If they see this topic, they may give you good information faster than I can.

Have you read the Wikipedia article "Solar cell"?  The theory of operation section there is excellent (for me, that means heavy on the math :-), and suggests to me that you're going to be better off being experimental -- build something, measure the output, and tweak until you get it right.  If you use enough PVs to get 6.5V out, then a 5V regulator should be fine.

By the way, there are a lot of "tell me how to do this" postings on I'bles, where the writer has done no research, or even search, at all.  Sorry I lumped you into that category!

Thanks for the tips, I'll read that article now. Wikipedia isn't something that occurred to me as a resource. Most of the articles written about things like this are a little over my head. Math isn't my strong point and I know only enough about electricity to do things like PC volt mods :) I updated my original post with more details from my reply. I understand, I know most people don't search before posting on forums in general.

Good luck, and don't hesitate to come back (either here, or in a separate topic or Question) if you run into specific issues there that you don't understand. The math they present is "basic" (to me, a professional physicist :-) ), and is not necessarily easy to apply to an actual build.

The easiest way to think of a PV in your circuit is just a battery.  It's a bit of a complicated battery, as both the voltage and the current output depends on the circuit you have connected to it.  Once you get past that weirdness, it's otherwise pretty straightforward.

I checked out the Wikipedia article, and I'm afraid most of that is far over my head. I'm looking around the net at what types of solar cells are available and I'm primarily focusing on the amp output. Unfortunately I can't find anything close to what I'm looking for.

This is the video I was watching that I was using as my primary reference. Those solar cells are the perfect size for my project and are durable. I need something about that size that will give me the power output I need. Everything I've found on Ebay and other sites seem to be either DYI kits for people who know how to build the panels themselves, or expensive all-in-one charging kits with a battery attached.

All of the detailed tutorials I can find focus on much larger scale projects than I'm trying to do. Things like full house systems.

I'm afraid I may just have to go to the hardware store and check out those lamps and hope they are powerful enough :(

As I understand it, as voltage increases amperage decreases? How would I wire these in a way that I ensure an acceptable voltage while providing enough amperage to charge? From what I just read, most USB devices will charge just fine at ~4V as long as the amperage is adequate.