Solar Phone Charger - Page 1
Forum section: electronics / projects

author: Exergy
author_rank: Newbie
timestamp: April 10, 2014, 02:33:23 am
content: I've got a week long backpacking trip coming up and wanted to bring my Galaxy S3 instead of a separate GPS, camera, flashlight, etc. So I bought a solar cell on eBay rated at 7v 300mA. Only half to a third as much power as the wall charger. To condition the power I took the guts out of a car usb charger and soldered it to the board with a diode on the positive electrode. The power out is about 5v 300 mA. I was expecting it to charge, just slowly. However, it doesn't. The phone charging symbols comes on but the charge percentage continues to drop. Doesn't matter if the phone is powered on or off. Yes, it's in full sun. Any ideas on what the problem might be? I'm wondering if the phone has a built-in current cutoff.This is the usb wire pinout configuration.Photo of project attached.

author: Skimask
author_rank: Super Contributor
timestamp: April 10, 2014, 02:44:53 am
content: Ya, it's probably rated at 7volts open circuit and 300ma short circuit. You can have one or the other, but not both. Put a load on that solar panel, and it as likely that the voltage drops to well below what's need to charge anything at a current that's well below any amount to do any good.

author: Exergy
author_rank: Newbie
timestamp: April 10, 2014, 03:11:16 am
content: When a multi meter is measuring current is it inducing a load? So if I use two meters at the same time, one to volts and the other to current, it seems I'll then have enough information to calculate the power out. Or do I need to measure across a resistor?

author: theatrus
author_rank: Frequent Contributor
timestamp: April 10, 2014, 06:04:54 am
content: QuoteYes, but its measured in mega-ohms.QuoteIf the current measurement is in-series with your load (voltage regulator + phone). However, nothing is going to work unless the phone says near 5V.It would be simpler to measure the voltage to the phone when plugged in. I'm betting its significantly less than 5V.

author: Skimask
author_rank: Super Contributor
timestamp: April 10, 2014, 06:38:49 am
content: Hook up the original setup and see what you phone draws for an average charge current.If it's 300mA, as you said in the first post, then put a ~16-17 ohm resistor across the solar panel output (5v / 16 ohm = 312.5mA), put a meter across the resistor, put the solar panel out in the bright sunlight and see what the voltage is. As the post above said, I'd be willing to bet it's going to be far less than 5v.Not saying that your phone won't charge at 300mA. It might charge at far less than that. Increase the value of that load resistor until you get to point where you can keep 5v at the output of the original charger. Then you'll have a good idea of what that solar panel can actually put out for you.With all that being said, I think you're forgetting about one important thing....the dropout voltage of the regulator itself, which, if it's a cheapo chinese regulator, probably uses something like a 7805 or LM317, both of which have dropout voltages of around 2v, meaning you have to put in at least 7v (give or take) to get 5v out.

author: amyk
author_rank: Super Contributor
timestamp: April 10, 2014, 09:28:46 am
content: Quote from: Exergy on April 10, 2014, 02:33:23 amThat looks like a typical buck converter, and since a car charger is designed for nominal ~13V input, 7V might be too low for reliable operation. I quickly looked online at some car charger specs and most of them are 12-24V input, with some slightly wider at 10-26V, but there don't seem to be any that go below 10V. An LDO might be a better option.

author: Exergy
author_rank: Newbie
timestamp: April 10, 2014, 04:58:34 pm
content: Thanks for all the suggestions. I'd say wish me luck but the laws of physics don't care! Will let you know the results.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: April 10, 2014, 06:05:38 pm
content: Solar panels are constant current sources. Just about any DC/DC converter you find (like the one out of your car charger) expect a constant voltage input, it's not going to work well with CC.Why? Because when a basic DC/DC converter needs more power to feed the load, it lowers its input resistance to pull in more current and more power. With a CC source, this has the opposite effect since the current won't change, but the voltage will drop, reducing power, causing the DC/DC converter to lower its input resistance even more to get more current, lowering the voltage even further, and so on. You'll get stuck in a vicious cycle where the DC/DC converter is bouncing off of its lower voltage limit and sucking out what little power it can from the panel, likely with very unpredictable and irregular output/charging voltage.You need a solar panel controller, which usually has built in DC/DC. It could be MPPT, it could be PWM, but it needs to be something that operates the panel near peak power as best it can, without just trying to short out the panel to get more power when the load requests it.The only way I can see a panel working properly with a regular DC/DC converter is if the panel is significantly oversized so the DC/DC doesn't have to try to short it out to get more power. But having never built a solar powered phone charger with a regular buck DC/DC converter that's just a guess. Of course, as amyk said, you also need to be using a DC/DC converter that can operate down to the panel's peak power voltage, which is probably around 5v.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 10, 2014, 06:42:08 pm
content: All i can say is, i hope you get good sunlight coverage.With a panel that size i'd say it will only just provide your rated power under best case conditions. Also, that looks like a 34063, not exactly the best tech as far as switchers are concerned - it will probably use a large chunk of your power by itself.If it were me, i'd have just bought a LiPo and stuck a small efficient step-down converter on it. A 70Wh lipo pack can be had for $25. That should easily keep your phone charged for a week.

author: Exergy
author_rank: Newbie
timestamp: April 11, 2014, 05:06:16 pm
content: Quote from: peter.mitchell on April 10, 2014, 06:42:08 pmThis trip is to the four-corners region where a cloudy day in April is rare and sunshine is the whole reason for goingNeedless to say I can't wait.Quote from: peter.mitchell on April 10, 2014, 06:42:08 pmAboslutely right, it is a 34063. It seems the consensus is this was not a good combo to start with.Quote from: Skimask on April 10, 2014, 02:44:53 amQuote from: theatrus on April 10, 2014, 06:04:54 amVoltage actuals:Pre-converter 6.98vPost-converter no load 5.24vPost-converter with phone plugged in 4.00vDue to all the hot glue I didn't check current with the phone plugged in but it's obvious that the voltage has dropped too low.Quote from: peter.mitchell on April 10, 2014, 06:42:08 pmGreat idea. I have plenty of LiPos around from flying planes. However, the weight penalty is high since I like to keep my pack weight < 30 lb. A 24 Wh Lipo weighs 3x as much as this solar panel, so enough battery for a week will be 6-9x. Yeah, yeah the solar panel is inop so it's a bad comparison. If I can't find a way to make solar work I'll probably go this route.Digikey has a DIY solar charger plan that I'm considering but the panel is larger than what I'd like to carry so I'm looking at their charge controller which uses a V7805-500. The minimum input voltage is 0.5v under what this panel produces. Am I going to run into the same problem without additional panels?http://www.digikey.com/web%20export/mkt/general/solar/solar-charger-instructions.pdfhttp://www.cui.com/product/resource/v78-500.pdf