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Solar Panel Cell Phone Charger

Discussion in 'General Electronics Discussion' started by Dante, Jan 3, 2011.

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  1. Dante


    Jan 3, 2011
    Hey, it's a place to start! Hello, I'm Dante, and it's a pleasure to meet you. I'm working a cool little apparatus to help power my cell phone (because I have a horrible memory and can't remember to plug it in - and it will drop dead in power in a couple of hours. So, given that I have a cheap little boost-mobile phone, I figured this would be an awesome trick to try.

    It's essentially just a pair of solar panels, a 200 ohm resistor to avoid short-circuiting and a micro-usb cord mounted on some foam. I've got it outside on a bread-board running current through a 1k resistor (Ha ha, I'm wasting power XD).

    Anyways, there were a couple of problems I ran into (which embarrasses me given that this is nothing more then a solar-battery connected to a resistor), and I was wondering if I might be able to learn something from this from others that certainly have more experience in electronics, and may have seen similar situations.

    1) The solar panels have something called "solderable foil" on the back, but I had the most difficult time in the world soldering anything onto it. The solder wouldn't stick to the foil, and it wouldn't stick to the glass either. It would just bead up with the wire on it then fall off :(. I eventually got it to stick after a couple of hours, but I believe that's only because I soldered tabled dust, wire plastic, hot glue and burnt flesh from my fingers onto the glass... with a good contact still for current to flow. On top of this, one of the pieces of foil fell off and I had to try and solder it again. I know there HAS to be something I'm doing wrong here, but I'm not sure what.

    2) It hates diodes! I originally had a plan to stick a diode in this circuit to limit voltage and to avoid back-flow damage into the solar panels. Given that in series they put out far more voltage then I expected (or they were rated at) it should have no trouble dropping down .6-.7V. But instead it just goes open circuit on me. I've checked the direction of current and the voltage is properly biased across the diode, so I'm wondering if there is some kind of low current caveat that I'm ignoring here, or if I just have a dud diode.

    3) The voltage is high, really high. When I bought these panels, they claimed that each one put out about 3V and the description acted like it was only the current that changed like you would get from a battery. Instead, the voltage and current seems to change - and in the middle of winter on a cloudy day, I'm getting up to 8.5 V out of these two panels in series (4.25V a panel) and a low of about 6-7 V. I could try and figure out the thevinin resistance of my cell phone and throw a resistor on there to kill off some voltage, but that would be wasting valuable energy. So instead, I've thought of sticking them in parallel.

    What I'm wondering is if anyone else has tried something like this, and if (if the panels have some weird upper limit of about 4.25V) their cell phones needed the full 5V to charge?

    Thank you,
  2. Resqueline


    Jul 31, 2009
    Solar panels are inherently constant-current devices and as such they don't need any short-circuit protection.

    1) Nickel strips are easy to solder, though not as forgiving as tinned copper. You probably heated them too much before applying fresh tin, thereby oxidizing them.
    Just scrape off the oxide layer and try again. Soldering on the conductive layer on glass is riskier, as the layer is easily damaged/dissolved by the soldering action.
    What kind of soldering tools & tin are you using?

    2) Can't tell what you're doing wrong w/o seeing what you're doing.. Panels aren't damaged by back-flow current btw., it's only about batteries possibly being drained.
    I've even seen products with these kinds of panels where the manufacturer simply had omitted the diode due to the panels drawing so very little back-current in the dark.

    3) These panels seems to have about 10 cells each, for a maximum voltage limit of 4.5V per panel.
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