Reality check for 6v solar charger

Discussion in 'General Electronics Chat' started by sam452, Sep 2, 2013.

  1. sam452

    Thread Starter New Member

    Sep 1, 2013
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    Hello, new forum poster seeking guidance for project.

    As cell phones boom across Africa, the people in outlying areas tend to get left behind. I'd like to end up with a project not unlike bootstrapsolar.com which has an open-source project to be able to charge your cell phone. I like the idea of using cheaper 6vDC panels to feed a battery. Having bought many cheap solar power banks I find that none were really able to fully charge a cell phone. More wattage seemed to work. Ultimately, it would be a pipe dream to seed a community leader with a panel and a charger circuit to feed into a battery they can acquire cheaply rather than dump a device and leave town. That is to say, it's better for a recipient to be involved in the project so that they're not helpless when we leave town. And leave a "plan" for a circuit so that an inquisitive community member could conceivably build their own and perhaps build a side business charging others' phones.

    So I bought a 6vDC solar panel at 3w and some 2500ah NiCd D batteries to bring it up to 6v in line with a 7808 regulator. Mainly to be able to store enough energy so that more than one phone could be charged at night. Also I was wanting to use a storage medium that is easier to acquire in the countryside rather than providing an Lithium battery that they are less likely to acquire inside their country. Reading up elsewhere and here I'm thinking that NiCd is not the way to go. It appears no matter what my source and storage option is there's more to it than dumping juice into batteries. I'm told that NiCd and car/motorcycle batteries are reasonably available there.

    Searching this forum I found this, but the comments after this post confused me as they downplayed the circuit for reasons that weren't clear to me.

    http://forum.allaboutcircuits.com/showthread.php?t=56158

    Perhaps I need to reconsider using a 6v panel and power storage. Given that I'd like to build an inexpensive means for people to build their own grassroots businesses so they can build their own economy, how would you suggest I approach this project? thanx, sam
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Why charge at night? Sure, you get to sleep while your phone is somewhat tied up. but to do that you need an extra battery. That battery costs both money, but also energy as charging a battery is never 100% efficient. So you need a larger solar panel just to keep things even... then you need two chargers, one for the night time battery, and one for the phone.

    Here in the USA phones frequently come with USB chargers that work with computers... or ANY other source of 5 volts. Combine one of those with your solar cell and you are done. Locally all people may have to do is cut and strip some wire to fit the cord to the solar cell. You may have to experiment a bit to see how high the voltage can safely get and not knock out the charger; ideally you don't want to use a voltage regulator (less parts is cheaper).

    Oh, and make a long wire so your phone can be inside at your desk while the solar cell is outside in the sunshine.

    That's my advice for you: charge in the daytime with an existing charger.
     
  3. sam452

    Thread Starter New Member

    Sep 1, 2013
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    Thanks, it may turn out that way. The idea is that while the community leader is out with his/her phone during the day, it would not be attached to the panel charger, thus the phone could recharge when the sun has gone down.

    I'm was hoping that the storage source would get enough to charge more than one cell phone and that a repeatable circuit/setup could be created so that others could set up their own charging service.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    OK, gotcha. So a two step charge (day & night) is a hard requirement.

    I've never build a charger but I understand NiCads are particularly sensitive to being overcharged, so I would *buy* a good charger there.

    As an alternative, I work on a lighthouse where we maintain some power from batteries. We have several cells charging a bank of lead acid batteries with no charger whatsoever. These cells have a built in diode (to prevent night time discharge thru the panel) and they charge at a low enough rate that the batteries don't seem to mind.

    Now I would by far prefer to have a charge controller there the system seems to work as is, though I cannot get good data on how long the batteries last. But this may be a simple inexpensive and workable scheme for your use.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    Maybe instead of solar you could consider a way for someone with an engine to carry - and charge - an extra battery as they go about their normal daily driving/riding activities, then swap out the charged battery for a discharged one and start over. They could make some small fee for the trouble and the extra gas that is required to charge the battery, and the battery "manager" would have a steady supply of charged lead-acid batteries with which to operate a charging station.

    Solar is cool but it tends to be more of a plaything for the rich than a useful utility for the poor. It's just too expensive as long as there are other options.
     
  6. sam452

    Thread Starter New Member

    Sep 1, 2013
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    This approach was what I was hoping the NiCds could fill, but it appears they need a charge controller. Part of it is finding a simple enough circuit that could possibly be replicated. But lead-acid batteries may be the way to go. Thanks for pitching in.
     
  7. sam452

    Thread Starter New Member

    Sep 1, 2013
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    Thanks, this is part of the reality check I was hoping to get here, maybe I'm too stuck on "my" solution and should be interested in the real goal.

    But if someone can point me to a circuit that would work with a 6v panel, I'm still wanting to keep my eyes open. But I'll ask my contacts there if the small engine is ultimately a better approach, sam
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    I'll give you a few numbers if you give me one back: how much energy does your cell phone battery need?

    Everything starts from there, and ultimately ends up at the solar cell. Here the "load" is the cell phone battery, how big is it, and let's assume we go from fully discharged to fully charged each night.

    You can probably just read the rating off the battery in the phone you want to use.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    The project would be much easier if that was 12V instead of 6V. Any chance of that? I'm guessing you might need two panels anyway, once you work through the math of all the various power losses.

    I'm assuming you need a 5V, USB type charging port. Starting with 6V, at best, there just isn't much driving force using a 6V panel. There are all sorts of DC-DC converters you could use to overcome this, but you lose efficiency every time you touch the power stream.
     
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