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Solar battery charger

  1. May 16, 2013 #1


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    I want to build a usb charger that works with solar power (and later add a crank dynamo too). The photovoltaic cell will charge rechargeable batteries and they will charge the usb device, like my smartphone.

    The smartphone charges with 5V and 1500mAh. I'm planning to use 2 AA batteries 3.4V 3000mAh, but i don't know how sometings will work.
    First, what should be the voltage of the solar cell? What will determine how fast does the solar cell charge the batteries, is the current? I don't have much knowledge in batteries...
  2. jcsd
  3. May 20, 2013 #2
    The USB "spec" is 5 V - a basic way would be to use a LM7805 voltage regular between your source ( PV Cell, battery, etc) and the USB connector. As long as the voltage of the source is higher than 5 V - that part should be fine.
    Charging batteries is another matter - every different battery type is charged / treated a little differently. It looks like you are using NiMH AAs - reasonable durable, you should research the battery to determine how you would best regulate the charging current - then determin what circuit you would need to do this - for NiMH I'll bete there is an IC out there for just that purpose since they are so commonly used.
  4. May 20, 2013 #3


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    i was thinking in using a 5V or 6V PV cell (5V is 140mAh and 6V 400mAH, so i think 6V will be faster to charge) to charge two serie 3000mAh 3.7V batteries and then regulate the voltage of the batteries to 5V to charge the phone.
    The problem is, how can i charge a 7.4V battery with a 6V PV cell...
  5. May 20, 2013 #4


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    With fancy circuitry, that, if you can't understand a battery, you will not understand either.

    I try to keep my solar projects as simple as possible. I solar power my laptop by hooking my 17vdc no load solar panels to a deep cycle battery, and hook a 12vdc to 120vac converter to that, to which I plug my laptop power supply into.

    It works great!

    I see no reason to re-engineer the world, when stuff is just lying around.
  6. May 20, 2013 #5


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    Well, you have no problem charging the batteries, since your panel is 17V, i'll use a 5-6V to keep it cheap and portatile. I'd also avoid to use lots of converters to keep it small...

    Having solved the solar panel - batteries issue, the only problem will be regulate the output to 5V, right? There's no need for filters, i believe..
  7. May 20, 2013 #6


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    I believe Windadct answered that question. It's been a couple of decades since I built an IC voltage regulator.
  8. May 21, 2013 #7


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    no you havent really solved the solar panel and battery situation
    to get a regulated 5V the 7805 5V voltage regulator is going to need at least 7V going into it
    Also most of the voltages you see quoted for solar panels is the open circuit voltage ( that is measured with no load) That voltage will drop significantly once a load is applied. So you will need something a bit higher in voltage and charge the batteries in series you are probably going to need 3 x 3.4 V batteries and a 12V solar panel

  9. May 21, 2013 #8
    The 6V PV to charge a 7.4V Battery is not a standard situation and will require a semi-custom soluiton. Not really difficult for someone familiar, but not trivial ether. As Dave pointed out the 6V PV - is the open circuit voltage, I'll bet under load this will put out 4 to 5.5 V depending on the light available, to get this to 7.4 ( a little more typically for charging - like I said what is the battery spec ) - you need a boost circuit, and it needs some type of control because the Voltage Input, the PV Panel, is not constant voltage.
    Also note - most battery charging situations are for all of the batteries in parallel - you can chage in series, but this will also make the charging case more custom.

    Punt - Make the Battery to USB (5V) circuit / system first, use a convention charger. This is a good entry level project and you will learn a lot.... then on the next possession ( portion of the project - continuing my metaphor) - Develop a PV to Battery charging circuit. ... lastly - get them to work together = TD.
  10. May 21, 2013 #9


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    I didn't said i solved the PV - battery problem, or maybe i didn't express well. I wanted to say that after this problem is solved the only left will be regulate to 5V.

    Now, searching better on internet i found this thing:


    Initially i thought that these chargers weren't capable of storing energy, but i was wrong, and they do exaclty what i want. And altough they are very small and portatile, they seems to charge the batteries really fast like a solar panel much bigger.
    It says 8-12 hours to charge in the sun. Does this really works?

    Also, it says voltage output 5.5V, i believe i can use it to charge 5V USB electronics
  11. May 22, 2013 #10
    Your OP says "I want to build a usb charger that works with solar power"

    If you only needed the actual device someone may have already known about the item or similar you found.
  12. May 22, 2013 #11


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    Yes, but i'm searching for the parts to build it and it will cost like double the price of this thing, just in solar panel and batteries. Also, i couldn't find a small solar panel like this with more than 5V...

    I think building it would be much more fun and interesting, though. I'll keep looking for cheaper parts.
  13. May 22, 2013 #12

    jim hardy

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    Look to salvage parts. Solar yard lights are abundant and cheap. Each one has a small solar cell and battery in it. They should cost about a half dollar apiece in thrift stores.
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