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USB ports to 12V / 1amp?

  1. Jan 9, 2016 #1
    I have a portable solar panel (for camping, etc) that outputs to two USB ports (2 amp). I want to use this to power a small Li-ion charger. I know that there are products/adapters I can buy to achieve this but I am fascinated by the science of it and want to know *how* all this works. So I want to build the circuit myself.

    - My solar panel is 19.5 watts (in sunlight) and has two 2 amp USB ports for output.
    - My charger requires input of 12v 1amp.

    So, if I under stand this right, I only need 12 watts to power the charger and my solar panel *can* output up to 20 watts (5v 2amp x 2). So I think I have enough watts to work with but I don't know how to physically achieve the conversions I need. Should I wire the USB outputs in series (and double the voltage) or should I wire them in parallel (and double the amps)? Either way I will need to convert both for my output to match the input for the charger but I guess, I just don't know which approach is best and why.
    • Which devices do I want to use in my circuit to convert voltage/amps? I am pretty sure that a transformer would do the trick for AC but I don't know how to change voltage on DC? And are amps as simple as resistors/ohms law?

    • Is there a generally accepted "better" way to solve this problem - ie, is there a rule of thumb or something that says its always "better" to wire them in series and do your conversions working with high voltages or/rather go parallel and convert working with higher amps? And why?

    • Do I need to worry about my current not being entirely stable? I know that the output of solar panels can vary, should I be thinking about capacitors? Or?
    So those are my questions - thank you so much for your time. And I really do what to learn "why" so if you have any links that would explain these solutions in beginner terms, that would be fantastic! Thanks!
  2. jcsd
  3. Jan 9, 2016 #2


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    Hi there Jason
    welcome to PF :smile:

    you have given us not real info about the solar panel other than it produces around 19W

    we need to know what voltage the panel puts out and what voltage regulators other circuitry etc there is between the panel
    and the USB ports

    no, you will likely find that the two 5V USB ports are just paralleled from the regulator circuitry
    so there is ONLY 5V available

    so us photos of the PCB, both sides, WELL LIT and SHARP FOCUS
    tell us the name of this solar unit ... make and model, supply

  4. Jan 9, 2016 #3


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    I'll assume for the moment that all you have to work with is two 5 volt usb ports and they can each output 2 amps at the same time.

    That does, in fact, give you 4 amps at 5V, which is 20 watts.

    You cannot connect them in series since they are essentially two ports into the same 5V supply. Like trying to get 18V from one 9V battery.

    The standard solution for converting 5V DC to 12V DC is a boost converter. It is a high frequency switching converter that converts 5V to 12V.
    There are lots of chips and circuit examples you can look into before deciding how you might like to proceed. Just search for boost converter.


    The only issue you might have is connecting the two USB ports in parallel to get the 4 amps. The internal circuitry in your device might not load balance well. Not sure how to best proceed in that regard.
  5. Jan 9, 2016 #4


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    How about ignoring the USB outputs and using a battey charge controller chip fed directly by the solar panel? Seems like it would be simpler. I'm suggesting a battery charging chip to avoid damaging the batteries from overcharging. Those Li-ion batteries can throw a nasty temper-tantrum if they aren't treated nicely.
  6. Jan 9, 2016 #5


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    His charger requires 12V @1A. There are probably 5V input chargers available also. How he gets the voltage his li-ion charger requires is either the 5V ports he has, or some other method we don't know about because we don't have the system details.

    My suggestion was predicated on using the 5V ports, which is one of many potential (pun) solutions.

    I don't think anyone has suggested not using a proper li-ion charger.
  7. Jan 10, 2016 #6


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    indeed :smile:

    and it would be VERY helpful if @Jason porter returned and answered the Q's I posted in the first response

    The Solar panel may not even be capable of supplying 12V ???

  8. Jan 10, 2016 #7
    First off - THANKS! Now here are some details about the products. I am using:
    solar panel: http://www.ipoweradd.com/poweradd-195w-dualport-foldable-portable-solar-panel-for-samsung-s3-s4_p1322.html [Broken]
    charger: http://charger.nitecore.com/product/digicharger-d2

    However as others have noted I really don't want to damage/modify either the charge or the panels themselves. I want to build a device *between* them that will let me leave each of them more or less intact. Still, I took a swing at getting the housing off solar panels where the USB ports were (to see the insides and get you the details you wanted) but it seems like its going to be difficult without permanently breaking it. So, all of that said, advice? :-p
    Last edited by a moderator: May 7, 2017
  9. Jan 10, 2016 #8
    Yes - this exactly. I don't want to mess with the devices themselves, I just want to build a new device to go between them that can convert the power.

    So, then, I guess we're left with parallel? Although, as someone very new to science, could you explain that a little further just for sake of understanding. And then, if we're going to do parallel that means we'll stick with 5v but jump to 4 amps (20 watts?). Will the boost convert manage the amps too? Or does it just keep the amps as-is and then I use resistors to cool thing down?

    So, how likely is this scenario? :-p
  10. Jan 10, 2016 #9
    I don't want to damage/modify either the panels or the charger if possible.
  11. Jan 10, 2016 #10
    edit: Also, thanks for the info!!!
  12. Jan 10, 2016 #11
    I've looked and I can't find any? At least not any that can charge the few battery types I am looking to keep charged.
  13. Jan 11, 2016 #12


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    I have no idea. A higher quality charger may have current monitoring on each port. They probably used some USB power management chip, but I have no idea.
    I'm not even sure how to test it in any real definitive way other than load it down and look/hope. I think you need to contact the manufacturer and tell them you want to parallel them and draw more than 10 watts but less than 20 watts. A schematic might provide the answer. For example, maybe they used 2 of these: http://www.ti.com/product/TPS2540. Or maybe they just provide a 4amp supply and you can use it any way you want. No way to tell from here, though.
    You have to read the specs really carefully, because sometimes they play with words, and you can only charge 1 high current device at a time.

    For the boost converter, think of it as an 80% efficient (or whatever your converter is spec'ed at) 12V supply. If you draw 12 watts at the output, it will draw 12/0.8 = 15 watts at the input. So that will be 3 amps at 5V to supply 1A at 12V.

    I don't know how to explain "the science" of not being able to connect in series. It's like splitting a battery to 2 outputs. Or like connecting two batteries to a common ground. There is a common ground, so you cannot stack them. The only way that would not be true would be if there were some intentional isolation transformers in some 5V output switching supplies, which is even less than unlikely.
  14. Jan 11, 2016 #13


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    It's a neat solar panel. I think it's possible that the outputs are independent, each using 1½ of the panels. A multimeter would reveal whether the USB outputs share a common terminal.
  15. Jan 11, 2016 #14


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    I just saw the link for the panel, sorry I missed it. It looks like a really neat panel. It will charge the two devices simultaneously, so that is not an issue. It probably has a controller chip for each port. I have no idea how well it will load balance if you simply parallel them. I also don't know how smart the output ports are since it is tricky to be able to charge Androids, iPads, iPhones, and other devices at 2amps. Those devices all look for different charger characteristics on the DP and DM pins of the charger.

    I expect that you will probably get no help from support. But, it is worth a try.

    Maybe something simple, like a very small resistor in series with each output will be enough. But I'm just guessing at this point.
  16. Jan 11, 2016 #15


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    The external boost converter to 12V is still the best option other than to get a solar panel capable of supplying the correct power for what you are wanting

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