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Building a solar phone charger

  1. Mar 19, 2016 #1
    I am building a solar phone charger and have decided to use 3 Nimh batteries, a 5/6 v solar panel, a usb 5V rectifier and also a Lipo credit card rechargeable cell.

    My reasoning is limited for this build as the more I read, the more conflicting info I find...
    (Eg, charging AA batteries is easier with solar than Lipo's, then I read its harder)...

    So Im somewhat confused but my design has been decided on.

    What I am really after is info on how quick I can charge the Lipo cell off the AA batteries and will that cut off, overcharge function be built into the rectifier.... I am buying a rectifier specifically for phones. (Although not sure if it will do Iphone or not)

    And also any assistance I can get that would make this project amazing instead of just ok.

  2. jcsd
  3. Mar 19, 2016 #2

    Simon Bridge

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    Whether there is a cut-off/overcharge function on the "rectifier" will be a question for the manufacturer: but they are pretty essential for phones so anything specifically built for charging phones should have one. There is no way for anyone to be able to tell for sure here. iirc some phones have the cutoff built in.

    If your phone would normally charge directly off a USB port, then all your charger has to do is imitate the USM port output.
    From your description, the "phone charger" is a circuit powered by some AA batteries ... the AA batteries are, in turn, trickle charged from a solar panel.
    Suggest you test it using an old/cheap phone.
  4. Mar 20, 2016 #3


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    Rectifier appears to be the incorrect word there, did you really mean 5V regulator ?

    this doesn't really make sense unless you are referring to rechargeable AA style NiCad or Ni Metal hydride batteries ?

    Last edited: Mar 20, 2016
  5. Mar 20, 2016 #4


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    I cannot tell from your post what you are really doing (how you are going about it).
    Are you charging the NiMH with the panel, and then the LiPO from the NiMH?

    If so, you need a proper LiPO charger (see https://en.wikipedia.org/wiki/Lithium-ion_battery#Charge_and_discharge)

    If you post a sketch and part numbers, we can set you straight.

    This is two projects in 1:
    1. properly charge your batteries from a solar panel, and then
    2. properly provide 5V to the phone to charge it (or properly charge your LiPO batteries)

    Depending on the phone, #2 can be tricky if you want optimum performance (quick charge)

    There are plenty of these available cheap, and ruggedly built. There is some significant design work required to get it right.

    just google "solar usb charger"
  6. Mar 21, 2016 #5
    Yes it is Nimh AA batteries I am charging and my bad, I meant regulator.
    I have various solar cells ranging from 3v to 7.2v outputs so I've still to play around with them all.
    I also have 2 DC-DC regulator modules that say will take input from 2.5 - 5v and give a constant 5v 1amp usb output.

    So what I really want is 2 conditions, USB loaded or unloaded..

    For unloaded I want the solar panel to charge up the AA batteries (until full) then stop (and perhaps recheck every hour, for example)

    Until there is a load, in which case I want the USB module to take power from either the solar panel, or if not sunny enough, then take power from the AA cells.
    The complete system would also include a Lipo power pack to fully charge off the USB, like a phone, so this part is simply an additional feature rather than part of the main system.
  7. Mar 21, 2016 #6


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    The basic architecture seems OK, but you need proper battery chargers and overdischarge protection.

    Regarding the LiPO
    A 5V regulator can be used to power a proper LiPO charger, but cannot charge it directly. You will get fire if you don't use a LiPO charger. I previously posted a link to Lithium charging. (note that the phone has a very sophisticated Lithium charging system that runs off the 5V from the USB connector)

    Regarding the Phone:
    Connecting 5V to a phone does not mean it will charge. Depending on the phone you need to do special things to the DP and DN pins.
    That can be a simple a shorting DP/DN tohether (most common) or connecting proper resistor values (iphones) or other things.

    Regarding the NiMH charging:
    Matching the solar cell to the NiMH batteries and charging them properly is also a concern, especially avoiding overcharge. I don't have the answer for that
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