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Question for battery experts.

  1. Feb 7, 2014 #1
    Hi guys and gals,

    This may be a bit off topic, but your community seems like the most adept concerning my topic.

    In short, I want to build an emergency cell phone charger out of one AAA battery. Energizer has done this with one AA battery, so I assume that it is very possible. Link

    How are they doing this? If I just hook wires up to a AAA battery (let's say lithium) and hard wire that to a micro USB and plug it into my phone, would that work, being that the AAA battery is only putting out 1.225 volts and cell phone batteries usually put out around 3.8 volts?

    My goal is not to actually charge the phone battery, but to power the phone (a smart phone) for, say, 10-20 minutes.

    I know I could just purchase the Energizer contraption with the AA, but I really want to make my own and make it with one AAA.

    Any ideas, thoughts or advice will be greatly appreciated.

    Thank you in advance.
    Last edited: Feb 7, 2014
  2. jcsd
  3. Feb 7, 2014 #2


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    Hi Guysaker, welcome to Physics Forums.

    I think you'll find what looks like an AA cell connected to a micro-usb plug also includes some hidden electronics to boost the cell voltage close to the 5v level expected from a usb port.

    If the cell is lithium, its voltage won't be 1.5v, it will be more like 3.8v but that is still too low to charge the phone on its usb input, and some electronics is probably included to boost it to nearer 5v.

    That's my guess. But let's await the opinion of an expert.
  4. Feb 7, 2014 #3


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    Look at the charger for your phone. I assume it will be 5V 500ma. I'm also assuming it is a usb connector. This means you can power the phone with a 5V supply. Unfortunately I don't think there is any way to stop the phone from charging when you connect 5V.

    The next step is to build a boost switching regulator to convert 1.5V (or 3.8 if lithium) to 5V. This circuit would work for a lithium cell. http://uzzors2k.4hv.org/index.php?page=usbcharger

    Here is one for a 1.5V battery. http://www.electroschematics.com/6232/portable-5v-power-box/
  5. Feb 8, 2014 #4
    Thanks for the reply. That is what I am just figuring out now, that I would need to boost the voltage somehow. I wonder if they sell a single part, like a voltage regulator booster that would do the trick from one single AAA.

    I'm trying to build something and there is only room for one single AAA battery (probably lithium).

    Anyway, thank you for your help and the links.
  6. Feb 8, 2014 #5


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    You cannot do it with 1 part. You need a boost regulator. They either are capacitor based or inductor based and consist of a controller chips and a handfull of components. For this application I think inductive based is best (for the output voltage regulation and current requirement).

    You can look at all the 5V boost regulator controllers and choose the one that suits your fancy. Google images is generally a good way to see a lot of implementations quickly.

    Post what you decide on or more questions if you have issues deciding.
  7. Feb 8, 2014 #6

    jim hardy

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    Try a search on phrase "joule thief"
  8. Feb 9, 2014 #7
    LOL. Bulls eye my friend. I mean that was egg zachery what I was looking for. If I ever get this thing made, I'll let you know and send you one.
  9. Feb 10, 2014 #8


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    Actually the joule thief has issues for this application. (It's a cool design though for non critical voltages). Your charger needs to supply 5V +- 10% over a wide current range, up to 500ma. By the time you add voltage regulation and such to a joule thief you have a more complex, less acurate, less efficient circuit than using one of the simple switching converters.

    I left it to you to find a simple one, but .....

    http://www.intersil.com/content/dam/Intersil/documents/isl9/isl9113.pdf is the simplest I found
  10. Feb 10, 2014 #9


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    One problem with Li-ion cells is that you have to protect against over-discharge. Some cell packs have internal protection circuits. I don't think the AAA cells have such a thing.

    The charger circuits I have shown do not include that feature.
  11. Feb 10, 2014 #10


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    Many Lithium AAA cells are not rechargable. Are you planning on making this rechargable? Notice the voltage from Li-ION and non-rechargable lithium are different. (as are the capacities).

    Li-ion batteries must be protected from over-discharge, which the circuits I have been showing you do not do.
    Some battery packs have protection circuits built in, but not the AAA cells.
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