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Portable Power Supply

Discussion in 'Power Electronics' started by Braeden Hamson, May 7, 2017.

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  1. Braeden Hamson

    Braeden Hamson

    220
    13
    Feb 18, 2016
    Hello gents,

    I want to build a portable battery powered variable power supply. Not sure what I'd use it for but, I think it'll be one of those things that when you have it you don't know how you didn't have it before.

    So I got a laptop battery pack out of a smashed laptop. It has the standard lithium cells in it. My plan is to use a LM317 voltage regulator based power supply circuit, if anyone knows of a circuit that's more efficient than that one I'll take it. but if that is efficient then great. Efficiency is paramount in this project. Also is there a circuit that can regulate volts as well as amps? I was thinking that I could potentially help myself out by not directly wiring all the batteries together. Instead I could wire them all (4 of them) to their own transistor, probably a MOSFET. (Again if there's a better kind out there.) Then I could use an arduino to turn on and off cells, for example if I needed 2 volts, I could just use one battery, if I needed 12 I could use more. Additionally I could use different cells each time if I were using one cell a lot. So they all stay balanced. I'd also have undervoltage/overvoltage protection on the batteries as well as a proper lithium charging circuit. I don't know what circuits to use here, but I could probably find it on google in 5 seconds. Though the battery pack does have all of its own monitoring circuitry with it. But I'm feeling like it could be a pain to use.

    Any suggestions are welcome.
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    Any linear regulator will have the same low efficiency as the LM317T.
    To get more efficiency, use a buck convertor with the full battery voltage on the input. Should get at least 80% efficiency. This will be much simpler than splitting the battery.
    The convertor will regulate voltage, if you want to regulate current, then additional circuitry will be needed.
     
  3. 73's de Edd

    73's de Edd

    2,988
    1,261
    Aug 21, 2015
    My Reads . . . . .

    It has the standard lithium cells in it
    Meaning 3.6 VDC per cell

    Then I could use an arduino to turn on and off cells, for example if I needed 2 volts, I could just use one battery.
    One battery does not equal 2 volts

    Li Ion cells need special / extreme pampering, they like charging at a constant current but a cell will then charge upwardly from below . .or at . . . its standard 3.6V spec up to about the 4.25 voltage threshold and then MAYBE on up to the 4.30V threshold.
    If rising above that . . .then you are just waiting for that magic moment when your charge current instantly drops to zero and you have an open/dead cell on your hands NEVER-NEVER-NEVER-EVER to work again.
    And it DOESN'T care if it IS EVEN a brand new cell and cost you $40.
     
  4. 8bit_brain

    8bit_brain

    6
    0
    May 7, 2017
    i just built something similar based on a laptop battery pack. I used a buck convertor to 'lift' the voltage from 3.8V to 12V. I also have a solar panel attached to recharge the battery. I can post some pictures if you want.
     
  5. Braeden Hamson

    Braeden Hamson

    220
    13
    Feb 18, 2016
    Awesome I'll look into buck converters. If I'm not mistaken that circuit can boost voltages too correct? Additionally is there any benefit to using one cell for low voltages?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,448
    2,809
    Jan 21, 2010
    That would be a boost converter. A buck converter reduces the voltage. :)
     
  7. 8bit_brain

    8bit_brain

    6
    0
    May 7, 2017
    Sorry. It is a boost converter indeed.
     
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