HELP Reducing Li-po Voltage..!!!!!!!!!

Discussion in 'General Electronics Chat' started by Fernando1001, Jul 14, 2011.

  1. Fernando1001

    Thread Starter New Member

    Jul 14, 2011
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    0
    Hi all,

    Instead of using a 9.6V NIMH I'm trying to use a 11.1 Li-Po, however the li-po has just to much voltage when charged so I need to know how to reduce the voltage by 1.5V ONLY!!!!!

    This is a 11.1V li-po and 25C rating.

    I have been told to use resistors but not sure how to do it.....

    ANY HELP OR ADVICE PLEASE.!!!
     
  2. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    Are you sure you need to?
    A fully charged 11.1V Li-Po is actually around 12.6V (4.2V X 3 cells)
    A fully charged 9.6V NiMH pack is actually around 11.6V (1.45 X 8 cells)
    I would imagine most components could survive an extra volt (but don't sue me) and it is only for a short while because Li-Pos initially drop in voltage quickly.
     
  3. Fernando1001

    Thread Starter New Member

    Jul 14, 2011
    3
    0
    Hi Markd77...

    Yes they do, the issue is the li-po at full charge is just too much much voltage and the 25C rating is a lot higher than the NIMH. If I could somehow just drop the voltage 1.5V it would be perfect...any ideas.
     
  4. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
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    Isn't there some voltage regulation after the battery? If so, what is the input specifications to that regulator?
     
    Last edited: Jul 14, 2011
  5. #12

    Expert

    Nov 30, 2010
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    First, a couple of power diodes in series with the voltage will lose some voltage. You need to know the current range to decide whether to use 1 amp diodes, or 3 amp diodes, etc. Second, I believe 25C means "room temperature", 77 Fahrenheit.
     
  6. tom66

    Senior Member

    May 9, 2009
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    "25C" is the (max) discharge rate, it means 25 x nominal capacity. In this case the OP hasn't specified the battery capacity, but I know you can easily pull 15+ amps from a good LiPo; (I have a few); some do 50-60A continuously and 100A+ surge.
     
    Last edited: Jul 14, 2011
  7. #12

    Expert

    Nov 30, 2010
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    D'oh


    !!!!!!!!!!!!!!!!!
     
  8. Fernando1001

    Thread Starter New Member

    Jul 14, 2011
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    tom66....Correct all the way...I think max is around 30AMP draw.....So maybe 3 to 4 power capacitors.

    Solder them on a board maybe between battery and electric motor..?

    Again any advice is appreciated.
     
  9. #12

    Expert

    Nov 30, 2010
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    Circuit boards generally don't do well with 30 or 40 amps.
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    I stumbled onto this thread and noticed that it was in "Off Topic". I think it belongs in "General Electronic Chat".

    hgmjr
     
  11. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
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    It's not impossible to design a PBA to deal with this sort of current, but you would need to take copper weight and track section seriously to keep resistances down.

    More to the point, if the motor really is going to draw 30A, dropping 1.5V in diodes would represent 75W of losses - which is wasteful and makes a lot of heat to be got rid of. You would be better to use some more efficient means of controlling the motor drive, like maybe PWM?
     
  12. tom66

    Senior Member

    May 9, 2009
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    Unless specifically designed to do so, for example with a motor controller.
     
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