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3.85 V li-ion battery to replace 3 V lithium cell in bathroom scales?

Discussion in 'Power Electronics' started by seanspotatobusiness, Apr 21, 2016.

  1. seanspotatobusiness

    seanspotatobusiness

    193
    4
    Sep 11, 2012
    I have a couple of old phone batteries which I would like to put to use and I have bathroom scales which use a single 3 V lithium coin cell. Would the electronics in the scales be likely to be damaged if connected to a lithium-ion battery? The scales have an LCD screen in case that helps.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,380
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    Nov 17, 2011
    Your chances are 50/50. Without knowing the internal circuit of the scale we cannot say for sure what will happen.
    If you want to be on the save side, use a silicon diode in series with the batttery (observe polarity), This will reduce teh li-ion' voltage by ~0.6V to 3.25V. This is now less than 10% more than the nominal 3V and should be fine.

    In case you want to do it real nice, use a low drop 3V regulator to ensure max. 3V to the scale's innards.
     
  3. seanspotatobusiness

    seanspotatobusiness

    193
    4
    Sep 11, 2012
    Thanks, Harald. I already have some diodes so I guess I'll just use one of those.
     
  4. Alec_t

    Alec_t

    2,695
    712
    Jul 7, 2015
    A fully-charged Li-ion cell could be 4.2V, so even with a dropping diode the voltage would be a tad high. Your scales would probably survive, though.
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

    9,380
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    Nov 17, 2011
    I didn't take that into account. The 3V regulator definitely is the better choice then.
     
  6. Externet

    Externet

    710
    145
    Aug 24, 2009
    Just use two diodes in series... 4.2 minus 0.6 minus 0.6 = 3V

    +4.2--------|>|--------|>|----------+3V
     
  7. seanspotatobusiness

    seanspotatobusiness

    193
    4
    Sep 11, 2012
    But then when the potential drops to nominal voltage, it will be too low. I think a voltage regulator is the only way but I can only find them for 3.3 V except a few SMD packages. I think I can work out a resistor to use by measuring current consumption at the rated voltage.

    Is a regulator dead simple to use? It looks like it would require no external components; just the battery, regulator and a resistor to drop ~0.3 V?
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    9,380
    1,912
    Nov 17, 2011
    3.3V is probably fine witholut additioonal measures. However, 3V regulators are available.
    You'll need a so called low drop regulator which can regulate to 3V from 3.8V (regulators not labelled as low-drop typically require > 1V voltage diffference between input and output). It is a common requirement to have a small capacitor (100nF) from each input and output to ground (0V) for stability and noise supppression. The datasheet of the regulator of your choice will tell you details.

    Also note that you should select the regulator for very low quiescent current. This is the current that flows even when the load (scale) is off. You'll want this current to be very low to minimize drain on the batteries while the scale is not in use.
     
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