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Homemade Battery monitor for 4 x 3.2V cells

Discussion in 'Power Electronics' started by Jimyb5, Oct 18, 2014.

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  1. Jimyb5

    Jimyb5

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    Oct 18, 2014
    Hi all,

    I have a PLC which is 12V powered. This voltage comes from 4 cells (3.2v) in series. I want to monitor the voltage of each cell individually so I can set up alarm points on my PLC.
    I have an analog Input card that will accept 0-10v (or4-20mA) single on each channel. So I can use a direct input from the first 2 cells
    Cell 1 3.2 To 3.8 v Max --- --- direct channel input
    Cell 2 3.8 To 7.6v Max --- --- direct channel input
    Cell 3 10.4 v --- --- Maybe used voltage divider to half voltage then double it in software
    Cell 4 13v --- --- Maybe used voltage divider to half voltage then double it in software

    Does anyone know of a voltage isolator I could use for this application that would be a nicer design?
    I have tried 4-20 isolator but the company tells me this wont work here.
    I am now looking at MornSun chip.
    MORNSUN Science & Technology Ltd F0505M/N-1W
    http://www.sunyuansz.com/china-4_20..._high_linear_distribution_sensor-1828165.html

    Any help would be great..
     
    Last edited by a moderator: Oct 18, 2014
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    What battery type are you using and what capacity.
    Thanks
    Adam
     
  3. Jimyb5

    Jimyb5

    2
    0
    Oct 18, 2014
    3.2v Lipo cells giving 700ah..
    thanks,
    jim
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Hi there and welcome to Electronics Point.

    The link in your post doesn't work - gets a server not found error.

    Do you have a link to the data sheet for that IC? Or can you post the data sheet as an attachment to your post using the Upload a File button?
     
  5. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
  6. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    But if you only have 12V how do you plan to measure upto 13V. Will you be using the potential divider mentioned in your first post?
    Adam
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Ah, it worked just then too. The site must have been down temporarily.

    I guess that's an option. There are other ways that might be cheaper, or smaller, or simpler, depending on your priorities. Here are a few that come to mind.

    1. "Flying capacitor" method using one DPDT relay per cell. Google flying capacitor relay isolation for details. Accurate; bulky; expensive; limited lifetime; simple.
    2. Transformer with secondary clamped to battery voltage. One transformer per cell. Energise the primary so a voltage is produced at the secondary. Connect the secondary through a series diode to the cell. When the secondary voltage exceeds the cell voltage plus the diode drop, the diode will conduct and clip the voltage at the secondary, which will be reflected back to the primary and the primary voltage will be clipped as well. Use an analogue multiplexer so one generator/measurement circuit can be switched to all transformers. Inaccurate; bulky; somewhat expensive; somewhat complex.
    3. Cell-powered op-amp circuit generating output current proportional to cell voltage. Have a small circuit using a low-voltage, low-current op-amp, that is powered from the cell and enabled through a low-current path to ground or via an optocoupler. When the circuit is enabled, it sources a small current into a grounded shunt resistor; this current is proportional to the cell voltage. All circuits can share the same shunt resistor. Accurate; compact; somewhat expensive; somewhat complex.
    4. It's late. That's all I can think of at the moment :)
     
    Arouse1973 likes this.
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I like number 2, only because I posted this a while ago on another post. Measuring lots of series cells can pose issues with high common mode voltages.
    Adam
     
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