measure multiple batteries with isolation

Discussion in 'General Electronics Chat' started by bug13, Aug 24, 2016.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Hi guys

    Some people here may already know, I am working on a battery monitoring device for mini computer.

    Here is a little background info. The computer is powered by 12VDC and backup by a 12V SLA battery. I need a way to tell the computer the main is lost so the computer can shut down safely. This part is solved.

    But there are other batteries (backup for other system) that I will need to monitor too. Ideally I don't want these batteries to share a ground. The best I can come up with is this (below image), is there a better way to do this??

    Thanks guys!!

    IMG_1174.JPG
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,969
    388
    Looks good to me. You could also do it with transformers but that is more complicated.
    If you want to send the actual voltage, rather than just whether it is above some limit, you can use the same circuit by flashing the LED at a speed determined by the voltage.
     
    bug13 likes this.
  3. tcmtech

    Well-Known Member

    Nov 4, 2013
    2,041
    1,673
    So why can't the batteries share a common ground?o_O
     
  4. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    There are multiple devices connected to the computer via RS232 and USB, and they all individual powered by 12V and individual backup by a 12V battery. We have some grounding problem before, depending on how they connected together.

    Keeping the power supplies and battery isolated seem like a good option for me. Sharing a common ground will create some ground loop there, that is something I would like to avoid.

    NOTE: I have no control over how all the devices are powered and backup, or how they connected together. My role in this project is to design something to monitor all the batteries, and tell the computer to shut off safely when main fails and/or batteries low.
     
  5. crutschow

    Expert

    Mar 14, 2008
    13,053
    3,244
    Do you need to monitor their actual voltage or just indicate when the voltage is low?
     
  6. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
    1,208
    38
    Knowing the actual voltage is ideal, it makes me look good :p

    I think just indicate when the voltage is low is good enough. I actually didn't give this any thought, I will have to discuss with others. What do you have in mind if only low voltage indication is needed? Comparator with hysteresis or something like this?

    Your question make me think...
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,053
    3,244
    Yes, just a comparator with a reference voltage would be all you need.
    A low cost, single device that can do that is the TL431 voltage reference circuit (see pg 21).
    That will give a very stable trip point for any changes in supply voltage or temperature.
    The nominal trip point is 2.5V so you would put a voltage divider at the input to get the desired battery voltage trip level.
    The input resistor can be a 10kΩ pot (wiper to input and one end of pot to common) if you want to be able to adjust this level.
     
  8. crutschow

    Expert

    Mar 14, 2008
    13,053
    3,244
    Here's a comparator circuit using a TL431 with an opto isolator output.
    The output goes low when the battery voltage drops below the set point (set to near 12.06V in the simulation by pot U2, which is about 25% of the battery charge left).

    upload_2016-8-24_23-30-5.png
     
    bug13 likes this.
Loading...