Cell Voltages in Series

Discussion in 'General Electronics Chat' started by fredp, Sep 28, 2009.

  1. fredp

    Thread Starter Active Member

    Sep 24, 2009
    41
    0
    Hi,

    I am trying to simultaneously measure the voltages of individual cells in a configuration of cells in series.

    When I connect digital panel meters such that the negative input of one meter is connected to the positive input of the next meter, and both are connected to a leg connected to both cells, it doesn't work.

    The panel meter vendor says that the meters are interfering with each other.

    Does anyone know any alternative way of measuring all the cell voltages simultaneously?

    I've attached a schematic of what I've tried. The power source to the meters is independent of the cells being measured.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I have locked your earlier, identical thread.

    Myself, I would use a two pole rotary switch and one set of resistors and a single meter.

    If this is not satisfactory, please wait for more input, and not open yet another thread.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The power source to the meters may be independent from the cells, but I have a feeling you're using a single source to power all of the meters; and the meters may be using one of their leads to establish a ground reference point. See the attached modification to your schematic to get an idea of what I'm talking about. The resistors next to the meters are actually inside the meters; there wasn't room to show them in that manner. Their value is unknown.

    To avoid that situation, you'd have to use independent floating power supplies for each meter, or use switches/relays to only monitor one battery at a time.

    [eta]
    I suggest that this thread is not identical to the first; additional critical information has been supplied. However, the additional information does render the prior thread obsolete.
     
    Last edited: Sep 28, 2009
  4. fredp

    Thread Starter Active Member

    Sep 24, 2009
    41
    0
    Thanks for the advice SgtWookie. The meter itself has many connections on it. I was under the impression that it was designed for use in an application like this. It has a com, inlo, inhi, and rel connection, which are all tied together and connected to the negative side of the cell being measured in the wiring diagram provided.

    http://www.circuitspecialists.com/images/CX101-1.pdf

    I'm wondering if connecting these pins in some alternative configuration would have any effect.
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    From an E-mail by the OP:
    First, please keep your inquiries in the thread rather than via E-mail or PM. If you send members E-mail or PMs, it fills up their PM and E-mail boxes, and denies you of knowledgeable replies from other members.

    The modified schematic that I posted was merely my concept of what is happening with your circuit. As I previously mentioned, I believe the panel meters are referencing themselves somehow to one of the 5v meter supply or returns.

    If you want to be able to use all of the meters concurrently, you will need four separate 5v power supplies that are truly independent of each other; no interconnecting grounds/wires.
     
  6. fredp

    Thread Starter Active Member

    Sep 24, 2009
    41
    0
    Thank you for the info. I am going to purchase a DIN mounted 5 V power supply for each of the meters. Hopefully that will resolve the problem. Now I understand what you were trying to illustrate with your schematic.

    Unfortunately, it is important that we are able to monitor all the cell voltages simultaneously--switching isn't really an option for the application.

    Thanks again all who responded. I will update on success or failure with the power supplies
     
  7. fredp

    Thread Starter Active Member

    Sep 24, 2009
    41
    0
    So, I've been reading about this issue and I'm wondering about the concept of Galvanic isolation. Would two separate switching power supplies, both connected to mains, be considered Galvaniclly isolated?
     
  8. Tesla23

    Active Member

    May 10, 2009
    318
    67
    Why don't you find a meter that can stand a reasonable common mode voltage. For example http://www.datel.com/data/meters/30lcd.pdf running from 9V give 6V of common mode range, which should be plenty as if you rearranged your resistor network slightly (swapping the 1.8M and 200k resistors on one end network) you really only need 3.3V of range. Then only one power supply for the meters.
     
  9. fredp

    Thread Starter Active Member

    Sep 24, 2009
    41
    0
    Well, in the complete system, there are actually 10 cells and 10 panel meters. I just included enough in the schematic to provide the concept.
     
  10. Tesla23

    Active Member

    May 10, 2009
    318
    67
    Then you need 15V of common mode range. If you can't find a suitable panel meter, why don't you simply use 10 instrumentation amplifiers running off something like +/- 15V, to produce 10 voltages all referenced to the one common voltage, and measure those?

    Alternatively, I don't know what sort of accuracy you want, but you could alter your resistor network to divide the common mode voltage by 10 as well, then the 15V drops to 1.5V. The trade-off is accuracy, worst case the end cell voltages could be in error by something like 15mV if you used 0.1% resistors.
     
    Last edited: Sep 28, 2009
Loading...