Saturday, December 1, 2018

Testing BMS Voltage Sensing

In this article on building a BMS, one comment I received cautioned about "phantom" voltage reasons if one of the battery cell voltage sense wires broke, or a fuse blew.  His experience was that the phantom voltage was essentially the average of the two adjacent cells, and could appear to be a healthy battery when it's not.  That would be a problem, so I wanted to test it out.

Thankfully, my BMS doesn't exhibit that problem.  The three pictures below show the cell voltage readings, first with the sensor wires for cell #4 connected correctly, and then with each of the sense wires disconnected.

All sensor wires connected, voltage read correctly

Positive sense wire disconnected from cell #4.  Cell alarms and shows "phantom voltage of 17mv.

Negative Sense wire disconnected from cell #4.  Cell alarms and shows phantom voltage of 17mv
There is a small phantom voltage, but no where near enough to not realize there is a problem.

I think this all comes down to the internal circuitry of the sensing device, and in particular how much impedance there is between channels.

5 comments:

  1. Hi Peter,

    I'm happy to be wrong! Very interesting, The one I used was:

    http://www.wayjun.net/index.php?main_page=product_info&cPath=3&products_id=43

    And at $87 obviously has some internal differences. I guess you get what you pay for?

    I owe you a beer (or two).

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    Replies
    1. Wow, that is an astonishing rip-off of the ADAM product. Other than performance differences, the only thing I see different is that you need to order for a specific voltage or current sense range where the Advantech units are configurable for any of the ranges. A few other details too, but not many.

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  2. Hold the train :)

    After a rum on the rocks...it struck me that you had voltage on the adjoining cell. Hmmm.

    As I understand your wiring, there are 3 "legs" to each cell tap?

    1) ADC C4+ to "common"
    2) ADC C5- to "common"
    3) "common" to C4+ and C5- via a fuse.

    If you disconnect 1 or 2 above, all will be fine. But if you disconnect 3 above, both C4 and C5 should read near zero.

    What line did you disconnect?

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    Replies
    1. I'm not following your description. If you can use the "Contact me" form on the right of the blog, that will get us connected via email and I can send you a schematic showing what I did.

      Delete
  3. I just did that. Look forward to the email. I can share my schematic as well that way.

    ReplyDelete

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