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info needed on nimh/nicad charging

Discussion in 'Electronic Design' started by prasad, Mar 27, 2005.

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  1. prasad

    prasad Guest

    Hello there,

    I understand that one of the common methods for termination of fast
    charge is the delta V technique. Kindly clarify the following :

    1. Is the voltage to be monitored while charging current is flowing or after
    switching it off briefly?
    2. If the latter, how soon ( or how much later ) should the voltage be
    measured after the current is switched off?
    3. How often should the voltage be monitered if the charging rate is around
    0.5C?
    4. What should be the optimum maximum voltage for nimh cells?
    5. If zero delta is used, should the charging be stopped immediately on
    finding that two consecutive readings are the same , or should we wait for a
    few consecutive readings?

    Thanks.
    Prasad.
     
  2. mike

    mike Guest

    This is a complex issue.
    If you measure it with the current on, resistive effects in the path
    come into play.
    IF you turn the current off, the voltage starts to drop, so you must
    measure at EXACTLY the same delay every time.

    I once built a charger that used a 555 as the timing and voltage
    detector. The voltage dropped for the entire off time, but since the
    off time was constant, the thing worked great.
    I tried to duplicate the operation using the PC parallel port for
    control. This FAILED miserably because the sample time latency was not
    constant. I leave the current on.
    This is another tradeoff area. First consider how much current you
    want. If the current is too low, the zero slope may not happen. Too
    much and temperature effects cloud the issue. I've been using C as the
    charging current with reasonable success.

    I monitor the current every minute with the current left on for the
    reason above. I terminate after five consecutive readings of the same
    value Or at the first lower value.

    Here are some of the reasons why.

    Noise is an issue if you try to measure small changes.

    My voltage measurement is just the voltage readback from a programmable
    supply. It has only 10mV resolution. It really should be better, but
    since I'm charging at least 4 cells most of the time, it works ok.

    For series cells, the zero slope happens at different times for each
    cell. In aggregate, it smears out.
    The problem gets much worse for NiCd cause the pack may never go
    negative. Most of my charging is for NiCd.
    The 5 minute compromise seems to work for me.

    You're gonna find this varies all over the map depending on the age and
    abuse of the cells. I use 1.6V/cell as an overvoltage termination, but
    sometimes, even that is not high enough for OLD computer packs.
    Don't forget to compensate for the resistance in the test setup.
    My wire and circuit breaker are 0.3 ohm total. Charging one cell at
    4A, the error term is almost as large as the cell voltage.
    Depends on how far apart the readings, see above.

    Don't forget to terminate on time. Once you've put 1.2X C into the
    pack, you might as well quit. Don't forget to terminate on temperature.
    If you start with a full pack or if something goes wrong, you want to
    shut it down before it explodes or catches fire.


    Cell life will be better if you're conservative.

    mike


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  3. On Sun, 27 Mar 2005 11:11:26 -0800, in sci.electronics.design mike

    snip

    I'd never considered the aggregate problem before

    Excellent summary Mike.



    martin


    Opinions are like assholes -- everyone has one
     
  4. Luhan Monat

    Luhan Monat Guest

    Yea Mike, thanks from me too. I've been tinkering with a custom charger
    for the large assortment of NiMH batteries I use lately.
     
  5. So all the cons should join the Republican party?
     
  6. keith

    keith Guest

    1) Nah, the Republicans want everyone to show pivture ID when voting.
    Post office pictures don't count.

    2) Since when have Republicans been conservative?
     
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