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Flooded lead acid cell impedance

Discussion in 'Electronic Design' started by [email protected], Dec 30, 2012.

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  1. Guest

    Could some electronics guru please shed some light
    on this ? What is an approximate impedance value
    for a flooded lead acid cell battery, e;g., a 12V
    32 AN cell. I have gone through a number of online
    sites and material, e.g., 'Battery Univerdity but
    that has not been of much help. Thanks in advance,
     
  2. Sjouke Burry

    Sjouke Burry Guest

    wrote in :
    Crancking amps mostly suggest battery impedance,
    Just devide the voltage drop(2-4V) by the cranking amps
    gives the internal resistance.
    It is a gamble what the voltage drop is, some define 10 volt
    out at CA, others define 10V at Ca.
     
  3. On Sat, 29 Dec 2012 23:41:53 -0800 (PST), the renowned
    Okay, I'll give this a shot.. dunno your specs for that battery, but
    here's a (presumably) similar battery:-

    http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf

    Page 68 has the discharge curves (voltage as a function of charge
    state for various output currents).

    The internal resistance is a function of output current, time,
    temperature as well as charge state.

    From the curves you can read off the "impedance" as delta-V/delta-I
    for various values of charge state and current.

    As you can see, the impedance increases as the battery is depleted--
    a change in current from C/3 to C/100 (almost open circuit) results in
    about a 1.9V drop at 10% charge. At 100%, about 0.9V, so you could say
    the internal resistance more than doubles. If 'C' is, say, 300Ah then
    the C/3 = 100A and C/100 is 3A, so delta-I is 97A, and the effective
    internal resistance changes from about 9mOhms to about 20mOhms. It
    will be worse cold.


    Best regards,
    Spehro Pefhany
     
  4. mike

    mike Guest

    That's a very vague question. What are you trying to
    accomplish?

    Do you really care about the impedance? Over what
    frequency range, state of charge, temperature, age???

    Maybe you're interested in the equivalent series resistance?
    But what is that really?

    Maybe you want to know how much the voltage will droop
    when you load it?
    That's easy...well...no it isn't.

    A battery that's been sitting will have some relatively
    high voltage. That voltage will drop rapidly at first,
    then settle down to some lower rate.

    The only way I've found to reliably gauge actual battery
    performance in an application is to emulate the application.

    Typically, I start with a fully charged battery.
    Let it sit for a while.
    Add the anticipated load and sit for a while.
    Add or subtract come current and measure the change in
    voltage. Use deltaV/deltaR as the dynamic resistance.

    The definition of "while" in each case depends...
    Graph the data and pick a time that works to get you
    the info you need.

    Realize that sulfation will dramatically increase the
    dynamic resistance as the battery ages.

    Another interesting experiment is to use a pulse load.
    There'll be an initial step in voltage that's sorta
    related to resistance and a more gentle slope that's
    sorta related to chemical reactions.

    When people ask that sort of question, they're often
    contemplating HIGH charge/discharge currents. That's
    a different bucket of worms.
     
  5. mike

    mike Guest

     
  6. Situation gets even more complicated when you charge the battery. If
    you assume a 'full' charge is at 13.8 volts, then you might expect that
    if you apply 14.8 volts ( ie a 1 volt difference ), then at say 10 mohm
    impedance, as measured rom the discharge current capability, you might
    expect 100 amps to flow into the battery. Not so !
    The battery voltage will 'float' up to 14.8 or even up as high as 15.5
    volts or so, but the current will fall to only a few ma.
    Electro-chemistry is not anything like a conventional ohmic resistance !
     
  7. Guest

    Thanks to each of you for your comments. I am aware
    of the complicated nature of battery chemistry. I
    am creating some first-cut SPICE simulations of a
    battery system, and was looking for am approximate
    value to get started with, before adding a full-
    scale battery model.
     
  8. Jasen Betts

    Jasen Betts Guest

    close enough to 0 that you don't want to short circuit it ever.
     
  9. Tauno Voipio

    Tauno Voipio Guest

    A good 12 V car battery has a short circuit current in excess
    of 2500 A, if you have a good enough short.

    Please be careful with rings and metal watch bands.
     
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