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Track depth of discharge by coulombs?

Discussion in 'Electronic Basics' started by BobG, Aug 8, 2007.

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  1. BobG

    BobG Guest

    Can I model a 100% charged lead acid battery as a store of X coulombs
    and use a microcontroller to integrate the current out through a shunt
    or hall sensor to keep track of depth of discharge? Or is there some
    other tricky factors to consider?
  2. Guest

    Yes. Technically straightforward but the gods are with you if you can
    estimate within say 100% of the true charge state. The buggering
    factor is the current not running pro rata with the remaining
    Yuasa have a good PDF handbook on their SLA batteries. The graphs
    show various discharge currents versus resulting time to a flat
    battery. There's about a 10:1 variation in apparent Ampere hours
    capacity, dependant on how fast you flatten the battery.

    Having said that and if you have the graphs for the particular battery
    in use and the insane patience to translate them into stored lookup
    tables + ambient temp'+ battery temp' monitoring. You may see a +/-
    25% estimate accuracy.
    It's much easier and probably as accurate, to just WAG from the
    battery voltage :)
  3. BobG

    BobG Guest

    I knew there had to be a catch. Nothing's quite as easy as it seems is
  4. Yes, there are tricky factors to consider.

    google "Peukert's Equation (Peukert's Law)".

    Peukert essentially says that the faster you discharge a battery, the
    less total energy you can extract from it.

    A lead-acid deep-cycle battery is rated in ampere-hours, assuming the
    battery is discharged over a period of 20 hours. If you discharge at
    twice the "20 hour rate", you will only get about 89% of the rated

    Peter Bennett, VE7CEI
    peterbb4 (at)
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  5. Jasen Betts

    Jasen Betts Guest

    if it's a wet battery (liquid acid and not gel) you may do better to
    measure the density of the electrolyte... yuasa make a filler cap that
    changes colour dependant on this...

  6. Circa Wed, 08 Aug 2007 19:10:12 -0700 recorded as
    <> looks like Peter
    And it gets even more complicated than that! Different batteries will have
    different time frames set for their final amp-hour rating. The lead-acid
    storage batteries I work with are rated to an 8-hour discharge rate, as
    opposed to your stated 20-hour rate. (The major difference is probably due
    to the difference of deep-cylce vs. backup.)

    An added complication is that the battery's capacity will change depending
    upon how it is discharged. I don't have the data at hand, so can't quote
    the equation, but basically, if the battery discharge is not uniform, and
    is heavier during one part of the discharge than another, total capacity is
    changed from the uniform 8-hour rate, and the change differs dependent upon
    whether a light draw is followed by a heavy draw, or vice-versa.

    The data for each particular battery should be obtained from the
    manufacturer, and as a previous poster noted, the simplest method of
    determining discharge state is to measure cell voltage and compare to data
    in hand, and the accuracy of this method is usually good enough.
  7. BobG

    BobG Guest

    Any idea how to read electrolyte density with a microcontroller? PH
    Hydrometer rigged up to a gearmotor to squeeze the bulb and a camera
    and frame grabber and dsp to see where the ball is floating? Or
    something like that? How to you get this rig over the cells? An x-y
  8. A digital refractometer would probably work best (if you
    could afford it):
  9. Jasen Betts

    Jasen Betts Guest

    I'd start with one of the yuasa magic eye filler caps and look for a
    way to measure the colour, (RGB LED and a phototransistor maybe)

    The cap incorporates a light-pipe and a 2 part floating hygrometer that
    exploits two parts of differing shape that change alignment in reponse
    to the density of the electrolyte.

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