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How to measure the amount of energy stored in a battery ?

Discussion in 'Hobby Electronics' started by Guest, Jan 20, 2005.

  1. Guest

    Guest Guest

    When I measure an old battery with a voltmeter I can read almost full
    voltage of the battery. But when I connect it to a load voltage drops. Is
    there a way of measuring the amount energy stored in a battery WITHOUT
    wasting its energy (without drawing to much current) during the measurement
    ?
     
  2. KLR

    KLR Guest

    as far as Im concerned - a battery should be tested under load to get
    its true remaining energy level. Preferably the load under which its
    expected to operate under most of the time in normal use.Unfortunately - this will use some of its energy
    even applying a voltmeter to a battery has to use some energy from the
    battery in the process.
     
  3. Ed [:-\)>

    Ed [:-\)> Guest

    assume the internal ris increases with reduced remaining capacity and
    you can work the rest out yourself
    the test discharge current pulse can be short if using some smarts.

    if it is a lead acid cell then the SG of the electrolyte can be measured
     
  4. Rod Speed

    Rod Speed Guest

    Thats only part of the story tho, particularly with sulphated plates.
     
  5. Phil Allison

    Phil Allison Guest


    ** That depends entirely of what sort of cell - common "dry" cells can be
    tested with a volt meter and a load resistor or just measured while in use.





    ................... Phil
     
  6. Put a resistor in parallel with your voltmeter. Choose the resistor to draw
    the same current as your appliance will draw. Connect to battery for a
    second or so while you read the voltage. Select the end of life point as a
    voltage - say 1.3V - below which your application will not work - and at
    which point remaining cell life and reliability is poor.

    This test is realistic, because it duplicates the conditions of service for
    the battery.

    In strict terms, it is very hard to estimate the actual *energy* stored in a
    battery.

    If you go to Radio Shack and buy a *battery tester* you will get a voltmeter
    with a load resistor wired across it - just like I suggest.

    Roger
     
  7. Mike Harding

    Mike Harding Guest

    So tell us the rest...? If you know?

    Mike Harding
     
  8. hadda hadda

    hadda hadda Guest

    NO
     
  9. Rod Speed

    Rod Speed Guest

    Measuring the electrolyte is only part of the story
    on remaining capacity with a lead acid battery, stupid.
     
  10. Mike Harding

    Mike Harding Guest

    So tell us the rest...? If you know?

    Mike Harding
     
  11. Franc Zabkar

    Franc Zabkar Guest

    An alkaline battery has a fairly linear discharge characteristic
    whereas NiCd and NiMH rechargeable batteries maintain close to 1.2V
    full voltage up to 80-90% of usage. They tend to go flat very
    abruptly. I can see how one could reasonably estimate the remaining
    capacity of an alkaline battery by measuring its terminal voltage at a
    certain discharge rate, but a rechargeable battery would be more
    difficult. That said, mobile phones can assess their battery's state
    of charge, probably by pulsing it with a predetermined load. My own
    phone doesn't do a very good job of this, though. The battery meter
    can be at 50%, but the "attention: battery low" warning still appears.
    <shrug>


    - Franc Zabkar
     
  12. Phil Allison

    Phil Allison Guest

    "Franc Zabkar"

    ** Impossible is the word.


    ** With devices like mobile phones or lap tops there is often a micro
    dedicated to monitoring the internal battery - the time duration and the
    rates of charge or usage are constantly measured, the data is processed and
    a **model** of the battery's likely condition held in memory and also
    displayed as a "fuel gauge".

    Sometimes the calculated **model** and the battery get right out of whack
    and the micro has to be reset while the battery is fully cycled -
    likewise if the battery becomes defective or dies of old age the model will
    be wrong.



    ............... Phil.
     
  13. Rod Speed

    Rod Speed Guest

    Wota terminal fuckwit you have always been, Harding.
     
  14. Ross Herbert

    Ross Herbert Guest

    There is (as yet) no method of accurately measuring the state of
    charge of a battery. All of the suggested methods will provide an
    approximation of SOC and even then there may be some reliance on
    interpretation of the results of any test.

    Cadex are probably one of the most innovative companies specialising
    in battery testing equipment and the history timeline
    http://www.cadex.com/about_history.asp will give some idea of how long
    they have been trying to solve this problem. Most of their equipment
    is concerned with batteries for portable quipment but their latest
    Spectro CA-12 may yet provide the closest approximation to the true
    SOC of a lead-acid battery http://www.cadex.com/prod_testers_ca12.asp

    This page http://www.cadex.com/prod_testers.asp has links to some
    articles which detail the issues related to determinining the SOC (or
    reserve capacity) of batteries.
     
  15. Mike Harding

    Mike Harding Guest

    So you don't actually know then, I'm not surprised.

    Mike Harding
     
  16. Rod Speed

    Rod Speed Guest

    Wota terminal fuckwit you have always been, Harding.
     
  17. Franc Zabkar

    Franc Zabkar Guest

    That's the prevailing technology for expensive lithium laptop
    batteries, but I don't believe my mobile phone battery works that way.
    If your model were to apply to my phone, then why would it display a
    50% battery meter while warning that the battery was low?

    With an old battery, I have observed the battery meter drop to about
    25% over a couple of days and then mysteriously increase. No calls
    were made or received during this time.

    I have two identical phones with identical, genuine Ericsson NiMH
    batteries. If I swap batteries between phones, one fully charged, the
    other at about 25%, then the phones correctly assess the state of
    charge of the replacement batteries ... almost. The fully charged
    battery from phone A shows only about 90% in phone B.

    My conclusions are that there is no intelligent IC within the battery
    pack, otherwise the SOC info would be communicated to the new phone.
    Secondly, the phone itself senses the SOC in some way, probably by
    measuring the terminal voltage at a certain discharge rate. That's why
    the battery meter is often updated during a call, especially when the
    SOC is low.

    One other observation about my phone is that it will refuse to charge
    a battery pack that it considers to be dead flat. To get around this,
    I trickle charge the pack using a DC supply and series resistor until
    the voltage is at a reasonable level. I then install it in the phone
    and allow the phone to complete the process.


    - Franc Zabkar
     
  18. Phil Allison

    Phil Allison Guest

    "Franc Zabkar"
    "Phil Allison"


    ** Frank - can you fucking read at all ???

    What does the very first line of my post say ???



    ** Your particular phone is your problem.





    .................... Phil
     
  19. Franc Zabkar

    Franc Zabkar Guest

    Ummm, the very first line says "Impossible is the word."

    impossible
    ADJECTIVE: Not capable of being accomplished.

    Sure, nothing beats counting the coulombs into and out of a cell, but
    clearly it *is* possible to estimate the SOC without any dedicated
    monitoring chips. The accuracy may not always be great, but it is good
    enough for most practical purposes. Mobile phone manufacturers have
    been doing it this way for years.

    In fact, here is an innovative, accurate, 100% analogue circuit tuned
    for a particular battery that relies solely on ESR to predict SOC:
    http://www.hagtech.com/pdf/fuel.pdf

    It has no ICs, smart or otherwise, just 6 transistors and a zener. It
    also shows the SOC while the battery is being charged.

    AFAICT, smart monitoring is generally reserved for lithium-ion
    batteries because they are less resistant to abuse. These chips do a
    lot more than just report the SOC.


    - Franc Zabkar
     
  20. Phil Allison

    Phil Allison Guest

    "Franc Zabkar"
    "Phil Allison"

    ** " With devices like mobile phones or lap tops there is often a micro
    ....... "

    Your bloody phone is not the subject !!!


    ** What BULLSHIT - you have ZERO idea if that design works accurately in
    practice.


    The "impossible" question is to determine, with a brief test, the SOC of a
    NiCd or NIMH battery that is presented for such test. The continuous
    discharge curve is of **no use** as a predictor when a battery is used
    intermittently - as is usually the case.

    Cells recover when at rest for some time, voltages rise and ESRs fall -
    this * voids* the accuracy of tests that rely on those two parameters. The
    temperature and age of the cells also has a major effect on those same
    parameters.

    The upshot is that a NiCd or NiMH battery placed on a brief test will show
    an ESR and voltage reading consistent with near full charge when in fact the
    true SOC is as little as 10 % of capacity.




    ............. Phil
     
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