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Charging lead acid batteries.

Discussion in 'Electronic Design' started by adrian, Nov 11, 2004.

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  1. adrian

    adrian Guest

    We have a charger that chargers a lead acid battery to 90 percent
    after 4 hours and 100 percent after 8 hours. Does anybody know what
    formula we should use for plotting a graph of Voltage versus time?
    please reply to .
  2. Ban

    Ban Guest

    You guys cannot use any formulas, because the actual voltage will depend on
    temperature, age of the battery, charge history and on the charger used.
    To charge a lead acid battery you can do the following: a power supply of
    14.4V (best is a corrected value according to a temperature sensor) will
    charge pretty fast with the max. current (Ah/3h). When the current starts
    dropping to 1/20th of the Ah-rating, you continue with that current until
    15.0V(formation), then trickle charge at 13.8V. So here we have 4 phases:
    constant current/constant voltage/constant current/trickle. The formation
    bit is done only once a month. Your charger will probably do something like
    this. Just measure with your voltmeter, take a reading every 10min. or so
    and draw that diagramm, which is valid only for that time.
    To find out the state of charge of the battery, you have to constantly
    monitor incoming/outgoing currents and calculate the charge with an
    efficiency multiplicator. There are dedicated ICs available for this kind of
    Look up the different charging methods on google.
  3. budgie

    budgie Guest

    How have you determined that it's 80% after 4 hours and 100% after 8 hours?

    The only "real" way to determine SOC is temperature and electrolyte SG for
    non-sealed non-gel lead acid batteries. And then refer to the manufacturer's
    data - it does differ. And it also depends on how precise the electrolyte mix
    and fill were at the factory.

    Which voltage are you seeking - the actual terminal voltage on charge, or the
    rest voltage for that SOC? Either way, you are going to need to measure the
    voltage rather than trying to determine a formula. If you really want to
    determine the rest voltage you are going to have to disconnect the charger (and
    any load) and allow a rest period of several minutes minimum before measuring.

    You need to explain exactly what your objective is.
    Nope, replies go to the group.
  4. Rich Grise

    Rich Grise Guest

    If you want to plot it, you won't get a formula until you do a
    curve-fitting on your plot. To _get_ the plot, measure the voltage
    every 10 or 15 minutes (the same interval each sample), and draw
    a graph.

    What is it you're trying to accomplish?


  5. Terminal voltage can be very useful for charge determination of Lead Acid
    batteries, but you have to take care :

    1. If the battery is left standing for several hours, you can use tables
    from manufacturer's websites - stuff like this :

    Open Circuit Voltage vs. State of Charge Comparison*
    (taken from EAST PENN VRLA (sealed) Manual)

    % Open Circuit
    Voltage Charge Flooded Gel AGM
    100 12.70-12.60 12.95-12.85 12.90-12.80
    75 12.40 12.65 12.60
    50 12.20 12.35 12.30
    25 12.00 12.00 12.00
    0 11.80 11.80 11.80

    NOTE: Divide values in half for 6-volt batteries.
    * The *true* O.C.V. of a battery can only be determined after the battery
    has been removed from the load (charge or discharge) for 24 hours.

    (taken from checking_battery_condition.pdf)

    100% 12.75 & higher
    85 to 100% 12.60-12.74
    75 to 85% 12.40-12.69
    50 to 75% 12.20-12.39
    25 to 50% 12.00-12.19
    12.00 & below fully discharged


    After standing a few hours and at a specified temperature, SG and terminal
    voltage are locked together, so voltage is a quality indicator of charge.
    For a sealed battery, voltage is the only indicator of charge. For all your
    further development, you can check other estimates of charge by taking out
    the battery, resting it and measuring voltage. Temperature compensating
    terminal voltage is possible when measuring capacity, but you are never
    certain, so I recommend you sit the battery in a water bath at 26.7 C (18 F)
    temperature - a good reference point for battery measurements.

    2. It gets more complicated if you can't stand the battery. If your
    application has a steady load which is not too heavy, you can still use
    voltage, but offset the voltage a bit - record voltage with load on, then
    pull out the battery, rest it and measure voltage.

    3. If you just want to know when your battery is fully charged, then forget
    all of the above. Get a decent voltage controlled charger and leave your
    battery on for as long as you can.

    If your cycle is brief charge followed by discharge, then your battery will
    have a short life - the longer you can charge on a voltage controlled
    charger the better.

    Don't use of a cheap, simple charger which just pumps current in until you
    turn off - your battery will die young, because you inevitably overcharge or

  6. as others have said, quite difficult to quantify, but here are a few

    A few links


    Serious error.
    All shortcuts have disappeared.
    Screen. Mind. Both are blank.
  7. Mac

    Mac Guest

    I guess you meant 80 F?

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