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A question on Lead Acid Batteries

Discussion in 'Electronic Design' started by martin griffith, May 24, 2007.

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  1. may help
    After full charge, remove the battery from the charger. If float
    charge is needed for operational readiness, lower the charge voltage
    to about 13.50V (2.25V/cell). Most chargers perform this function
    automatically. The float charge can be applied for an unlimited time.

  2. GregS

    GregS Guest

    When bringing up batteries to full charge, overcharging is standard
    practice. While on constant float, it does not get that overcharging
    which helps get rid of sulfation, and increases storage.

  3. Noway2

    Noway2 Guest

    I have a question regarding lead acid batteries, specifically regarding
    the concept of holding them at a float voltage.

    I am wondering what are the advantages and disadvantages of holding the
    batteries at a float voltage as opposed to letting them self discharge
    for a short while, say 24 hours, and then performing a charge top off.

    The batteries are in a system that requires that they be maintained in a
    readiness state, which is why I suggest only letting them self discharge
    for a few days at most before performing a top off.

    As I said, I am curious about the relative advantages and disadvantages
    of the two methods. Does anyone have an insight or experience that may
    help me decide which method to use?
  4. Martin

    Martin Guest

    My understanding is that for battery health, either float or isolation
    with periodic recharge are both approximately equally acceptable.

    In either case, you will need to initially fully charge the battery,
    so the equipment for that will be equivalent.

    After that, float is MUCH easier to do automatically ...

    to float, you hook up to a constant voltage source and you're done.

    with periodic recharge, you have to have some kind of timer or voltage
    sensor, a way to connect to the battery,. a way to tell when the
    battery is charged and disconnect again.

    If you have parasitic loads connected to the battery, it doesn't
    affect the float scheme at all (assuming the float source can handle
    the extra current)
    The periodic recharge scheme would perhaps need the float interval
    length adjusted.
  5. Gibbo

    Gibbo Guest

    With both flooded lead acid and Gel or AGM lead acid batteries there is
    a slight advantage with constant float charge as it prevents the build
    up of sulphation. Each time the battery is discharged, no matter how
    little, and no matter whether it's by way of a load or self discharge,
    some of the sulphate remains as sulphate even after a full recharge.
    This is just a part of the natural battery ageing process.

    Maintaning a constant float charge reduces the extent of this effect
    dramtically over the extent when the battery is allowed to stand then

    With wet cell (ie flooded) lead acid batteries there is a slight problem
    in that during float, the electrolyte stratifies and the first discharge
    cycle after an extended float does not produce the full battery
    capacity. This effect doesn't happen with periodic recharge following a
    few days with no charge. Well it does happen during the standing period,
    but the following full charge solves it.

    For this reason many advanced chargers run float with a periodic full
    charge at some interval between about 48 hours and 28 days. Different
    manufacturers have different ideas what works best.

    As regards battery life, the difference between the two methods is
    actually quite minimal. But the effect of the reduced first discharge
    following extended float may sometimes cause problems.

    With AGM and Gel cells there is no doubt that maintaining a constant
    float is the better option.
  6. Pete D

    Pete D Guest

    We use the method of float charging with a constant regulated 27.6V (2
    series 12V gel type) We have encountered no problems, some battery
    manufacurers recommend a salightlu lower voltage for float charge but as
    I said we've had no problems with approx 2000 units out in constant use.
    We do recommend replacing the batteries sooner than battery
    manufacturers suggest, but the application is a fire safety system and
    needs to be reliable.
  7. Noway2

    Noway2 Guest

    First off, I would like to thank everyone for taking the time to reply
    to my inquiry. The responses have given my some ideas to consider on
    both sides of the fence.

    This application is also part of a fire safety system and reliability
    is my foremost concern, which is why I can make arguments for both
    ways. One of my biggest areas of concern is float voltage versus the
    temperature, which can vary wildly with different installations.

    As it stands, I am thinking about defaulting to a float condition, but
    leaving the option for a periodic top off charge instead as a
    programmable / selectable feature.
  8. James Beck

    James Beck Guest

    We do the same sort of thing.
    A current limited bulk charge with a float at the end and you can leave
    it floating if/all you want.
    Lots of units in the field, no problems.
    I went with the classic 13.7V for the float, but with resistor
    tolerances it can go +/- .1V anyway.

  9. Rich Grise

    Rich Grise Guest

    It wouldn't do any harm to monitor the batteries' temperature, albeit it
    would add complexity to your circuit. But a battery with a float voltage
    that's adjusted for temperature is a very happy battery. :)

    Good Luck!
  10. g

    g Guest

    The TI chip I used several years ago, took all that into
    consideration. It was a Unitrode chip and just need a pass transistor.
  11. g

    g Guest

    The TI chip I used several years ago, took all that into
    consideration. It was a Unitrode chip and just need a pass transistor.
  12. James Beck

    James Beck Guest

    The TI's I've seen need a bit more than that.
    Resistors to set the bulk charge rate, peak voltage, float voltage, and
    so on.
    The UC3906 is a prime example.
    Nice IC's use them in our upper end stuff, but they do need a bit more
    than a pass transistor.

  13. rebel

    rebel Guest

    Indeed they do, but IMOE they are one of the best SLA/gel charge controller
    chips around. I highly recommend the O/P read the TI/Unitrode appnote U-104
    for them, to be found at:
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