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Sealed lead acid battery maintenance?

Discussion in 'Electronic Basics' started by John Doe, May 28, 2010.

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  1. John Doe

    John Doe Guest

    I would like to learn something about proper care of sealed lead
    acid (SLA) batteries. Is there any good authoritative science on
    how SLA batteries should be cared for? I am searching... I suppose
    battery makers/distributors would be one of the best sources of
    information about that?

    Specifically... I wonder why instructions that come with SLA
    batteries so strongly emphasize charging the battery immediately
    after use, without specifying how much use. Are they concerned
    that the battery might eventually self discharge too deeply? Or
    does it degrade an SLA battery to rest it in any state but fully
    charged?

    Thanks.
     
  2. Phil Allison

    Phil Allison Guest

    "John Doe"

    ** If an SLA battery is left in a discharged condition (ie less than 2 volts
    per cell ) for a long enough, ie weeks or months, it will corrode internally
    and be ruined. Same goes for most lead-acid car batteries too.

    Not true for Ni-Cd or NiMh cells though.


    ...... Phil
     
  3. All lead-acid batteries degrade with time. The degradation rate goes
    up with increasing temperature and/or decreasing charge state.

    In other words: Keep them cool and fully charged.

    Note that proper charge voltage is dependent on temperature, so if you
    keep the battery in the freezer, your charger must be able to adjust
    the voltage accordingly.
     
  4. John Doe

    John Doe Guest

    Is the degradation versus charge state rate linear? I guess not.
    Does the act of charging an SLA battery also degrade it? I have
    always thought that charging a battery degrades it, but maybe that
    is a misinterpretation of the maximum number of charge cycles.

    Anyway... Keeping it fully charged is impossible here since it has
    to be used. But of course degradation takes place over time, so I
    guess that is why they emphasize recharging the thing immediately
    after using it. And the deeper the discharge, the more important
    that is.

    Thanks.
    --
     
  5. Sjouke Burry

    Sjouke Burry Guest

    Very long trickle charge will kill one. That sulfates the plates,
    and is difficult to recover from.
    Also leaving a small consumer on for the winter period is a great
    killer.
    Often someting like a radio, a voltmeter, a computer chip, etc.
    Best to remove one cable from the battery, to make sure.
     
  6. The long and short of it:

    Don't discharge the battery.
    Don't overcharge the battery.


    That kinda impedes on your actual use of the battery, so, a more useful
    compromise would be:

    Don't discharge the battery any deeper than you can help it, and charge
    as soon as possible afterwards.

    While charging, don't exceed the maximum charge rate for your battery.

    Use a float charger, and ensure the maximum float voltage is not higher
    than the maximum spec. This varies with temperature, so may be an issue
    if the environment changes widely.




    As an example, we used 100A/hr or so SLAs in an application where they
    were deep discharged (over a week or so) as a matter of course. Perhaps
    left for a couple of days in that state, then charged afterwards.

    We'd get end of life after about two years of constant abuse like this
    - about 80% or less service capacity left before it was no longer
    acceptable in our application.

    Which is not bad, considering two years is considered the service life
    of SLAs in more critical applications like UPSs and such, in an
    environment SLAs must love.
    Not quite, at deep discharge states, the plates under go irreversible
    chemical reactions that prevent an ideal charge from taking place
    afterwards.

    There's more that can go wrong too. Which is why I said "don't
    discharge the battery" not entirely tongue in cheek. :)
     
  7. :)

    Just because you *can* do that, doesn't mean you *should* do that.

    The recommendation is two years, to ensure the battery doesn't
    magically fail soon afterwards. In fact, in those conditions, they're
    probably almost guaranteed to last double or more of that.

    Problem is, "almost" doesn't cut it in a mission critical application,
    thus the two year replacement recommendation.

    This is why some UPSs have an automatic scheduled "test", where they
    shut down the server but leave it powered on, go to battery backup,
    ensure it lasts pre-determined amount of time and power up again.
    Otherwise you have no way of knowing a battery has failed.

    I should qualify my quoted constraints assume normal domestic SLAs.
    Higher grade UPSs use higher grade batteries that can last many years
    longer before the recommended replacement.
     
  8. Martin Brown

    Martin Brown Guest

    And it is at least worth the occasional visual inspection of the
    batteries for obvious signs of swelling and distortion of the case.
    UK replacement is annual test and replace on fail or 5 years old
    whichever happens sooner. Mine live in a fairly cool environment and
    would still hold a decent charge when retired. I have seen some 8 year
    old ex UPS cells that I took to the recycle centre in a bucket because I
    was unsure if the plastic cases would hold out. They were very obviously
    mangled shapes with big bulges but still not leaking.
    Exactly. No point in having an emergency back up that doesn't actually
    work when you need it. Same applies to emergency lighting.
    Even so it makes sense to check them annually. You don't really want a
    failed SLA spewing corrosive electrolyte over your UPS.

    Regards,
    Martin Brown
     
  9. Unfortunately, that's not an effective enough test. Most fail at end
    of life looking like new. So a visual test alone is not enough.

    Some of the large commercial scale units perform an internal battery
    test with a limited load, with the marketing blurb claiming it's good
    enough to detect a dud battery even while the unit is live and running,
    never having to go offline for the test.
    We were so poor, we had to used other's disposed of batteries as our
    new ones.

    We went so far as trying to recover open circuit SLA cells. We dumped
    10-15+ volts into the things (single 2v cells!), till they took a
    charge, then they somehow magically worked. For a while anyway.

    That's on good quality (albeit old) cells. I've never really had any
    luck reviving cheapies.
    True. I've seen batteries go short, overheat, dry out and become
    basically useless. A real chore to get the buggers out because they had
    warped so much.

    No-one noticed because the power had never gone out in that time (many
    years) till someone thought it would be a good idea to actually look at
    it...
     
  10. legg

    legg Guest

    Check the Yuasa website for VRLA operation and maintenance
    recommendations:

    http://www.yuasabatteries.com/literature.php
    http://www.yuasabatteries.com/pdfs/TechMan.pdf

    There used to be some good stuff from Exide, but I've lost links to
    the electronic format manuals.

    RL
     
  11. legg

    legg Guest

    This server only provides ~ months back postings.

    If it's archived somewhere, I'd be interested.

    RL
     
  12. Jasen Betts

    Jasen Betts Guest

    dejanews (now called google groups)



    --- news://freenews.netfront.net/ - complaints: ---
     
  13. I've chatted with and watched quite a few large (tens to hundreds of kVA)
    UPS maintenance techs.

    the standard procedure seems to be a visual check, retighten all
    connectors, measure voltage across all batteries, then again with the UPS
    in test mode where there is a real load across the batteries.

    They can usually spot a bad battery before it would cause problems.

    larger UPSes have redundant strings of batteries so even with a dud you
    can still operate.
     
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