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Q about Battery Charging

Discussion in 'Electronic Design' started by Norm Dresner, Mar 2, 2006.

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  1. Norm Dresner

    Norm Dresner Guest

    A company called ZBattery.com is offering a "6V Sealed Lead Acid Battery
    Charger" which, judging by the picture at
    http://store.yahoo.com/zbattery/6v-chg.html
    appears to be nothing but a 6V DC wall wart. Questions:

    1. Is this product likely to charge a (6v sealed) lead acid battery?

    2. Is this product likely to damage a (6v sealed) lead acid battery?

    3. If the answer to #1 is "yes", would any reasonable 6V DC wall wart be
    usable to charge a (6v sealed) lead acid battery?

    TIA

    Norm
     
  2. Yes!
    Yes!
    Yes!

    john
     
  3. Rich Grise

    Rich Grise Guest

    It's an ordinary wall wart.

    1. yes, it probably can, "for capacities up to 18Ah".

    2. Did you read any of the words there?

    "Note:
    Charger should not be left on battery indefinitely.
    Optimum charging time can be calculated by figuring the following:

    Capacity / 0.6= maximum charge time (assuming completely drained
    battery)

    So for a 4.5Ah battery use the following equation - 4.5 / 0.6 = 7.5
    hours maximum charge

    Please note that overcharging a sealed lead acid battery can permanently
    damage the battery, or reduce the capacity of the battery."

    So, yes, it's likely to damage the battery if you neglect to follow the
    instructions.

    Good Luck!
    Rich
     
  4. Macgyver

    Macgyver Guest

    Interesting that the picture actually shows an AC adaptor with the
    diagram on the front for a standard DC appliance plug on the output.
    This, plus the fact that the unit is only rated for 6V 600mA (you are
    not going to be able to charge an 18AH battery properly with this)
    leads me to believe either the picture is wrong or that the Marketing
    department were a little hung over when they wrote it (or more likely
    both).

    Misleading data supplied next to picture seems to indicate a 600mA
    constant current sourceSomething is definitely not right.

    Avoid this product as if it were a pile of dead chipmunks!

    And no, you cannot use a standard wall wart to charge a sealed lead
    acid battery, without additional circuitry to correctly charge and
    protect the battery. At least they got the fact that the battery
    should not be left on this "charger" indefinitely.
     
  5. Chris Jones

    Chris Jones Guest

    If you want to charge a lead-acid battery without damaging it, then I
    suggest one method is to build yourself a regulated power supply with a
    LM317 or similar, which could then be powered by some kind of wall-wart.
    You can adjust this to the voltage which the battery manufacturer
    recommends, e.g. look on the Yuasa web page, they have quite a bit of info.
    If the power source which is driving your LM317 would be damaged by a high
    current flowing then my approach has been to put some form of current
    limiting between the power source and the LM317, at its simplest this could
    just be a resistor in series with the input terminal of the LM317, which
    causes the LM317 to drop-out when the battery draws more current than the
    power source could safely supply. I also was worried that the battery
    might be discharged if the mains power failed during the charging process
    so I put a diode between the LM317 and the battery, but note that if you do
    this then the voltage from the LM317 definitely needs to be increased so
    that you get the right voltage after the diode. It is very important to
    measure the voltage at the battery terminals and check that this is correct
    according to the battery maker's instructions. The LM317 will probably
    need a heatsink. I suggest you put a fuse in series with the battery
    terminal to reduce the risk of fire if something in your circuit gets
    shorted.

    Chris
     
  6. kell

    kell Guest

    The wall wart puts out a nominal 600 mA at 6 volts. That's
    approximate; those ratings they print on wall wart labels are not real
    exact.
    6 volts is pretty flat for a "6 volt" lead acid battery, but if a
    battery really is that low and you put it on the wall wart, it would
    deliver ballpark 600 mA into the battery.
    As the battery takes on charge and its voltage rises, the current
    coming from the wall wart decreases. If the wall wart is pretty well
    matched for charging "6 volt" sealed lead acid batteries, then as the
    voltage gets around 7.2 to 7.5 volts, there will still be some current,
    but nowhere near the original 600 mA.
    This is called "taper charging," which is a fancy word for CHEAP
    UNREGULATED CHARGER.
    The problem lies in the fact that open circuit voltage on the wall wart
    is likely to be significantly higher than is healthy to expose the
    battery to, which is why you can't leave a battery on a "taper" charger
    indefinitely. It might go up to 10 volts eventually and damage your
    battery. And in the worst case, maybe this could happen even without
    the battery getting a full charge in the process!
    Proper charging for lead acid batteries calls for putting enough charge
    into them at a limited current until the voltage gets up to 2.4 volts
    per cell (flooded type) or as much as 2.5 for AGM batteries (sometimes
    called sealed or valve regulated). Then the charging protocol calls
    for holding a constant voltage on the battery for some time; the
    current flowing into the battery will decline eventually, and when it
    gets to about 3% of the amp hour rating of the battery, the battery is
    considered charged and should be taken off the charger or maintained at
    a float voltage of about 2.2 or 2.3 volts per cell.
    I charge batteries all the time with just a transformer and rectifier,
    but I know what I'm doing. Timing the charge isn't really the best
    approach.
     
  7. I've found the following site useful...

    http://www.gotbatteries.com

    They have "Powersonic" lead-acid chargers in a variety of capacities.
    They also sell a 6 volt lead acid in a "lantern" form factor. I've
    used these for several years and the PSC-61000A charger.

    Jim Veale


     
  8. Ted Wilson

    Ted Wilson Guest

    Sealed lead-acid batteries should be charged at constant voltage and a 6V
    SLA requires a fixed voltage of nominally 6.8V. (If you really want to do
    it properly, the voltage should be temperature compensated - can't remember
    the mV/C adjustment required OTTOMH, but a quick Google should throw up all
    the info you need. This would provide optimum protection for the battery
    for long-term use, but charging at 6.8V at normal room temperatures is good
    enough for most applications).

    Any half decent charger will usually include some form of current limit, but
    this is usually for the protection of the charger itself, as opposed to the
    battery, which can handle quite large currents.

    One of the appeals of sealed lead-acid batteries is that they can be left on
    'float charge' on a suitable charger pretty much indefinitely, with the
    charger and battery taking care of themselves without the need for
    intervention or monitoring.

    However, the charger shown in the link doesn't come close to being a 'good'
    charger and should only be used with care - the SLA would almost certainly
    be damaged if left connected to such a supply for any length of time.

    Ted
     
  9. kell

    kell Guest

    6.8 volts is good for float, but if you want to charge a battery in any
    reasonable time you need a higher voltage than that..
     
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