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Charge gel cell with car charger?

Discussion in 'Electrical Engineering' started by Mike Cook, Feb 24, 2013.

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  1. Mike Cook

    Mike Cook Guest

    A friend has a battery powered Ryobi lawn trimmer ("weed whacker") which
    doesn't work. The 12vdc wall wart puts out only ~ 12.4v.

    I charged the lead-acid gel cell using a car charger. The charge rate
    (selectable) was 0.5A. Afterward the trimmer seemed to work as new.

    Can the car charger be used to charge this battery? He can install a timer to
    limit charge time if that's something required.

  2. Mike Cook

    Mike Cook Guest

    The gel cell is 12v.
  3. Guest

    Followups set to .

    Probably. Do you know what the amp-hour capacity is of the battery that
    is in there now? A little Googling *1 suggests that the original 12 V
    battery might have been six Gates/Hawker/Enersys "Cyclon" 2 V cells in
    series; the replacement that one site sells looks suspiciously like the
    common or garden 12 V 7 amp-hour "gel cell", plus a couple of brackets
    to make it fit. If the battery capacity is around 6 or 7 amp-hours,
    then the 0.5 A charger will work OK, as long as you don't mind it taking
    12 or more hours for a full charge.

    One manufacturer of these batteries *2 says this is how you charge the
    12 V, 7 Ah size:

    "Limit initial current to 2.1A. Charge until battery voltage (under
    charge) reaches 14.4 to 14.7 volts at 68°F (20°C). Hold at 14.4 to 14.7
    volts until current drops to under 70mA. Battery is fully charged under
    these conditions, and charger should be disconnected or switched to
    "float" voltage."

    So... the 0.5 A car battery charger should be OK. The charger you have
    may not do the automatic float and shutoff - more below.

    The voltage output by the car charger will be slightly different than
    what a sealed battery needs; this is probably not too important in this
    application, but may reduce battery life a little. If you want, you can
    get an "official" charger for sealed batteries for $35 or so; *3 has
    0.8 A output and will work a little faster. Or, get one of the
    automatic car battery maintainers like a Deltran Battery Tender or
    Schumacher Battery Companion; these usually have 1.0 to 1.5 A outputs
    and will automagically switch to a low-current charge once the battery
    is full. These are easy to find at local stores.
    If the charger doesn't have any kind of automatic shutoff, then this
    might not be a bad idea. The million dollar question is "how long is
    enough". The answer depends on how discharged the battery is. The
    simple answer (for a 7 amp-hour battery) is 7 amp-hours / 0.5 amps =
    14 hours. But: the charge won't be 0.5 A for the whole time. It may
    start off that way but it will go down as the battery charges up, so
    a full charge will take longer.

    At a guess, I would get one of those 12 hour spring wound timer switches
    (often used with attic fans) from the hardware store, and put it in an
    outlet box along with an outlet and a cord. Maybe set it for 2 or 3
    hours the first time, then when it shuts off, measure the battery
    voltage to find out how full it is. If not full enough, repeat. After
    a few runs you will know what to set the timer to initially.




    Standard disclaimers apply; I don't get money or other consideration
    from any companies mentioned.

    Matt Roberds
  4. Phil Allison

    Phil Allison Guest

    ** So you have no fucking idea what really happens.

    ..... Phil
  5. SoothSayer

    SoothSayer Guest

    No most 12 volt car battery chargers operate at 13.6 VDC. That is the
    CHARGE Voltage, not the voltage the battery ends up carrying. In order
    to charge a battery, one must apply a higher voltage than the battery
    operates at.
    You're an idiot. A car battery or 12V gel cell that reads 12volts IS
    fully charged and will put out its declared rate.
    You're an idiot. Perhaps you should provide a wiki citation or such.
    Right now you appear as a joke.

    Cross posting fucking retards.
  6. SoothSayer

    SoothSayer Guest

    Which are NOT meant for overcharge relief.
  7. Jamie

    Jamie Guest

    THe OEM charger most likely is unfiltered. Put a cap on the output but
    do not connect it to the battery and then do a voltage check.

    If you see a jump in voltage then this means the battery is on its
    last leg and you really shouldn't push it.

    A good test for this condition is to fully charge it with your auto
    charger and disconnect it and test the voltage a day later or few hours
    later to note the discharge.

    It is possible the charger is bad but I am going with a
    bad battery. You can always get a weak battery to appear good with an
    extra current source but the charge will dwindle.

  8. Guest

    Why are you here?
  9. SoothSayer

    SoothSayer Guest

    I wasn't saying you said something wrong. I was merely iterating that
    those vents are not for charging pressure relief, per se.
  10. SoothSayer

    SoothSayer Guest

    No shit, dumbfuck.
    No shit, dumbfuck.
    But you very likely have no clue as to why.
    It is also a FACT that a battery at 12 Volts is NOT discharged as you
    claimed, DORK.

    It has nothing to do with rudeness, it has to do with your lack of
    I don't need a primer from a dipshit who cries like a wimp when he gets

    And almost everyone knows that rechargeables and alkalines are not at
    the same voltage.

    AND, asshole, they are NOT "1.5 Volt NiCads". They ARE 1.2 volt

    so, I am "rude", but YOU are "stupid".
  11. Phil Allison

    Phil Allison Guest


    ** Most do not - eg:

    An acquaintance had a 12V gell battery like the one pictured explode
    violently while doing just what the OP here is contemplating. He was
    sleeping nearby at the time an got one HELL of a scare !!!!

    Makers all warn against using unregulated ( ie raw, rectified AC ) chargers
    with gell or SLA batteries.


    Makers also warn against shorting the battery and often quote a max safe
    discharge current.

    The penalty for ignoring this advice is a dead battery when the deliberate
    (weak link ) fuse inside goes open.

    ..... Phil
  12. SoothSayer

    SoothSayer Guest

    Until you load it.

    Unloaded, an old xformer/rectifier battery charger always had
    significantly higher voltage without a battery on it.

    BIG ripple!

    But once a battery is charged and even peaked out, that higher voltage
    peak can blow the damned things up, so all charging sessions had to be
    monitored to keep them from exploding. Then better chargers came out.

    Now, I wouldn't be surprised if there wasn't one you could operate or
    observe with your iPad. There are Wi-Fi and/or bluetooth handheld meters
    now. That would be enough to know when to go shut it off.
  13. SoothSayer

    SoothSayer Guest

    For NORMAL battery utilization. NOT a vent for charging pressure
    relief.. The device and or person doing the charging has to keep the
    process below any overt gassing level.
  14. Guest

    Ah Ha!
    Oohhh! I'm impressed. You took a *real* FCC license exam. Did you
    So you know it all. Great!
  15. Phil Allison

    Phil Allison Guest

    ** Kindly point this out in the pic I posted - smartarse.

    The battery will simply blow apart if overcharged.

    .... Phil
  16. Jamie

    Jamie Guest

    One of your fetishes?

  17. Guest

    I can't help it if you can't make up your mind.
  18. Jamie

    Jamie Guest

    Well, a lot of us love to watch a circus side show..

  19. mike

    mike Guest

    I understand how the light bulb works, but I'm very interested in
    hearing YOUR explanation.
  20. mike

    mike Guest

    So, you're saying that a light bulb can work very well for current charging
    as long as you pick the voltage and the bulb correctly.

    Also, a light bulb can work well to limit the maximum charge current
    to stay within the limits of the battery and charger, even in the event
    of shorted cells or other long as you pick the voltage
    and bulb correctly.

    You can do better. You can abuse any charge method.
    IMHO, the bang for the buck of a light bulb current limiter is
    second to none when properly applied.
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