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1200mA Power supply and 12Ah battery

Discussion in 'Electronic Basics' started by jaffy james, Aug 28, 2004.

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  1. jaffy james

    jaffy james Guest

    Hi,

    I have a mini LCD TV screen which requires a power supply of 12v -
    1200mA. I would like to power the device using a battery. As i don't
    want to blow up the TV i wanted to make sure the battery I use is
    correct.

    The voltage is correct, but I don't know what 12Ah means, which are on
    the batteries. Can someone clarify this please? Thanks!

    Regards,

    James
     
  2. John Fields

    John Fields Guest

    ---
    12AH is the "Capacity" (C) of the battery and it stands for "12 Ampere
    Hours" which means that if the battery starts out at 12 volts, fully
    charged, and you pull 12 amps out of it for 1 hour, its voltage should
    fall to a certain voltage, (the cutoff voltage) usually about 10
    volts, at the end of that time, and then you'll have to recharge it.
    If you don't, and continue to use the battery it'll get damaged. More
    or less, depending on whether it's lead-acid or NiCd or whatever.

    BUT, there's a catch with C. To get the full capacity out of the
    battery, you're only allowed to pull current out of it at some rate
    smaller than C, usually C/10 or C/20. That means that if you have a
    12AH battery and its rate of discharge is specified as C/10, you'll
    only get to use its full capacity if you pull 1.2A out of it for 10
    hours, then you'll have to recharge it.

    In your case, since you have 12AH batteries and you want to pull 1.2A
    out of them they should last for 10 hours (1.2 amps X 10 hours = 12AH)
    if they're rated for C/10 until they have to be recharged but, not
    knowing what rate they're allowed to discharge at to realize C, it's
    hard to say. If you can find out who made the batteries, though, you
    can go to their website and they'll probably have the spec's there.
    If you can't, but you know the chemistry of the batteries, then you
    can got to anybody's website who manufactures batteries with the same
    chemistry and get their specs for a similar capacity vanilla battery.
    Chances are the spec's for yours will be pretty close to theirs.
     
  3. andy

    andy Guest


    It means you can draw 12 Amps from it for an hour, or 1 Amp for 12 hours,
    or 0.5 amp for 24 hours, etc.

    Your TV would last around 12Ah/1.2A=10 hours. (But bear in mind that most
    lead acid batteries shouldn't be discharged fully, as it damages the
    plates. I think the rule of thumb is not to discharge below about 11V,
    but this would depend on the battery. Ones sold as 'deep cycle' or
    'leisure battery' are designed to cope better with this. Car batteries are
    the worst. I asked a question about how to protect against over discharge
    just recently, so you could look through some of the designs people posted
    in reply to that if you want to build a circuit to do it, or else just use
    a voltmeter.)

    The only catch might be if the TV needs a voltage very close to 12V - lead
    acid batteries go from about 13-14V on full charge down to about 10-11
    when they're running out.
     
  4. jaffy james

    jaffy james Guest

    Thanks for the info, much appreciated :)

    Regards,

    James
     
  5. jaffy james

    jaffy james Guest

    Thanks for the info, much appreciated :)

    Regards,

    James
     
  6. 12Ah means that the battery will be able to supply 1.2A ( 1200mA ) for 10
    hours.
    What you need to know is the internal impedance of the battery ( or maximum
    current that it can supply ).
    At 1.2A and 12V you will require an internal impedance of much less than 10
    ohms in order to be able to supply the necessary current and voltage.
    You could test the battery out by connecting a power resistor of 10 ohms
    accross the battery. The resistor will need to be a 20 Watt or more power
    resistor.
    They are usually large ceramic covered wire wound resistors. If the battery
    can still supply 12 Volts with the power resistor hooked up then it can
    supply the necessary current. Your probably going to need something of the
    size of a car battery ( or atleast bigger than a lantern battery ) to do
    this. Don't try it wih anything smaller than a lantern battery because it
    will definately not be able to handle the current and will be at risk of
    exploding if it gets too hot.
    By the way, there are products on the market that will do this. They put a
    small car battery in a plastic container ith some electronic circuitry to
    boost the current ability and they usually have 12V outputs for accessories
    such as TVs. Canadian Tire sells the Eliminator.

    Mike
     
  7. Rich Grise

    Rich Grise Guest

    I have a 5" portable, that can take 8 x C cells. They last about 3
    hours.

    Cheers!
    Rich
     
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