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12v Lead Acid Battery Charger aka I need to stop killing my batteries!

Discussion in 'Electronic Basics' started by Kevin Walton, Mar 14, 2005.

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  1. Kevin Walton

    Kevin Walton Guest

    Hi

    I have killed a 12v lead acid battery, and would like some help
    understanding why, and how to stop it:

    I currently have a 'plug in the wall' style 12v lead acid battery
    charger that I use to recharge a sealed 7ah 12v gel cell.

    The cell is generally used one afternoon a week to start model
    aircraft engines (i.e. high discharge rates for short periods) and is
    left in a cold garage the rest of the time, usually on charge. After
    less than a years use the capacity of the cell is less than half that
    of new.

    The charger I have just researched a little more, no load it outputs
    12v, loaded (ie charging the battery) the voltage is 15v. The current
    flow is a constant 400ma (all measured using a electric flight watt
    meter) and does not change between a discharged battery and a fully
    charged battery. I opened up the wall charger and it is simply a
    transformer and a rectifier - nothing else.

    I don't understand the theory of above, how come a current is flowing
    when attaching the battery 'appears' to raise the voltage across the
    charger and the rest voltage of the cell is higher than the rest
    voltage of the charger?

    So, the reason I am killing the battery is that I routinely over
    charge it for long period of time. Once I understand the theory above
    I hope to add something to the output of the existing charger to stop
    me killing the battery - be that turning it into a float charger by
    limiting the voltage (so all I would need to do here is to add a 13.x
    v regulator to the output?) or by something more sophisticated.
    Options welcomed.

    Thanks in advance for the help :)

    Cheers
    Kev
     
  2. Externet

    Externet Guest

    Hi Kevin.
    Lead acid batteries are to be charged with constant voltage. If you are
    using that constant current charger, they are being killed.
    A 7812 regulator with a couple of 1N4004 diodes in series to its ground
    reference pin will raise the regulation to about 13.3 V, good enough.
    Miguel
     
  3. Eric R Snow

    Eric R Snow Guest

    Greetings Kevin,
    Though I'm no great shakes in electronics I think I can answer your
    questions. First, since there is no filter on the charger it is
    delivering pulsed DC. When measured with your meter it will show the
    wrong voltage. Connecting to the battery puts a filter in the circuit
    and your voltage reading is now closer to correct. Second, use a
    resistor to limit the current. 15 volts should be OK for the battery.
    It's the high current that kills the battery. Does the battery maker
    have a website or a phone number? If so, maybe you can ask them about
    the voltage and current requirements to keep the battery charged
    without overcharging.
    Cheers,
    Eric
     
  4. John Fields

    John Fields Guest

    ---
    If your meter is average-reading and is reading 12VDC out of the
    charger when the charger isn't connected to the battery, then the peak
    voltage out of the charger will be about 19V even though your meter
    only shows 12V. However, as soon as you hook it up to the battery the
    battery will soak up the peaks and your meter will then be reading the
    steady DC voltage across the battery terminals which, when you
    measured it, was 15VDC. If you were to let the battery discharge to
    the point where it had about 11V across it, under load, and then you
    removed the load and started to charge the battery, the voltage would
    jump up a little, maybe to 12V or so at first, but then it would take
    much longer to get to its fully charged voltage if it was a good
    battery. If you have a discharged battery and and it reads 15V as
    soon as you put the charger across it, then its internal resistance
    has risen to the point where much of its capacity will be lost.
    ---
    one of the easiest ways to do this is shown in the middle illustration
    on page 19 at:

    http://cache.national.com/ds/LM/LM117.pdf

    You should contact the battery manufacturer to find out what the
    terminal voltage should be if you're going to float charge the battery
    for a long time or if you want to cycle-charge it when you're using it
    frequently. They'll be different voltages, and they'll vary a little
    between manufacturers and with temperature, so you may want to use a
    pot to select the voltage out of the LM317 to suit your situation.

    Also, it would be a good idea to heat sink the LM317.
     
  5. Kevin Walton

    Kevin Walton Guest

    Hi, Thanks for the reply
    I think you missunderstood my posting a little, it is not a 'constant
    current charger', it is a charger designed for 12v lead acid
    batteries, just that measuring the current flow from the charger
    during the charge cycle it is a consistant 400ma.
    Sounds like a nice simple option to improve the wall charger that will
    also physically fit inside the wall charger.

    Thanks
    Kev
     
  6. Kevin Walton

    Kevin Walton Guest

    Actually, after just doing a discharge test it barely has 10% of its
    original capacity, so alhtough this thread is relevant as I need to
    not kill the next one, I dont think I will be geting much use out of
    this one again unless I can rejuvinate it
    (http://www.shaka.com/~kalepa/lowpower.htm)?

    Cheers
    Kev
     
  7. The current is pulsatile, but its average value
    is approximately constant under the conditions
    that exist while it is destroying your battery(s).
    See the post by Mr. Fields to get a better idea
    why that current is approximately constant.
    It is a decent solution, but it will dissipate
    more power than the diodes and tranformer
    presently do, so you need to be sure that
    the circuit does not overheat in that location.
     
  8. John Fields

    John Fields Guest

    ---
    Yes but, unfortunately, if it's putting 400 mA into the battery when
    it's discharged and also putting 400 mA into the battery when the
    battery is fully charged then it is, in fact, a constant current
    supply!

    Lead-acid batteries generally want to be charged using a
    constant-voltage supply because as the battery voltage rises when it's
    charging, the current being put into the battery tapers off with time
    because the difference between the battery voltage and the charger
    voltage gets to be smaller and smaller, so the current being forced
    into the battery gets lower and lower as the battery voltage goes
    higher and higher.

    However, I suspect that what's happening in your case is that as the
    battery voltage increases when it charges, the diminished load on the
    transformer causes its output voltage to rise as well, forcing the
    voltage difference between the battery and the transformer to remain
    more or less constant, forcing the current to remain more or less
    constant. Also, I believe that as the battery charges its internal
    resistance decreases somewhat, which would aggravate the effect. With
    a regulated (or voltage-limited) supply, though, the change in the
    battery's internal resistance would be more than compensated for by
    the rise in battery voltage, so eventually the system would come to
    rest with only the trickle charge current flowing into the battery.
     
  9. Kevin Walton

    Kevin Walton Guest

    Ahhh, ofcourse, RMS and all that.

    If 12v = average, then peak = 12/0.637 = 18.8v, rms = 1.11 x 12 =
    13.32v

    Not quite the 15v seen but then without putting a scope on it, it does
    explain it.
    Yup, I have now realised that this battery is pretty much dead and
    have been experimenting with a battery that does have 50% of it's
    capacity left. Charging the second cell (12ah) started with a lower
    voltage and a current of around 600mah, the voltage slowly rose
    through the charge to 15v and then sat at 400mah current.
    What part does the Rs play in that circuit? I appretiate it sets the
    output impedance, which has an effect on the current flowing. If I
    know the internal resistance in a battery limits the initial charging
    rate, and the voltage is only set at the appropriate float charging
    voltage, when the battery is fully charged the voltages will equalise
    and the current flow will become zero?

    Cheers
    Kev
     
  10. John Fields

    John Fields Guest

    ---
    What's your point? The 15 volts you saw, you said, was when you read
    the DC battery voltage with the charger hooked up to the battery.
    That means that was the voltage the battery was limiting the 19V peak
    input DC pulses to.
    ---
     
  11. Kevin Walton formulated on Monday :
    I keep an automatic motoecycle battery charger in my flite box. never
    killed a battery that way
     
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