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solar lead acid battery controller

Discussion in 'Electronic Design' started by walle62, Jun 9, 2005.

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  1. walle62

    walle62 Guest

    dear friends

    i design a solar lead acid battery controller.

    it feautures:

    battery nominal voltage: 17 V surge load current: 15A or 30A

    it does not need a regulator

    it should protect against:
    over-, undercharge, short circuit, solar cells reverse current, battery
    and load reverse polarity

    it must contain a gas gauge (battery charge measurement)

    i must have a self explaining display of the battery charge in Ah and
    of the charge current in A

    the design must be simple and cost effective

    my idea is to use bipolar transistors for measurement of the over-,
    undervoltage etc. and drive power-mos-fets. or exists a good ic that
    features all conditions? perhaps i could also use a multi-op-amp device
    as the LM 324 and c-mos-digital-ics for threshold, driving etc.
    but i have no idea how to implement the gas gauge. can i determine the
    battery charge via battery voltage?

    i would be very glad if one of you could help or give me a hint!
    thank you very much!

    if i find a good design, i will report you here in the forum!

    with kind regards

    walter nussbaum, zuerich, switzerland
     

  2. Can we get a sample each then ?

    Rene
     
  3. walle62

    walle62 Guest

    hi rene

    surely! i try to post a schematic, as soon as i have a good circuit.

    bye

    walti
     
  4. what research have you done so far?


    martin
     
  5. Ken Smith

    Ken Smith Guest

    For a lead acid, this is basically a voltage regulator with a bit of funny
    back lash added. When the charging current it flowing, you allow the
    battery voltage to rise a bit above the "float" voltage and then drop back
    down to float.

    --------- D1
    ------! LM317 !--+--+/\/\/---+-->!-------battery
    --------- ! ! !
    ! / \ !
    ! \ / !
    ! / \ !
    ! \ / !
    ! ! ! !
    ! ! \! !
    ! ! !-------
    ! ! /! PNP
    +------+--
    !
    \
    /
    \
    /
    !
    V
    --- D2
    !
    GND

    D1 and D2 are the same sort of diode but very different sizes.
     
  6. walle62

    walle62 Guest

    hi martin

    that´s a good device, the bq2031. perhaps i can use it.
    thanks!

    walti
     
  7. Ross Herbert

    Ross Herbert Guest

    Given that you are doing the design based on a client specification
    then you have no choice. I therefore agree that you are bound to give
    him what he wants despite any recommendations as to the necessity of
    certain functions.

    It did occur to me that modern deep cycle storage batteries using
    Absorbent Glass Mat (AGM) construction does make it impossible (and
    unneccessary) to check electrolyte levels and SG, so some other means
    of determining the battery condition may be desirable. I still don't
    think a great deal of assistance can be gained by having gas gauges
    and SoC readings even with AGM batteries. If all batteries in the
    string deteriorate at the same rate gauges will likely indicate this
    symptom but I think their readings will only tend to confirm what is
    already detectable using local knowledge of the system. If only one
    battery fails in the string, gauges are not really going to give any
    reliable indication of the problem. You still need to get down to
    basic checks on each battery in the string in order to find the
    culprit.

    Good luck!

    Ross Herbert

    SNIP
     
  8. walle62

    walle62 Guest

    hi herbert
    yes, that's my problem: i'm bound on these specs.
    that's interesting: so you will say that the charge of cells in a
    string can be different despite charging/loading current is the same?

    thanks for your good advices?

    i wish you a nice week!

    walti
     
  9. mike

    mike Guest

    Solar charge controller doesn't have to be very quick or precise.
    If response time on the order of minutes is too slow, you've got way
    too much solar or way too little battery.
    Use a PIC (or other processor) with A/D. Use a PWM shunt (charge dump)
    regulator. PWM rate can be SLOW, SLOW...no problem with doing it in
    software.

    For a domestic system, dump the excess energy into a big resistor in the
    water heater.

    And the PIC can do all your integrating, storage, display.
    And it'll be cheaper than trying to do it analog.
    And easy to change in firmware when your beta test discovers it's all
    wrong.
    mike

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  10. Ken Smith

    Ken Smith Guest

    If you simply open the connection between the charger and the battery,
    there is no excess energy dumping needed.

    If you make your solar pack such that it outputs a little more voltage
    than a charged battery, a very simple system will do. There is not much
    to be gained by adding a switcher. If you want to get the last bit of
    energy, a very simple bucker will do nicely.
    The micropower PICs can work at low clock speeds over a wide voltage
    range. This means you can use a very bad regulator to run the PIC.
     
  11. mike

    mike Guest

    Well....since insolation varies many orders of magnitude
    over a 24 hour period, just how do you propose to make a solar pack that
    outputs "just a little more voltage than the battery"????

    Problem with a buck switcher is that it is lossy ALL the time.
    The shunt switcher is lossy only when you have more solar energy than
    you can use.

    Now, if you were advocating a higher voltage solar pack and a buck
    switcher that operates the solar pack
    at maximum power output for any level of light input, you might have a
    point...
    My research indicates you'd get more benefit from sun tracking than
    from peak power sensing.
    mike



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  12. I think I came to the same conclusion, once.
     
  13. Ken Smith

    Ken Smith Guest

    Check out the voltage out vs light in of a solar panel. You will find
    that the open circuit voltage is far less than linear with the light
    input.

    The short circuit current is nearly linear with the light input. As a
    result, the optimum voltage (point of greatest power) remains high over a
    very wide range of light inputs.

    Please explain what "shunt switcher" means in this context. I took it to
    be a simple booster driving some large resistor.

    If this is what you mean,I think the idea is clever but needlessly
    complex. Unless this is a huge installation a few transistors as a simple
    shunt linear regulator would be able to take the power.

    Yes, this is what I'm suggesting. The bucker could have very low losses
    since its output is usually almost exactly its input. The catch diode and
    inductor losses only apply to the voltage difference.

    Yes, but sun tracking involves moving parts and electric motors etc. This
    is sci.electronics.design not sci.mechanical.design. Over there I would
    have suggested a system that uses the solar power to aim the panel.
     
  14. mike

    mike Guest

    Sounds like you have more experience than I do. I understnd about the
    short-circuit current. Does the same apply to current into a 14V
    battery? I did some experiments with a small solar cell. I determined
    that the available charge current went down dramatically for small
    changes in incident angle. If you don't track the sun, there's a
    significant part of the day when the EFECTIVE light input is way down.
    Yep, all you need is a transistor and a big resistor to ground.
    Simple. But when you do the math, 15V max voltage 40A max shunt
    current, the maximum power in your transistor is 7.5V X 20A = 150W.

    What we actually did for the mountain top repeater was to build a
    10A switching shunt and switched additional 10A resistors in as needed.
    Once we had the pic for voltage measurement, all the other stuff became
    trivial. Very low power in the switching FETS and BIG resistors mounted
    on the wall to dissipate the heat. Also limits the surge currents in
    your batteries without having to add inductors or caps. The pic can
    also let you correct voltage points for temperature. BIG temp changes
    on a mountain top. Been running about a year now without a hitch.

    Oh, there was another complication. This site also had wind power.
    Couldn't just disconnect the load with the wind at 60MPH.
    I'll need more explanation of this. I always considered the inductor
    loss to be mostly due to it's resistance and therefore the current.
    For the diode, a diode drop is a diode drop. Yes, there are also
    dynamic losses. What's the voltage swing at the input to the inductor
    when the input and output voltages are close?

    mike


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  15. Ken Smith

    Ken Smith Guest

    I'm not sure I understand the question but I'll take a shot at it.

    A fully discharged 12V lead acid battery is right about 10.5V. A fully
    charged one is about 13.7V.

    To be able to fully charge the battery, the solar cells need to be able to
    make something like 13.7V.

    In a simple system, enough solar cells are connected in series to charge
    the battery and connected nearly directly to it until the battery is
    charged. In the short term, the terminal voltage of the lead acid battery
    is nearly independant of the charging current (ie: it has a low
    impedance). This will work to your advantage if you are making a switcher
    for the charging circuit. If you use a PIC, you can dither the PWMing up
    and down to find the point where the greatest current is flowing into the
    battery.

    Yes it falls of with angle faster than a cos() curve. The sun hitting the
    face of the panel follows a cos() curve. The percentage that refects back
    away from the surface increases with increasing angle too.

    If you point the panel towards the sun, there will be about 1KW of power
    falling on a square meter of panel. 10% of the power can end up as
    electrical power. The rest ends up as heat.

    It comes down to a question of cost whether tracking makes sense or not.
    Any money you spend on gears and motors could also be spent on another
    panel. You need to price the situation both ways to see which is better.
    At one time the motors and gears where always the lower cost way to go. I
    don't know about today.

    [...]
    I'd be inclined to use many TO-220 resistors for the power resistor. At
    20W each, they would spread the heat around nicely.

    [....]
    The "real world" is never as nice as the lab bench.

    [...]
    The AC resistance of an inductor is always more than the DC resistance and
    the more the difference between the input and output, the more inductance
    you need.

    This means that the losses in the design, end up determined by the input
    to output difference. (If the difference was zero, the inductance would
    be zero and so could the resistance)

    Well not quite. If you don't use a very fast diode, the storage effect
    adds to the losses. Also remember that the diode only conducts for a
    small fraction of the total cycle.
    Until they are almost exactly equal, the peak swing remains the input
    voltage. The RMS of the AC part of the waveform decreases as you near
    equal. There is a bit of a bummer in that the high harmonics decrease
    slower. The losses at the higher harmonics are proportionally larger.
     
  16. Ross Herbert

    Ross Herbert Guest

    Yes. In large battery installations such as in telephone exchanges the
    most common cell used was a 2V, 1000 or 2000Ah flooded lead acid type
    configured as 24 cells in series. Despite the fact that all cells were
    from the same manufacturing batch when installed it was not uncommon
    for a single cell to lose capacity after a few years of service which
    then had to be replaced. The only way to detect the weak cell was to
    perform periodic testing of individual cells.
     
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