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Trickle Charge Battery

Discussion in 'Electronic Design' started by west, Nov 19, 2005.

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  1. west

    west Guest

    I know this question has probably been beaten to death, nevertheless I hope
    you will still indulge me.

    During a power outage, which occurs frequently in Florida, I would like to
    use a battery to power some communication gear. Although the battery is 12v
    and not 13.8v, I think that the gear will still work OK (hopefully).

    Question 1. I believe that car batteries have a low internal resistance
    because their primary function is to provide a lot of current for a short
    time. There are other 12v large battery types that will last as long as a
    typical car battery. They are designed not to provide peak current, but to
    supply steady state current for a long time.
    For the life of me, I can't remember the name of those type batteries so I
    can't do a google to find a supplier. I hope this is making some sense.

    Question 2. Where can I find articles & schematics for a solar cell to
    provide a trickle charge for the aforementioned battery?

    Thank you very much.

  2. The voltage of a fully charged battery will be over 12V, somewhere
    around 13ish.

    You are talking about a "deep cycle" or marine battery.
  3. Guest

    solar array.... diode.... battery

  4. Phil Allison

    Phil Allison Guest

    "Robert Scott"

    ** Nor a cheaper way to obtain a trickle charger to use with the solar
    panel than to buy one from the same store that sells the panels. That panel
    is going to be the big expense.

    ......... Phil
  5. Robert Scott

    Robert Scott Guest

    Even if such batteries exist, they won't have a cost-effectiveness
    advantage over car batteries. You are thinking that by trading in
    peak current you are going to gain in overall amp-hours. It just
    isn't true. You won't find a cheaper way to store X amp-hours than a
    car battery.

    -Robert Scott
    Ypsilanti, Michigan
  6. Pooh Bear

    Pooh Bear Guest

  7. Mac

    Mac Guest

    Deep cycle batteries are more expensive than starting batteries. The
    advantage to them is that they are designed to endure many
    charge/discharge cycles, where the level of discharge is high, compared to
    what a starting battery endures.
    If you size the solar cell correctly, you can just hook it up to the
    battery with no other circuitry. The way this would work is that when you
    have power, you keep the battery charged with a charger, and leave the
    solar cell circuit open.

    When the power goes out, open the charger circuit, and close the solar
    cell circuit. There may be some danger of overcharging the batteries
    with the solar cell if it is very sunny and/or you don't use the equipment.

    If you are only trying to keep the batteries topped off, and not trying to
    run indefinitely, then you can just use a very small solar cell, and there
    will be no danger of over-charging under any circumstances.
    Good luck.

  8. Ian Stirling

    Ian Stirling Guest

    Nominal voltage is 13.8 fully charged. "12V" is just the sticker value.
    Deep cycle.
    Generally from who you buy the solar cell.
  9. You will need a blocking diode to prevent the solar panel from becoming a load
    at night.
  10. Mac

    Mac Guest

    AIUI, the diode consumes more power than the solar cell. I don't think you
    need the diode.

    I lived on a boat with solar cells for 2 years, and I had no diodes. It
    didn't seem to be a problem.

  11. Jim Thompson

    Jim Thompson Guest



    on the S.E.D/Schematics page of my website.

    ...Jim Thompson
  12. Guest

    Some panels will eat when unilluminated, some dont.

  13. Guest

    other than using something from the junkbox.

  14. Phil Allison

    Phil Allison Guest


    ** Then you don't understand much.

    ** A series Schottky diode is often built into the solar panel itself.

    There is a 0.5 volt loss during charging, but this is trivial as panels
    typically output 20 volts in bright sun.

    ......... Phil
  15. Pooh Bear

    Pooh Bear Guest

    Please explain how they can output 20V when loaded with a lead-acid battery.

  16. Guest

    8kW of panels on a 6Ah SLA might manage it :)

  17. Kevin

    Kevin Guest

    A solar cell can be modelled as a current source with a current
    proportional to the amount of light falling on it, in parallel with a
    silicon diode.

    As such it tends to have an output voltage that is proportional to the
    logarithm of the solar intensity that drops with increasing temperature
    (~2mV per degree). The individual cell voltage will be a maximum of
    about 0.5V and because of the logarithmic effect of the diode does not
    drop greatly as the intensity reduces.

    A typical solar array will consist of 36 cells in series giving an open
    circuit voltage of about 18V. When this is fed to a 12V lead acid
    battery through a diode the voltage will drop to about 13-14V depending
    on charge level - it will act as a constant current generator so the
    series diode will have little effect on charging current except at low
    intensity levels or high-temperatures.

    At nighttime the solar panel will act act like 36 forward biased
    silicon diodes in series - with ~13V from the battery each diode will
    only have about 360mV across each and the current will be pretty low,
    usually the array temperature will be lowest at night so this increases
    the voltage required for the diodes to conduct helping reduce the

    So it is usually acceptable to not have a reverse blocking diode but it
    will not significantly affect the charging current except when the
    array temperature is high (under midday sun).

    I like to have a blocking diode as a safety measure to avoid dangerous
    currents from the battery if the array wiring that is exposed to the
    elements has a short ( you should put a fuse in series with the battery
    physically close to the battery as well - even a small battery can have
    short-circuit currents of hundreds of amps).

    Some solar arrays do have diodes included in the array but they are
    usually in parallel rather than series - this is to avoid problems when
    arrays are placed in series and one array gets shaded while the others
    don't. Without the diode the shaded array would be reverse biased by
    the voltage from the other arrays and be damaged (a similar problem can
    occur with series cells in a battery when one is discharged before the

    It is usually advisable to have a charge controller - if the voltage
    goes above ~14V the electrolyte in the battery will be electrolyzed
    away pretty quickly - in the case of sealed lead acid batteries there
    is no way of replacing it and so the battery will be damaged.

    There are some solar arrays available with fewer cells that claim to be
    self regulating - these usually just have fewer cells (33 rather than
    36) so that the charging current drops off as the battery is charged.
    These are not usually completely effective.

    These days small charge controllers only cost $20-30 and are easily
    available - my local Fry's has them.

  18. Jasen Betts

    Jasen Betts Guest

    are those cells connected directly to the battery?

  19. Ken Smith

    Ken Smith Guest

    Or less pannels on a totally dead 24V SLA of any size.
  20. Rich Grise

    Rich Grise Guest

    They do, and they do, and they do.

    They're called "deep-cycle" batteries, or "marine", or sometimes "golf
    cart" batteries. They're designed to provide moderate current for a long
    time, whereas car batteries are designed to stay at the ready on float
    while in use, and provide hundreds of amps of starting current when needed.

    Please don't mislead the children.


    You are thinking that by trading in
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