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9v battery min voltage

Discussion in 'Electronic Basics' started by lerameur, Dec 31, 2006.

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  1. lerameur

    lerameur Guest

    HI

    I am building a unit that will have a small 9v battery for power. I
    will put a voltage comparator and it will activate a led to warn for a
    low voltage battery. Iwould like to know at what voltage dos the
    battery need to be change, when is it dead ? is it 7.5 v ? or 8?....

    thanks
    ken
     
  2. That's up to you to decide - how low a voltage can _your_ device
    tolerate?

    I think somewhere around 0.9 volts per cell is considered pretty dead
    for disposable batteries - that would be 5.4 volts for a 9 volt
    battery.



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  3. jasen

    jasen Guest

    if it's non-rechargable when it's too weak to use.
    if rechargable when the voltage drops 1.2V below the full charge
    voltage.
     
  4. chuck

    chuck Guest

    A bit of a challenge here: if the battery can power the led, then
    perhaps it isn't really dead.

    An alternative approach is to use the led to indicate a "low" battery,
    rather than a dead one. It is much better (usually) to know that
    changing the battery is now prudent, than to know that the battery
    should have been changed earlier because it is now dead. ;-)

    If you take that route, 7.5 or 8 volts might be just enough to operate
    your device and indicate time for a battery change. If I recall, battery
    voltage drops fairly quickly once the battery begins its decline in earnest.

    Chuck
     
  5. lerameur

    lerameur Guest

    ok good, and what would be the voltage for a 1.2v NmHI batteries ?
    0.9 v ?

    ken
     
  6. chuck

    chuck Guest

    Well, you will have to lower the supply voltage to your project by steps
    to determine the approximate voltage at which it fails to operate
    reliably (or at all) and then set your warning light (comparator) to a
    voltage somewhat higher than that.

    Chuck
     
  7. lerameur

    lerameur Guest

    yes thats good, thanks
     
  8. ehsjr

    ehsjr Guest

    No - not good. The project may keep working, but
    discharging the cells too far risks damaging them.
    When the cells discharge to 1.0 volts per cell,
    it is time to recharge, regardless of whether your
    project is working.

    Watch out for "9 volt" rechargeable batteries.
    They are really 7.2 volts. There may be
    exceptions to that - I don't know.

    Ed
     
  9. chuck

    chuck Guest

    Ed has pointed out yet another design parameter. The problem is not that
    the batteries would be damaged: every charge/discharge cycle decreases
    battery capacity somewhat. Discharging below 1.0 volts causes greater
    decreases.

    One's purpose in life, however, is usually not the preservation of
    battery capacity.

    As the designer, you get to choose whether you want to keep the
    batteries for as long as possible, at the price of frequent recharges;
    or you want to recharge less frequently because it's a pain, with the
    knowledge that your batteries might not last for as many recharges? as
    many years? etc., etc.

    Electronic design is full of tradeoffs. Haven't even mentioned
    alternative battery chemistries, charging techniques, voltage
    regulation, etc., all of which will affect battery life; and the
    appropriate cost-benefit analysis. ;-)

    Chuck
     
  10. ehsjr

    ehsjr Guest

    The problem most certainly *is* that cells may be damaged.

    See "Discharge Termination" in the Energizer
    Battery Application Manual, pages 14 - 16.
    http://data.energizer.com/PDFs/nickelmetalhydride_appman.pdf

    Ed
     
  11. chuck

    chuck Guest

    Thanks for the interesting reference, Ed.

    I see that you were referring to a potential situation involving
    series-connected cells in which one or more cells caused a voltage
    reversal on one or more of the other cells. This could cause damage to
    the reversed voltage cell.

    Good point, and thanks for the correction.

    I still would argue, of course, that such a consideration is one of many
    that would enter into a full-blown project design that involves numerous
    tradeoffs. While the "cure" is to ensure that the total voltage remains
    above 0.9 volts/cell x number of cells, there is a real cost to doing
    so. And what is the expected benefit from this precaution?

    Unfortunately, that information is not given in the reference. "May"
    cause damage is not sufficient information to evaluate the
    cost-effectiveness of a fix. "May" could well range from a probability
    of 0.01 to 0.99. Even if the probability is near 1.0, the extent or
    nature of the "damage" is unstated. Does the damage mean catastrophic
    component failure? Does it mean a 10% reduction in capacity (truly a
    form of damage) or a 90% reduction?

    Charge and discharge rates are further examples of tradeoffs involving
    possible damage and costs.

    I would repeat that the purpose of this project is surely not the
    preservation of the batteries. It is to accomplish some other purpose.
    Whether the inconvenience of more frequent battery charges would offset
    the benefit of reduced (but unknown) risk of battery damage (of unknown
    consequence) is a design decision. Good engineering practice REQUIRES
    that such tradeoffs be considered, but it does not require that
    batteries be made free of risk of damage, although that is always an
    option.

    Having said all of that with much huffing and puffing, I must admit that
    a possibly more conservative "battery low" criterion is probably a
    pretty costless step and a good suggestion, Ed.

    Chuck
     
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