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measuring project lifetime of a battery — Parallax Forums

measuring project lifetime of a battery

ArchiverArchiver Posts: 46,084
edited 2000-11-11 23:35 in General Discussion
How can I measure how long my breadboard project will last with a fresh 9V
battery? Obviously I can let it run until the battery is dead and use the
time lapsed as an educated guess, but there must be a much more scientific
way of estimating the lifetime.

TIA for any suggestions,

/* Andrew */
----
Andrew Tucker
Software Engineer
Aegis Software
206-447-4175 x120
mailto:andrewt@a...

Comments

  • ArchiverArchiver Posts: 46,084
    edited 2000-10-30 19:08
    I have done this at work, but the method I used is very expensive for the
    hobbiest. The biggest problem is that it is a non-linear function. As the
    voltage decreases from the battery, current demand from the circuit
    increases. With that in mind, as the battery gets drained it loses charge
    exponentially faster. Some battery manufacturers have graphs of this and you
    can make an estimate. But you'll have to know at what point (voltage) your
    circuit will stop operating. Also, don't forget at this lower voltage, your
    circuit is demanding more current to run. So it's also important to figure
    out if the battery can supply that much current at the lower voltage. I
    apologize this is not very straight forward, but since the function is not
    linear it is much more difficult to calculate mathmatically than we would
    like.

    If you want more info on the bench set up I did to test our batteries, just
    contact me offline.

    Jason Linscott
    PCB Layout/Engineering Technician
    ILX Lightwave
    jlinscott@i...
    (303) 516-8877
    Original Message
    From: Andrew Tucker [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=lAz540qnx1lzaNPV691ILZxbu3jfCrN0cSEKYQmleNgNbShqNDaaElJtBFsBm7sCn4cUGogWXFFvyxc]andrewt@a...[/url
    Sent: Monday, October 30, 2000 11:27 AM
    To: 'Basic Stamp List'
    Subject: [noparse][[/noparse]basicstamps] measuring project lifetime of a battery



    How can I measure how long my breadboard project will last with a fresh 9V
    battery? Obviously I can let it run until the battery is dead and use the
    time lapsed as an educated guess, but there must be a much more scientific
    way of estimating the lifetime.

    TIA for any suggestions,

    /* Andrew */
    ----
    Andrew Tucker
    Software Engineer
    Aegis Software
    206-447-4175 x120
    mailto:andrewt@a...
  • ArchiverArchiver Posts: 46,084
    edited 2000-10-30 19:30
    >How can I measure how long my breadboard project will last with a fresh 9V
    >battery? Obviously I can let it run until the battery is dead and use the
    >time lapsed as an educated guess, but there must be a much more scientific
    >way of estimating the lifetime.

    Hi Andrew,

    As a rough estimate, you divide the amp-hour capacity of the battery by the
    average current drain of your project. That gives you the number of hours.
    A standard 9 volt alkaline battery has a capacity of 500 milliamp hours.
    (A 9 volt lithium version is 1100 milliamp hours, a NiCAD much less) So if
    you have a BS2 drawing 8 milliamps continuously, you can expect about
    500/8=62.5 hours. (Your milage may vary!)

    Using the NAP or SLEEP commands, the average current can be lower and
    lengthen the battery life. On the other hand, if your BS2 is driving LEDs
    or external power hogs, the average current will be higher and the battery
    life lower. The average current is estimated by factoring in the
    proportion of time your system operates at each discrete current level.

    -- Tracy Allen
    http://www.emesystems.com
  • ArchiverArchiver Posts: 46,084
    edited 2000-11-11 23:35
    Hi Andrew,

    The best way I have used to predict battery run-time is to download battery
    life curves from the internet, and estimate the run time based on current
    consumption. The curves are available from various manufacturers; just
    search under the name (e.g. Duracell).

    Keep in mind that this method is better than taking actual measurements from
    experiments unless you discharge THOUSANDS of batteries and average the
    data, because every battery is slightly different, and has a slightly
    different storage time versus temperature history, making every battery you
    buy perform differently.

    In addition, even if you had a battery of know quality, the actual use
    conditions vary slightly, so performance would vary slightly anyway.

    An estimated run time can be made, then, as I always do, set the run time
    specification for your device to be a safe amount LESS THAN the predicted
    value.

    Chuck

    Original Message
    From: Andrew Tucker <andrewt@a...>
    To: 'Basic Stamp List' <basicstamps@egroups.com>
    Date: Monday, October 30, 2000 1:27 PM
    Subject: [noparse][[/noparse]basicstamps] measuring project lifetime of a battery


    >
    >How can I measure how long my breadboard project will last with a fresh 9V
    >battery? Obviously I can let it run until the battery is dead and use the
    >time lapsed as an educated guess, but there must be a much more scientific
    >way of estimating the lifetime.
    >
    >TIA for any suggestions,
    >
    >/* Andrew */
    >----
    >Andrew Tucker
    >Software Engineer
    >Aegis Software
    >206-447-4175 x120
    >mailto:andrewt@a...
    >
    >
    >
    >
    >
    >
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