How to work out Battery Life

Discussion in 'Homework Help' started by JDR04, Jun 15, 2012.

  1. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Can somebody please help me figure this out.

    Lets say I have a circuit that consistently draws 10mA. I have a normal 9V (PP3) battery with no information as to its milliamps per hour.

    How do I work out how long the battery will power the circuit?
    Many thanks -JDR04
     
  2. jegues

    Well-Known Member

    Sep 13, 2010
    735
    43

    Do you know if it is an alkaline battery, or is it of different type?

    All energizer battery datasheets can be found here,

    http://data.energizer.com/
     
  3. JDR04

    Thread Starter Active Member

    May 5, 2011
    339
    4
    Its a Duracell 9V battery.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    A typical capacity of a 9V battery is 500mAh.
    Divide that by 10mA and you will get an estimate of 50 hours.
    You also have to take into account the drop out operating voltage of your device.
     
  5. wmodavis

    Well-Known Member

    Oct 23, 2010
    737
    150
    Step one...
    Get information on the battery in question. Any other starting point will lead to more guessing than anything.
     
    Last edited: Jun 15, 2012
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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  7. WBahn

    Moderator

    Mar 31, 2012
    17,768
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    The term "milliamps per hour" makes no sense. It is like refering to a car's mph per day. What you mean is milliamp-hours, the product of the current and the length of time it can supposedly sustain that current. The units for this are mAh.

    While using the nominal rating, such as 500mAh, and dividing by the current will give you an estimate, it is usually a really poor estimate. The higher the current, the worse the estimate will usually be. At 10mA, you are probably not too bad off.

    A big factor is what is called the cutoff voltage, which is nothing more than the voltage of the battery at which you determine that the battery is "dead". The lower your application's cutoff voltage, the longer it can supply current before falling to that level.

    Another big factor is temperature. As the temperature changes, so too does the internal resistance of the battery.

    Oh, and before you spend a bunch of extra money to get the high-end offering (the specific names keep changing) of a particular battery brand, look at the data sheets. You usually find that the high-end battery and the standard battery in the same line have identical discharge curves (even to the point of frequently being obviously the exact same data set). They just throw some shiny gold print and some marketing BS and charge a bunch more and people buy them in droves.
     
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