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Thread: Understanding Discharge Characteristics Curves of Batteries

  1. #1
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    Lightbulb Understanding Discharge Characteristics Curves of Batteries

    This to develop a basic and simple understanding of Discharge Characteristics Curves of Batteries.
    Discharge Characteristics Curves usually comes from battery manufacturer.

    This will give you a good idea about whether you are getting what you have paid for or not.

    or : How much load can you put on your UPS/Battery/IPS or anything that runs on batteries and how much runtime should you expect in reference to manufacturer specification.

    There are areas that i haven't been able to understand to the full extent. So, this is more of a collective learning approach.

    tagging @redwan hasan

    Please tag others who might have interest in the subject matter or well acquainted with the topic.



    below is a Discharge curve for yuasa SLA 12V battery (NPW45-12)
    If my understanding is correct then it can provide 12.6A or *1.4C for roughly 20 minutes.
    So, after 20 minutes of continuous discharge at 12.6A the battery will be completely discharged in perfectly controlled environment of 25c Temp.

    *(C = Battery Capacity in Ah so 1.4 x 9A=12.6A. NPW45-12 is a 9A Batt)

    Now I want to cross check my findings with Peukerts Constant but not sure what should be the correct constant.

    "For a lead–acid battery, however, the value of k is typically between 1.1 and 1.3. It generally ranges from 1.05 to 1.15 for VRSLAB AGM batteries, from 1.1 to 1.25 for gel, and from 1.2 to 1.6 for flooded batteries.[1] The Peukert constant varies according to the age of the battery, generally increasing with age" from wikipedia

    Determine Run Time for Specific Load with - Peukerts Constant

    Another calculator with more data projections

    Please feel free to ask questions/concerns/critics
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	NPW45-12.jpg 
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ID:	35280  
    Last edited by minitt; December 22nd, 2014 at 02:22.

  2. #2
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    Kisui bujhi nai. But it looks interesting. Following.

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    There is one more thing, from the curve its pretty clear that if u start drawing 1C which is 9A in this case i believe,the battery will reach 11.5V after 20 min and the low voltage protection circuit should kick in and turn off the system in order to protect the battery so in real life u r not getting 1.4C for 20 min but 1C for 20 min.


    About that law let me simply explain that, battery curves are not linear so 10Ahr might be an indication that u can draw 10amp for 1hour but its not guaranteed that u can draw 100A for 6 min, it will be much less than 6 min and if u draw 1A it might last longer than 10hour. Mostly the internal resistance wastes the power which is directly related to the square of current that's why it happens.


    Why does battery loose capacity over time? Simple the chemical reactions are not 100% reversible so it will loose some of its capacity over the time, going deep cycle is even more detrimental to the battery life.

    - - - Updated - - -

    There should be about 100%(of what i took) clonazepam in my blood so im not sure if i have answered properly so feel free to discuss more.

  4. #4
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    @redwan hasan thanks a lot for the input. Now I can guesstimate what should be the max usable time with a given load and observe terminal voltage as it reaches towards 11.5V

    But what do you think of this calculation?

    An online UPS has 2 NPW45-12 9AH batteries connected in series. So it's a 24V DC bus system with total 9AH capacity.

    @100% charge an 180 watt of AC load is placed. what is the DC amp draw from the 24V batteries?

    here is my calculation.

    To maintain 180watt AC load , 257watt of DC is drawn from the battery ( assuming 70% efficiency from DC->AC conversion) which means around 10.7A draw from the battery

    So, 1.18C where C= 9AH (1.18x9=10.7A) *10.62 actual

    Practical Data: (actual runtime tested by me)
    So after 8 minutes of roughly 180watt of load battery voltage came down to 24V as displayed on the UPS lcd screen. After 8 minutes UPS shows 55% capacity remaining.

    ** So, do I have a battery that is providing the expected performance ???

    Projection:
    If the above calculation is correct then is it safe to assume that, if I continue to use 180 watt of load the UPS will be running out of battery after 17.5 minutes?

    Have a look at the attached discharge curve and look for the projected thin yellow discharge curve and vertical yellow line that shows usable time.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by minitt; December 22nd, 2014 at 23:58.

  5. #5
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    1. Actual fully charged voltage of those batteries in series is more than 24V.
    2. Your power supply wastes energy too.
    3. Load varies so does consumption.
    4. Your rough estimation is fine.
    5. Temperature has something to do here too.

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    Quote Originally Posted by redwan hasan View Post
    1. Actual fully charged voltage of those batteries in series is more than 24V.
    2. Your power supply wastes energy too.
    3. Load varies so does consumption.
    4. Your rough estimation is fine.
    5. Temperature has something to do here too.
    These are new batteries.

    So, do you think they are putting out the expected performance in reference to the manufacturer provided specs?

  7. #7
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    Quote Originally Posted by minitt View Post
    These are new batteries.

    So, do you think they are putting out the expected performance in reference to the manufacturer provided specs?
    With time there will be a reduction in capacity.

    Yes i think they are.

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