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    Runtime of clock using alkaline D cells instead of AA batteries

    I have a wall clock that uses 2 AA batteries.

    I would like to use a transformer, but I get frequent power outages.

    The clock uses about 30 mAh.

    It lasts about 2 months before it starts losing time. (Around 2.0 volts for the 2 batteries.)

    If I go with 2 D cell batteries, what kind of run time would be typical?

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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    Do the math: Alkaline AAs have 2400mA-H of capacity; Ds have 13000mA-H. These values depend somewhat on the load.



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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    Quote Originally Posted by barry View Post
    Do the math: Alkaline AAs have 2400mA-H of capacity; Ds have 13000mA-H. These values depend somewhat on the load.

    I assume that Time(H) = Capacity(Ah)/Current(A) equals a final battery voltage of 0 volts which will not work in my case.

    I need a calculation for an ending voltage of around 2.0 volts.



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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    Quote Originally Posted by fixit7 View Post
    I assume that Time(H) = Capacity(Ah)/Current(A) equals a final battery voltage of 0 volts which will not work in my case.

    I need a calculation for an ending voltage of around 2.0 volts.
    Then you're going to need to get the manufacturers curves.

    Or, use some of your reasoning power . If the D has 13000 and the AA has 2400==> 13000/2400=x/2months



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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    For alkaline batteries, the capacity is NEVER calculated to zero volts, but an end voltage of 0.9 volts per cell.

    But from a battery datasheet, check the end voltage value. The 0.9 volts I gave you is from memory. It could be a little higher or a little lower.
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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    Ok, I found the data sheet. I see the capacity is 15,000 Mah.

    Click image for larger version. 

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    So, according to the table, if a device using 120 Mah, it would last about 80 hrs.

    Since the clock uses 30 Mah, that would be about 480 hrs.

    480 hrs = 20 days.

    That does not compute as AAs last around 60 days.



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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    15000mAh / 2400mAh = 6.25
    60days * 6.25 = 375days

    How this compute :D
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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    Hi,

    Please use correct units.
    * capacity is 15,000 Mah. --> 15,000 mAh
    * device using 120 Mah --> 120 mA (without "h")
    * and so on..

    device using 120 Mah, it would last about 80 hrs.
    How do you come to 80 h?
    My calculator says this is 120 mA x 80 h = 9,600 mAh ...

    Since the clock uses 30 Mah, that would be about 480 hrs.
    * are you sure the 30 mA are at 1.2V (as the 120mA in the chart)?
    * you need to recognize: the lower the current, the higher the usable energy.
    * the usable energy will differ from cell to cell
    * the usable energy depends on cell type (chemistry)
    * the usable energy depends on brand and exact type (model)

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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    I would like to use a transformer, but I get frequent power outages.

    The clock uses about 30 mAh.
    Doesn't this demand for a transformer supply with battery backup? The battery is only discharged during power failure then.



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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    I'm guessing the 2400 mah rating for AA is one battery. Fixits clock uses two AA batteries. Would this not be 4800 mah capacity.

    - - - Updated - - -

    Never mind, evidently it's 3 volt series connected batteries.

    - - - Updated - - -

    https://www.digikey.com/en/resources...r-battery-life

    With this calculator I plugged in 2400 mah and 1.16 ma. This gives a run time of 1448 hrs. , or roughly 2 months.



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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    Some name-brand alkaline D cells have a little C cell inside with the C cell capacity. Few D cells are sold today.
    Some Chinese alkaline D cells have anything that is cheap inside.



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    Re: Runtime of clock using alkaline D cells instead of AA batteries

    Thanks for all the responses. They helped.




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