# Runtime of clock using alkaline D cells instead of AA batteries

1. ## 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?

2. ## 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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3. ## Re: Runtime of clock using alkaline D cells instead of AA batteries

Originally Posted by barry
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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4. ## Re: Runtime of clock using alkaline D cells instead of AA batteries

Originally Posted by fixit7
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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5. ## 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.

6. ## 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.

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.

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

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

How this compute :D

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

Hi,

* 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)

Klaus

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9. ## 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.

10. ## 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.

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

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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.

11. ## 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.

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

Thanks for all the responses. They helped.

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