# Calculate run time of battery to a specified voltage

Started by September 20, 2019
```I want to calculate approx run time of a D cell with a capacity of 13,000 Mah.

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.
(At 2.0 volts, my clock starts losing time.)

Thanks
```
```On Friday, September 20, 2019 at 5:14:30 PM UTC-5, AK wrote:
> I want to calculate approx run time of a D cell with a capacity of 13,000 Mah.
>
> 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.
> (At 2.0 volts, my clock starts losing time.)
>
> Thanks

Maybe this would help stimulate some answer?

https://imgur.com/a/bbaNKzU

Andy
```
```On 9/20/2019 6:14 PM, AK wrote:
> I want to calculate approx run time of a D cell with a capacity of 13,000 Mah.
>
> 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.
> (At 2.0 volts, my clock starts losing time.)
>
> Thanks
>

I assume you have two D cells in series, since you specify an ending
voltage of 2 volts. A single D cell provides 1.5 volts.

Generally speaking the mAh discharge voltage for a single D cell would
be around 1 volt, not 0 volts, but that doesn't matter too much.

The equation will give you ONLY an approximation.

The rate at which a battery discharges varies with the amount of
current drawn. Unless the load is designed to draw constant current,
the denominator (Current) in your equation is a variable so you
can't solve it accurately.  Add to that the fact that the discharge
rate is not linear even at constant current, and you can see the
difficulty with getting a precise and accurate answer. Look at
some battery discharge curves to get a feeling for this.

Ed

```
```On Friday, September 20, 2019 at 3:14:30 PM UTC-7, AK wrote:
> I want to calculate approx run time of a D cell with a capacity of 13,000 Mah.
>
> I assume that Time(H) = Capacity(Ah)/Current(A) equals a final battery voltage of 0 volts which will not work ...

No, the mAh rating comes with an explanation of the test conditions,
you just need the full manufacturer's data sheets.

Sometimes there's a time/voltage graph, and you can chose the cutoff
voltage and measure area-under-the-I-V curve.
Sometimes there's a specified V when fresh, and V when dead
(alkaline, typically 1.5V fresh, 1.0V dead;  lithium, typically 3.6V fresh,
```
```whit3rd wrote:

------------------
>
>
> No, the mAh rating comes with an explanation of the test conditions,
> you just need the full manufacturer's data sheets.
>
> Sometimes there's a time/voltage graph, and you can chose the cutoff
> voltage and measure area-under-the-I-V curve.
> Sometimes there's a specified V when fresh, and V when dead
> (alkaline, typically 1.5V fresh, 1.0V dead;  lithium, typically 3.6V fresh,
>
-----------------------------

** The OP has indicted his load is a clock, dunno what kind but any battery clock will be engineered for absolute minimum DC current draw.

Standard Quartz mechanisms are phenomenal current misers, alkaline AA cells last a year or more. Average currents being in the order of 100uA.

It follows that a D size cell would last much longer, maybe 5 or 10 years.

IOW, its entire shelf life.

Matters more how fresh the cell is when you purchase it.

....  Phil

```
```On Sunday, September 22, 2019 at 2:49:28 AM UTC-5, Phil Allison wrote:
> whit3rd wrote:
>
> ------------------
> >
> >
> > No, the mAh rating comes with an explanation of the test conditions,
> > you just need the full manufacturer's data sheets.
> >
> > Sometimes there's a time/voltage graph, and you can chose the cutoff
> > voltage and measure area-under-the-I-V curve.
> > Sometimes there's a specified V when fresh, and V when dead
> > (alkaline, typically 1.5V fresh, 1.0V dead;  lithium, typically 3.6V fresh,
> >
> -----------------------------
>
> ** The OP has indicted his load is a clock, dunno what kind but any battery clock will be engineered for absolute minimum DC current draw.
>
> Standard Quartz mechanisms are phenomenal current misers, alkaline AA cells last a year or more. Average currents being in the order of 100uA.
>
> It follows that a D size cell would last much longer, maybe 5 or 10 years.
>
> IOW, its entire shelf life.
>
> Matters more how fresh the cell is when you purchase it.
>
>
>
> ....  Phil

The clock uses about 25 Ma and the AAs lasted about 3 months.

Andy
```
```AK wrote:
> On Sunday, September 22, 2019 at 2:49:28 AM UTC-5, Phil Allison wrote:
>> whit3rd wrote:
>>
>> ------------------
>>>
>>> No, the mAh rating comes with an explanation of the test conditions,
>>> you just need the full manufacturer's data sheets.
>>>
>>> Sometimes there's a time/voltage graph, and you can chose the cutoff
>>> voltage and measure area-under-the-I-V curve.
>>> Sometimes there's a specified V when fresh, and V when dead
>>> (alkaline, typically 1.5V fresh, 1.0V dead;  lithium, typically 3.6V fresh,
>>>
>> -----------------------------
>>
>> ** The OP has indicted his load is a clock, dunno what kind but any battery clock will be engineered for absolute minimum DC current draw.
>>
>> Standard Quartz mechanisms are phenomenal current misers, alkaline AA cells last a year or more. Average currents being in the order of 100uA.
>>
>> It follows that a D size cell would last much longer, maybe 5 or 10 years.
>>
>> IOW, its entire shelf life.
>>
>> Matters more how fresh the cell is when you purchase it.
>>
>>
>>
>> ....  Phil
>
> Sorry for the late reply.
>
> The clock uses about 25 Ma and the AAs lasted about 3 months.
>
> Andy

That makes it a 54 GAh battery then. Some battery!

M=mega, m=milli, and in ASCII text, u=micro, which I
guess is what you really meant. If so, 54 mAh is very
poor for an AA. A fresh AA should be able to deliver
about 1 Ah in a clock, maybe more.

Jeroen Belleman
```
``` AK wrote:

-----------
>
> >
> > ** The OP has indicted his load is a clock, dunno what kind but
> > any battery clock will be engineered for absolute minimum DC current draw.
> >
> > Standard Quartz mechanisms are phenomenal current misers, alkaline
> > AA cells last a year or more. Average currents being in the order of 100uA.
> >
> > It follows that a D size cell would last much longer, maybe 5 or 10 years.
> >
> > IOW, its entire shelf life.
> >
> > Matters more how fresh the cell is when you purchase it.
> >
> >
>
>
> Sorry for the late reply.
>
> The clock uses about 25 Ma and the AAs lasted about 3 months.
>

** Oh dear  -   ROTFL

So you only switch the clock on when you want to read the time - to save the batteries ?

25mA for 2160 hours = 54 amp hours.

What brand AAs do you buy ?

.... Phil

```
```On Friday, November 1, 2019 at 5:16:46 PM UTC-5, Phil Allison wrote:
> AK wrote:
>
> -----------
> >
> > >
> > > ** The OP has indicted his load is a clock, dunno what kind but
> > > any battery clock will be engineered for absolute minimum DC current draw.
> > >
> > > Standard Quartz mechanisms are phenomenal current misers, alkaline
> > > AA cells last a year or more. Average currents being in the order of 100uA.
> > >
> > > It follows that a D size cell would last much longer, maybe 5 or 10 years.
> > >
> > > IOW, its entire shelf life.
> > >
> > > Matters more how fresh the cell is when you purchase it.
> > >
> > >
> >
> >
> > Sorry for the late reply.
> >
> > The clock uses about 25 Ma and the AAs lasted about 3 months.
> >
>
>  ** Oh dear  -   ROTFL
>
>  So you only switch the clock on when you want to read the time - to save the batteries ?
>
> 25mA for 2160 hours = 54 amp hours.
>
> What brand AAs do you buy ?
>
>
> .... Phil

The clock is on continuously.

I buy whatever AA that is on sale.

Andy
```
```AK wrote:

--------------
>

> > > Sorry for the late reply.
> > >
> > > The clock uses about 25 Ma and the AAs lasted about 3 months.
> > >
> >
> >  ** Oh dear  -   ROTFL
> >
> >  So you only switch the clock on when you want to read the time
> > - to save the batteries ?
> >
> > 25mA for 2160 hours = 54 amp hours.
> >
> > What brand AAs do you buy ?
> >
> >
>
> The clock is on continuously.

** Crikey -  that must waste the battery !

>
> I buy whatever AA that is on sale.
>

** But where do you buy 54Ah ones ??

BTW: