Tom=E1s =D3 h=C9ilidhe wrote:
> Every single battery I see has its energy capacity cited in "milliampere =

> hours"; it'd be a hell of a lot handier if they were cited in "milliwatt =

> hours", or even joules.
>
> I need roughly a 5 V supply for my project but I'm thinking of using a =

> 1.5 V supply and stepping it up to 5 V for the microcontroller... but =

> I'm looking at batteries that have different voltages and so I've to do =

> some maths to change the milliampere hours to milliwatt hours.
>
> Anyway moving on...
>
> My project will consume a peak of about 800 mW or thereabouts. The =

> average power might be something like 300 mW.
>
> A single AA battery is 1.5 volts and has 650 milliampere hours... which =

> is equal to 975 mW hours. So I'd get about 3 and a quarter hours out of =

> one AA battery.
>
> It'd be cool if I could use one of those small button-shaped batteries; =

> does anyone of a very small battery that has a capacity somewhere in the =

> hundreds of milliwatt hours?
>
>   =

Your conversion factors are wrong... it is more complicated.

The rated 650mAh is for a specified fixed current. You must read the =

datasheet. But, just guessing, let's say a fixed 100mA was drawn =

continuously from the cell, and it was wossible to draw this 100mA for =

6.5 hours, then it has a 650mAh rating...

but, because nothing in life is ideal, the cell voltage at the start of =

the 6.5 hours was 1.5V. It dropped continuously for the full period =

until it was finally depleted at 0.75V. So, what is the 'power' rating =

(in mWh).....?

If the voltage curve over the 6.5 hours was linear, then it would be =

approximately 730mWh... but, the volt curve is non-linear, and, the volt =

value depends on the current draw too....

Take two identical AA cells, discharge one at 0.1 amp and the other at =

0.5A and you will see different volt curves... with the 0.5A curve being =

lower than the 0.1A curve.

Point is that, as it is, 'rating' a cell as 650mAh is only a ball-park, =

it depends on the current drawn from the cell as well as the cell =

chemistry. Then, you add additional variables as you try and turn the =

mAh rating to a mWh rating. All of these ratings are dubious anyway =

unless your project exactly duplicates the test that was used to rate =

the cell itself.

As for using one of those button cells, typically, here's a basic way to =

rate the capacity of a cell.... only buy from a reputable distributor, =

like Mouser or Digikey. Then, take the format you want the cell in, =

(like AA or AAA), then sort the results by price. You will find a good =

rule-of-thumb for relative energy densities in the cells.

Finally, everyone has AA batteries at home, but not so many have button =

cells. May I suggest that you use AA for the device because it is much =

easier to change them when needed.... what batteries do you have spares =

of at home? It may look 'cool' to use a button cell, but, you will be =

changing it 5 times as often, and will have to make a trip to the store =

for each one....

Rolf
-- =

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