I want to power a circuit using a 9V battery, that should stay active for at least a couple of years. These are my calculations.

General info on 9V batteries, as I found doing a quick search:

• Lithium: 1200 mAh
• Alcaline: 500 mAh
• 7.2V means that it's weak and reaching the end of its life
• 5.4V means that it's practically dead

So, a couple of years have 8,760 hours in them. That means that a Lithium battery should consume no more than 137 µA and an Alcaline 57 µA for the design to meet my requirements.

• Are my numbers correct?
• What other factors would be involved? (there always are... and usually turn out to be a serious pain) ... I'm guessing temperature is one of them.
• Should I consider some sort of safety factor for my design?

 

If "a couple of years" is two years, I get that there are closer to 17,532 hours in that length of time.

You should probably factor in self-discharge over that length of time. Alkalines (according to Battery U) have about 2% to 3% self-discharge per year, so you probably should allow for at least 6% and probably more like 10% over two years to be conservative. So that leaves you with 450 mAh to work with (assuming that the 500 mAh is to 7.2 V), giving you about 25 μA to work with assuming you want no additional margin (which is probably not a good idea).

Temperature is definitely a factor, but hot and cold. What kind of environment are you looking at the battery being in?

Yes, you want a safety factor. How much depends on the tradeoff between how expensive having it is compared to how expensive having a failure due to not having it is.

 

Does it have to run continuously? Can function be changed to intermittent?

"The MCUs reach the industry's lowest power consumption of 170 nA in lowest power mode with SRAM retention. Wake-up times have also been taken into consideration and are as low as 3.5 μs from Stop mode."

 

If "a couple of years" is two years, I get that there are closer to 17,532 hours in that length of time.

I hate arithmetic... yes, I forgot to multiply by two... let me pay closer attention to your observations and then I'll get back with more specific info... thanks for the help

 

I hate arithmetic... yes, I forgot to multiply by two... let me pay closer attention to your observations and then I'll get back with more specific info... thanks for the help

There are a few 'life calculators on the net.
https://oregonembedded.com/batterycalc.htm
https://www.microchip.com/SWLibraryWeb/product.aspx?product=nanoWatt XLP Battery Life