CR2032 battery on EMX board is lasting less then a year


I was hoping someone could give me an idea how long a CR2032 should last on an EMX board for RTC if the unit is used 4 hours a day 6 days a week? A general range / or educated guess would be great. Thank you.

So when your device is NOT being used, is it in low-power state, or is it just turned off?

If you think about the amount of time it is NOT in use, that’s 20hrs x 6 days + 24 hrs, 144 hours a week, roughly 7488hrs per year. A CR2032 has roughly 225mAh capacity (Wikipedia value, likely less depending on brand, source, age etc), so the rough numbers show if it lasts a year the RTC would be drawing ~33mAh. Is that too high? I haven’t measured it in real life, but is that something you can do?

@ Brett - check you decimal point. you are off by three orders of magnitude. :slight_smile:

Actually, I may not be :slight_smile:

So if we’re running for 7488 hours, and flattening a 225mAh battery, then to understand CURRENT DRAW we need to end up with mA, which is 225mAh/7488hrs, which equates to 0.030mA.

(now the bad part, I did 7488/225 = 33.28, but that is the inverse of mA. That’s somewhat worse than just needing to check my decimal point placement ! :slight_smile: )

@ Brett - I wondered how you got 33ma, but I figured you were doing rough calculations. been there etc

yeah, I am prone to do rough calculations on calc.exe which is obviously as good as the input it gets :stuck_out_tongue:

I had a look at some RTC chip datasheets and i asume the 33mA should be 33µA

page 54 of the LPC2478, Ibat is 20µA. RTC can not oscillated if Vbat is below 1.6. To be safe, I would say 2.0V. Most of CR2032 contains 240mAh till 2.0V. Let’s compute…

240mAh / 0.020mA = 12000h
12000h / 24h = 500 days

If the battery does not last then are you using a name brand battery? What temperature are you running at?

I’m using Panasonic brand batteries from Digikey and have also tried some Energizer batteries with the same results. The opp temp of the units are in standard room temp variances so nothing extreme. The only thing I can think of now is that we do see some noise on the 5V line during opperation being injected by some of the other components in the system. If this noise on the 5V line was severe enough, could it affect the CR2032 battery life?

For those who wants ever more: CR2450 can be a good solution