I'm trying to find a suitable coin cell battery to run a small PIR motion detector module.
The specs on the module say it operates on 4-15V @ 45uA (microamps) when no signal is detected (standby), and 20mA when it senses motion and sends a logic output.
Since this operates at just above the standard 3 Volts that most coin cell batteries are I'm looking at this 6V battery from Digikey, capacity rating on this guy is 165mAh @ 30µA.
My question is, at 45uA how long will this battery last? I expect to see motion only a dozen or so times a day so the majority of it's life will be in standby mode.
I'm guessing the capacity will be proportionally less if you draw 45uA continuously (as it is for sealed lead acid bats), so let's say 165*30/45 = ~110mAh. Not knowing how long the 20mA draw lasts makes precise calculation impossible, but if we guess 5secs at 10 times a day then the average sensed-motion current is 5*10*20/(60*60*24)mA, i.e. ~ 12uA. So the battery life should be 110*1000/(45+12) = ~1900hrs = ~80 days.
If you can estimate the amount of time the detector will actually detect something, you can get a more accurate number. Let's say it's activated 5 percent of the time: your consumption is then
Impossible to say how long it will be in 'detection' mode for the life of the battery. I do know the detection dwell time and output signal is less than 1 second for each occurance (couple dozen occurances a day???).
I'm basically going off the standby time. From there I can deduct an estimated percentage to account for the time it will be in detection mode.
Based on the guesstimate above I was hoping to get more than 5 months. Hoping for a year or more.
Coin cells have high internal resistance so may have trouble supplying the 20 ma without a large voltage drop. If you can spare the space I would look for something like a AAA type or a couple of lithium ion types.