I am considering powering pro mini sensor nodes from a coin cell, and as a result I started looking into using capacitors to handle peak loads:
The problem is, I am struggling with the basics :) My most significant peak load is 20mA for 35ms, happening every 3mn (radio sends ping).
The first question is whether to use a charging resistance, and if so what size. I am thinking that it wouldn't make sense to replace the sensor peak load with another peak load when starting to charge the capacitor, so I am coming up with:
- nominal draw for a Sony cr2032 -> 0.2 mA (https://www.sony.net/Products/MicroBattery/cr/pdf/cr2032_02.pdf)
-> charging resistance = 2.7V / 0.2mA = 13k (a bit more at 3V but then I assume a bit more than nominal current should be ok).
That will still charge even a 1000uF in a little over a mn so it looks like this should work.
Does this make sense so far?
The next step is to pick the capacitor; the first link above gives me a value of 1000uF through a calculation I don't fully understand...
If instead I assume that the discharge load is 2.2V / 20mA = 110ohms, for a 500uF I get a time constant of 55ms which should be plenty enough? In fact if I discharge down to 2RC I could use 160uF.
Finally I read that electrolytic capacitors have significant leakage, so I am considering this kind of ceramic capacitor:
So, what are your thoughts on the plan above?