Hi all,

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:

http://www.ti.com/lit/wp/swra349/swra349.pdf https://electronics.stackexchange.com/questions/43306/pulse-powering-heavy-loads-with-a-coin-cell

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?

Thanks! Franck