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Super capacitor with PCF2129 instead of a battery

Question asked by Fabian Romo on Aug 25, 2019
Latest reply on Dec 23, 2019 by Fabian Romo



I have read in the data sheet of the PCF2129 there is the possibility of using a super capacitor as an energy backup instead of a battery.


My question is:


If I use a super capacitor of 47000 uF (200 Ohms ESR), how long will the data and time be kept before a power outage?



There is no information about how much current it consumes when the battery is keeping the data and the internal clock. There is data about battery leakage current that is 50 nA and supposedly the minimum battery voltage for correct clock operation is 1.8V, so the following equation may be correct:


Vc = Vo(1 -I*t/C) 


Where Vc is capacitor voltage as a function of time.


Vo is  is capacitor voltage when charged to maximum value (3.0V)


I is the currente that needs the clock and de ram on the PCF2129 (Assuming it is constant)


C is the capacity in farads and


t is the time in seconds.


Then the equation would look like this:

1.8 = 3.0(1 - 50 nA * t / 47000 uF)


The time that the capacitor can provide power to the RTCC is 376000 seconds