Capacitor Discharge and DS1307

Hi. I need help on calculating the time it will take to discharge a SuperCap of 1.5Farad charged at 3,7 Volts down to 2,3 volts at a rate of 500nA.

This SuperCap is charged through diodes and a resistor from +5Volts and upon power failure, it will back up a DS1307 through a BAT85 diode.

Thanks for any help,
John

q=It
q=C(V1-V2)

It = C(V1-V2)

t=C(V1-V2)/I

t=1.5(3.7-2.3)/0.5e-6 = 4.2e6 sec

4.2e6 sec * 1 day/86400 sec = 48.6 days

Assuming no leakage current within the supercap. The cap's leakage rate will likely be greater then your 500 nA load current. There may be a little back leakage through the original supply blocking diode also.

Thanks for the replies. According to Maxim, 500nA is the Max current needed to hold the RTC ticking. Min is 300nA.

Data sheets do not show the exact internal leakage in nA, but give an estimate of 1 volt drop after 500 hours (from 5 to 4 volt drop). And this is dependent on the temperature of course.

Have charged the cap and will see how it goes.

In any case over a month is very good for zero cost maintainace RTC (no battery, that is).
John