>
> I'm considering using a 5.5 VDCC NEC "Supecap" Capacitor to back up a
> Dallas DS1302 Real-Time Clock.  This device draws less than 300
> nanoamps at 2.5 volts.
>
> I need some help in determining what size of capacitor to use verus the
> number of hours of backup.  I need to be able to spec at lest 30 days.
>
> Thanks
>   -barry
>

I would think that this is a simple RC discharge circuit.
The R component I would think could be modeled by

  R = E/I => 2.5V  /.3 uA => 25V / 3uA  => 8.333 MOhms

The voltage for an discharing RC circuit is modeled by the equation

          -t
  V  e
   0

where t is the number of time constants that have passed since the
voltage source was disconnected.
                                                      -t
Assuming V  = 5 and V = 2.5, we would come up with a e  = 0.5 (i.e. 2.5/5).
              0          f

Taking the natural logrithm of 0.5 = 0.6213 for the portion of a standard
time constant that is necessary to decay down to 50% of the starting
voltage, we will multiply this time the standard time constant to determine
the hold time of the circuit.

to calculate the standard time constant:

R X C = 1 F x 8.333MOhms = 8.333 million seconds  ~= 96.45 days

This number times 0.6213 ~= 59.92 days

Disclaimer: I am a computer programmer, not an electrical engineer!

-- Dave Ritchie N4DJS
der@hpuerca.atl.hp.com