I was waiting for someone to correct me on that one :-)
Actally it all boils down to 63%.  The magic value of charge / discharge.
The formulae t = R*C gives how long it will take to discharge the cap from a
full load, down to 100%-63%= 37% of the charge.  That may be far too low for
your concern.  Here is where the step-up voltage converters come in handy.
Find one that will give a nice voltage for you rapp.  Then see it's specs
for max/min.
Now have fun calculating.

    KreAture


----- Original Message -----
From: "William Chops Westfield" <billw@CISCO.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, November 28, 2002 3:31 AM
Subject: Re: [EE]: NiMh and solar charging


>     Find your load, then your needed operating time.
>     Then you have your size from the formulae t/R = C.
>             :
>     Fairly basic stuff, but it's nice to have it on the list.
>
> It's not quite so simple.  The RC time constant is how long it takes the
> capacitor voltage to decay to 1/e of it's original value, but the
> interesting number is how long it takes to decay to a voltage where your
> circuit stops working (which is a somewhat ambiguous number in itself.)
>
> I thought cooper electric had a little excel spreadsheet you could
download
> that would figure out the needed capacitance based on assorted "normal"
> circuit criteria, ie:
>
>
http://www.cooperet.com/documents/PowerStor_Aerogel_Capacitor_Calculator.xls
>
> Enjoy
> BillW
>
> --
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>
>
>

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