On Thu, 24 Nov 2011 21:46:48 -0800, Jeff Liebermann
wrote:

The next question is for how long will it run? *Assuming the calipers
can handle 3.0VDC without damage, how long will a junk 100UF
electrolytic cap run the calipers?

a) How long will it run? Not nearly long enough, and b) 3.0VDC is
waayyy too risky for my blood. 20uA will discharge 100uF from 2.0V to
1.35V in 3.25 seconds.

Trying the same calc using the super-cap formula from Pg 6 of:
http://www.cde.com/catalogs/EDL.pdf

t = C delta V / I
t = C[V0-(i*R)-V1] / (i+iL)
whe
t: Back-up time (sec)
C: Capacitance of Type EDL (Farads)
V0: Applied voltage (Volts)
V1: Cut-off voltage (Volts)
i: Current during back-up (Amps)
iL: Leakage current (Amps)
R: Internal resistance (ohms) at 1 kHz

For this example, I'll use a 0.1F (type F) 5.5V 100 ohm cap.
The low end of the tolerance range might drop this to 0.08F.
V0 = 2.0V, V1 = 1.4V, i = 15uA, iL = 2uA

Plugging in:
t = C[V0-(i*R)-V1] / (i+iL)
t = 0.08F[2.0V-(15uA*100ohms)-1.4V]/(15uA+2uA)
t = 2800 sec = 47 minutes.
Not bad.

I guess the protective case that most calipers use will need a clear
plastic window to keep it charged. Maybe another window on top of my
toolbox.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558