Mark Rages <markrages@gmail.com> wrote:
> On 8/25/05, Matthew Miller <namiller2@naxs.net> wrote:
> > I placed a 4700uF cap in parallel with the 3V battery, and measuring the
> > voltage across the cap, wait for the voltage reading to stabilize. This
> > voltage is V1.
> > 
> > Next, I do my best to unhook the battery and start a stopwatch at the same
> > instant. When the voltage reading drops to around 2.2V, I try to press the
> > hold button on the volt meter and stop the stopwatch at the same time. This
> > voltage is V2.
> 
> That's a clever method, but you should probably measure the 4700uF
> cap's actual capacitance. (electrolytic caps can vary by a lot).  Use
> the same method, but substitute a high-value 1% resistor instead of
> the test circuit.  Solve for C instead of I.

You'll also want to know its internal shunt resistance (leakage current),
so measure the time constant with no external load, or a second precision
resistor of a different value.

-- Dave Tweed
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