On 19 Aug 2003 11:36:37 -0700, (Tom Bruhns) wrote:

"Bruce W.1" wrote in message ...


I found a good way to measure the capacity of a single cell. This Radio
Shack multimeter:
http://www.radioshack.com/product.as...%5Fid=22%2D812

It logs voltage (or current) and its software can output the log to a
text file. Now all I have to to is write a little computer program to
calculate the capacity.

I have a similar meter that seems pretty accurate, and because the
voltage is around half of one of the full-scale ranges, you don't
sacrifice much because of poor resolution (as you would at, say,
2.1V).

An easier way (for those of us who don't want to deal with programming
access to the info) than writing a program is just to import the text
file to a spreadsheet. You then have a column of voltages at uniform
time intervals. If you know the discharge resistance (load
resistance), then I=V/R and you can make a column of that value. The
power at each interval is just V*I -- or just go to that directly as
V^2/R. Then the total energy is the integral of the power over
time...in watt-seconds, just the sum of the power column, if your time
interval is one second. Divide by 3600 seconds/hour to get
watt-hours. Sum the amps column to get amp-seconds and divide by 3600
to get amp-hours.

That is *exactly* what I suggested in another version of this thread.
(Not claiming anything except that at least TWO ppl find that way
easy).

I've done exactly this sort of thing with my RS-232-interface
voltmeter. Works fine.

Cheers,
Tom