Listers,
The cold cranking amp rating of a battery is supposed to be the current you
can draw for 30 seconds at zero degrees Fahrenheit without dropping below
7.2 volts. I was a bit incredulous earlier in reference to Cullen's 850 CCA
battery, but they apparently do exist, and even higher for submarines,
diesel truck engines, etc. But as a practical matter, especially with the
long length of cable we have in Tigers, I'll bet we are lucky to get more
than maybe 200A or so to the starter. I have read that a starter would like
to draw maybe 300A. Suppose we can deliver 300A @ 10V to the starter which
is 3,000 watts. One horsepower is 746 watts, so in terms of horsepower, and
assuming 100% efficiency of the starter, that's 4HP. The starter is
probably at least 75% efficient, which would mean 3HP delivered. Now, maybe
one of our cold climate buddies can take a torque wrench and measure the
amount of torque it takes to turn over their engine on a cold morning.
Then, assuming maybe 100 or so rpm of cranking speed needed to start, we
can calculate the HP and current needed to accomplish it.
I have been looking for, but have not found any information on just how the
CCA is measured. My guess is they plot a curve starting with much more
modest currents, voltages, and times, and extrapolate out to the highest
amperage, but there is undoubtedly an SAE standard procedure for
determining the rating. If you want to know more about batteries, here's a
few good links:
http://www.off-road.com/4x4web/faqs/batteries.html
http://www.njautosite.com/garage/subsys/baelba05.html
http://sura1.jlab.org/~grippo/ccatemp.html
Happy motoring,
Bob
Robert L. Palmer
UCSD, Dept. of AMES
619-822-1037 (o)
760-599-9927 (h)
rpalmer@ucsd.edu
rpalmer@cts.com
|