nudder thing.....
Most fan manufacturers rate their fan CFMs above battery voltage. Spal, as listed here, rate theirs @ 13.0 volts. I believe Flex-a-Lite rates theirs even higher. If you have an "undersized" alternator that goes into discharge at idle then your electric fan isn't providing it's rated CFM. If you're driving the typical "cooling challenged" Cobra, that's not a good thing. With everything on my alternator provides a constant 14 volts, at the battery OR the fan. If your car's wiring "infrastructure" is also inadequate, which a LOT are, then the problem's compounded. With your car running, at idle, run a voltage test at the cooling fan. A lot of people would be suprised. With my Lone Star's painless harness and provided 65 amp alternator I was getting 10.2 volts to my cooling fan till I rewired the whole circuit.
Dave
http://www.spalusa.com/fans/automate.../1516-1517.pdf