Ok, I am actually a wind tunnel guy.......and an engineer (I know, boooooo)......I have worked in automotive and Air Force wind tunnels for the past 10 years primarily in cooling system testing. And while I wasen't the technical test engineer I am around a lot of those aero guys (I know poor me, these guys are worse than us regular engineers
).
If the fan is not running it can likely run as fast as the airflow pushes it (assuming there are very little mechanical losses in the fan). But you still do get a little bit of back pressure (reverse blockage) that increases the faster you go (I don't know how fast you need to go for this to be a problem except that at Mach 3.0 it's a lot). You would have no current draw in this situation. Infact if you were super smart you could actual use the fan turning to charge your battery (or boost your ingition power.....)
If the fan is running and the vehicle speed is less than what the fan is pulling then the fan will "make-up" the difference in airflow and increase your cooling (using less current draw the faster you go....to a point)
If the fan is running at the same air speed as the vehicle then in theory the fan would have little to no load on it thus drawing little to no current from the charging system. So you won't need to worry about your battery draw. And you are getting little to no "help" from the fan.
However if the vehicle air speed is faster than the fan air speed and the fan is running the fan will likely continue trying to hold the max speed the fan is designed for (because most of these fans do not have a clutch). Thus restricting airflow and increasing current draw on the charging system. Of course this assumes that you will never be behind somebody who is slowing/disrupting the air in front of you.
If the fan is mounted on the front of the cooler it will likely block at least some airflow even when running. I would not recomend this unless you do most/all of your racing at speeds less than what the fan pulls. I'm not trying to say anything here but some of us maybe be that slow
I was wondering if there is a fan out there that has a clutch that would allow it to spin faster than it's max without increasing load or damaging the fan? That would be ideal otherwise I'm thinking a fan is only useful if you remember to turn it on and off properly (which I could never do during a race this past year)
If I get a chance I will try and run some calculations on what air speed the fans are pulling the see how far off we are. Does anybody know what cfm these fans pull? I'm thinking arounf 500cfm (for an 8-9inch) but I'm not sure.
Gerad