Itâ€™s unlikely that youâ€™ll have to limit the current. This is best shown by example:
The worst case set of conditions with regard to cooling typically occur late in the day, around 2:00 pm.
Assuming an outside temperature is 120 degrees F, and the engine temperature is to be regulated at 180 degrees F, the allowable temperature rise is 60 degrees F.
During this time, it is unlikely that your headlights (15 A) are on, and even more unlikely that your fog lights (10 A) are on. So, although the fan may be drawing 30 A, the charging system is relieved of 25A worth of lighting. So with the air conditioning fan, the stereo system and everything else running on high, youâ€™re left with a 5 A deficit.
Letâ€™s say that by time the sun sets, the temperature has dropped to 100 degrees F. The allowable temperature rise is now 80 degrees. Three fourths of the cfm , or 12A of current is now needed to maintain the 80 degree temperature rise. Although your headlights are on, it is unlikely that there is fog at 100 degrees F, your deficit is now 2 A.
By time the fog has set in at 60 degrees F, a 120 degree rise is allowable, 2150 cfm and 4A current draw, a 4A deficit.
The battery should be able to sustain a 5 A current draw for a number of hours. Realistically, it is unlikely that 4300 cfm will ever be needed, but if it is needed, the trade off for 5 A or having to shut off the stereo system in lieu of overheating is certainly worth while.
A series resistor would dissipate well over 100 W. Most of the efficiency of the system would also be lost. Check the links in this thread for the necessary controller.
A PWM greatly increases the longevity of the fan motor, as there is no current peak at startup, nor is there the wear on the bearings byway of the corresponding torque.
Here's a link to some performance numbers