ALUMITECH REPRODUCTIONS said:
as far as electrical systems go , your drawing off the battery yes and in most the casses the fan wont run for long periods so there may be no need to up grade the alternator .
I have to take exception to this with the highest respect to Don. Don provides great advice to many on this subject, but I offer this different take....
It's obvious that you want as much air flow as you can muster for both airconditioning (if you have it) and engine cooling....and at high cruising speeds fans usually are not needed at all to aid and abet the airflow thru the radiators. However, these big cooling fans can commonly take in excess of 30 amps and some as high as 40 amps plus. While it might be easy to think that these fans only run intermittantly at low speeds, alot of these cars spend most of their lives at low speeds and idle conditions....in very hot conditions and the fans run a lot! This is typical at large car show events where street cruising and long traffic lines are the rule...and not the exception. Even in everyday driving situations. Most fan systems can run 75% of the time in normal cooling applications believe it or not.
Don is correct, when you don't have enough electrical output from the alternator the excess demand calls on the battery to provide extra amperage/wattage to fill the gap. This IS what a battery is obviously for. But, when you get to the point where the demand is most of the time or even alot less than that, the battery becomes discharged all the time with no "charging buffer" for the system to ever fully recover. This commonly produces low DC bus voltages which leads to dim lights and questionable restarting conditions...especially when the engine gets hot further making starting difficult. Low DC bus voltage also leads to inaccurate gauge readings (because this voltage is used for reference in most gauges) and of course the cooling fans themselves are "derated" due to the low voltage....meaning less airflow.
To complicate all this, the "rating" system for alternators are at PEAK output which occurs at highway rpms....so commonly a claimed "63amp" alternator only produces 50% of the rating at low and even moderate speeds. Even the most simple of automotive electrical applications can demand as much as 35 amps most of the time....and that's just for lights, a moderate stereo, ignition and throw in a blower motor once in awhile. So you can see how you can get into trouble real fast and/or how narrow the "charging margin" is without a sufficient output from the alternator, ....then add a 30+amp fan. Well, you get the picture...
So, assuming you want to build a system for the "peak demand" of the conditions one would expect, you have to upgrade your electrical system in the wiring department and higher output alternators. With the proliferation of electrical components these days....the "100" amp alternators are very common and reasonably priced. The "high ouput" series such as the CoolCharger series from TuffStuff will provide as much as 80 amps at idle....more than enough for this type of upgrade.
Also, pay attention to the DC bus/wiring when upgrading the alternator. You can easily pop a fusible link and/or burn up a feeder or connection with increases in amperage thru these connections.
I hope some of this sinks in and provides some insight to why one wants to upgrade the electrical system as well.