Here's an easy scenario that indicates how your battery and alternator interact.

Lets say it's night time and you're driving with your bright lights on, the AC is on high, the windshield wipers are on, the radio is playing loudly. Look at the amp meter. If the battery is fully charged, the amp meter will read 0 amps. All the current to the lights and accessories is being provided exclusively by the alternator. The battery provides power to start the engine and power when the engine is not running. (It will also provide power when all loads are on and the engine is idling at too low an rpm for the alternator to provide full power.)
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I connected my electric fan (single SPAL) to the alternator (via a relay of course). My only concern is; what if I start the engine when the block is 200 degrees F. Before the engine starts, the fan will come on and try to draw power from the battery via the fusible link. I hope this will not burn out the fusible link. I'm going to have to try to answer this question.

 

your relay should be powered off the ON circuit, not the start circuit

 

Right...in the start position...the fan will have no power. Once it slips back from start to ON, the fan relay will kick on. Either way, I wouldn't think you'd have to worry much about the link.

 

if the fan is running off the alternator, why do the engineers at SPAL recommend hooking the fan directly to the Battery??

 

Is there a web link stating that recommendation?

 

Spal USA website. directions are hard to find but they are there. try this http://www.spal-usa.com/fans/automat...85FH-195FH.pdf

 

They say to run the power from a batt because that's a solid constant source. The switch should be a relay turned on by the ignition...or 'On' and the 'Start' position won't spin the fan motor. The power for the fan should come directly from the battery or constant 12V source, but switched on by the relay.

 

Ya know what? The battery and alternator are connected. The fan isn't gonna have a clue where the power is coming from.

 

This source says "...within 12" of battery", the drawing shows directly connected to the battery. Here's what I'm thinking, since most cars batteries are located in the engine compartment the distance of the wire that feeds the battery from the alt is short compared to C3's. Since the resistance would be lower you might get away with it, just don't connect anything else this way or you'll run into problems.

In our case if you think of the wires as pipes and you run a 10ga wire from the fans to the starter lug you do have a nice dedicated pipe to supply those fans. However, when the car is running, the alternator will need to supply the fans through it's 10ga wire to the distribution lug. And from there the stock wire travels to the starter through the fusible link, which is 14ga, and ending in a short run through a 12ga wire that connects to the starter. So your going from 10ga to 14ga to 12ga over a distance of about 4 feet (looking at my '72 schematic), a rather long distance with allot of changes in piping diameter and allot of resistance!

Now if we get back to the original reason this pipe exists, to keep the battery charged, you can see that it'll be overloaded doing two jobs. Worst case scenario is you could blow the fusible link. A lesser problem that could surface is that it could run the fans just fine, but during this time the battery will be deprived. In the summer when your out and about in town incurring frequent start-ups you could actually be slowly draining your battery.

Again the best way to hook this up is connecting to the horn's distribution lug, it's where all of the car's accessories get their power and that is it's sole purpose in life. You don't need to upgrade the pipe from the horn relay to the starter, it's big enough to supply battery power to the fans if you have a fan controller that provides a brief fan cool down period after ignition shutoff (I have one of these). You definitely should upgrade the alternators pipe to this lug, and you should wire the fans such that when the ignition switch is in the start position those fans can't steal battery power away from the starter.

 

SPAL directions probly indicate battery connection because in most cars the battery and alternator are very close together.
In the Generals infinate wisdom, he put the bat and alt at different ends of the car on C3's. The thing to keep in mind is minimizing the path for the heavy current applications ( Fans, etc). Since the Alternator furnishes all the normal operating juice, and its under the hood, seems like the relays, fans, alternator should all be in the same place.
I could get all scientific, but its a lot of truble unless you want to understand. So if you want to save some $$ on parts, trouble, etc, build it cool, efficient, smart...I'll help you out

 

The alt post is a fine place to mount the fan power supply. It's a btter idea anyway so you don't experience a voltage or amperage drop from teh distance from the battery to the relay/fans.

 

Here's some info on upgrading the wiring. I used 8 ga. wire.

MAD Electrical

 

Since I'm installing a pair of 130s I'll be using either 4 or 2 gauge from the alt, to the starter, and batteries.

 

Z-MAN, great read!! answers my questions. I will run an 8 guage wire from the alternator to the horn relay distribution post and connect the fan power supply line to that post. Makes things pretty simple. THANKS.

 

It would be a good idea to run that fan power through a breaker, just to be safe. They carry them at most Oreillys.

 

I'm asking this here and on the "madness" site. What underhood source did you use for the ignition switched power source for the fan control??

 

I'm running mine directly from the alt post, but I will create my own junction on the driver's inner fender and will move it there.

 

the alt post is not ignition switched. It is always hot;

 

You're right. That's the constant. The fan controller should be switched from an ignition source and the fan power should be a direct line through a relay.

 

The Yellow wire in the wiper motor circuit is switched, tap it and just run an extra relay to supply large current switched 12 from the big wire you put on the alternator post