[Excaliber]

Even 65 amp should be MORE than enough to power the fan AND keep the battery charged.
A couple of things, therefore, come to mind:

1. The battery itself is unable to "hold" a decent charge in the first place.
2. The alternator is not working properly.
3. The starter is drawing TO many amps and pulling down the battery EVEN though the battery is good!

Testing:
ENGINE CRANKING battery voltage should not fall below 9 volts when it is fully charged. If it does this can STILL point to bad starter or a weak battery! A carbon pile battery load test will determine battery condition. An AMP reading of the starter will determine starter health.

Bear in mind a 65 (or 100) amp alternator will rarely IF ever actually output that much current during "charging". 20 to 30 amp output IMMEDIATLY after engine start is more like it. And THAT for only a few minutes under normal circumstances.

You may well need a "specialist" to get to the bottom of it. One who has the "tools", NOT some "pump jockey" that just replaces everything. Carbon Pile BATTERY load tester. INDUCTIVE amp meter to read starter drain druing cranking is what I would use. Some alternator regulators can be "shorted" to "force" 100% output of the alternator, then take a reading, it shoud be 65 amps (this does NOT hurt the alternator if done correctly). The alternator regulator is also one of the "usual suspects", not allowing the alt. to output as much as it should.

It is VERY unlikely your fan can outdraw your alternator output EVEN if it is a fan "from hell".

Ernie

ASE certified electrical systems technician in another life, long long ago, lol.