Avoiding battery charge interception with high-current devices
If the fan requires 40 Amps, it will draw 40 Amps from somewhere.
If the alternator produces more than 40 Amps, 40 Amps will go to the fan, and any excess will go to charge the battery.
If the alternator produces less than 40 Amps, the fan will still get 40 Amps, and the battery will provide the &quot;missing&quot; current required to provide the 40 Amps for the fan.
Where you physically connect the fan should not alter the operation.