I could use some help to certify an assessment of an ongoing automotive issue. The vehicle is equipped with a high-performance electrical fan (40 amp draw/10 AWG), and I suspect that existing wiring is intercepting the charge from the battery.

Here is the current wiring diagram:

  • There is an AGM automotive battery under the rear seat, and it's grounded to the chassis.
  • There is an electrical junction box located under the hood where the battery and alternator connect.
  • The positive battery wire runs there and connects with alternator output (+). Basically, this is where the alternator flow meets and charges the battery.
  • The aftermarket electrical fan is also connected to the electrical junction and pulls the power from it on demand when the vehicle reaches operating temperature.

All three positive leads for (battery, alternator, and electrical fan) are connected at that junction. The positive wire running the electrical fan is also shorter than the battery wire that runs all the way to the back in the car.

Is it scientifically correct that the fan, at full operation (40 A), will intercept the alternator charge from the battery? If so, how significant would the interception be, and would it actually impact the charging of battery during normal operation? Please note that the alternator on the vehicle is rated at 140 A.

If there is an interception, would re-routing the positive wire from an electrical fan directly to the positive battery terminal remedy this issue?

Thank you!

  • \$\begingroup\$ the fan could be pulling down the alternator output voltage to the point that battery will no longer charge ... the voltage could be getting lost at a high resistance connection ... check the output voltage at the alternator when fan is on ... connect directly from alternator to battery \$\endgroup\$
    – jsotola
    Aug 8, 2020 at 2:36
  • \$\begingroup\$ Are you sure about the fan rating. 40 A is an enormous amount of current for a fan. I found a 12V 3000 cfm fan on eBay with a 10-13 A draw. \$\endgroup\$
    – mhaselup
    Aug 8, 2020 at 3:20
  • \$\begingroup\$ @mhaselup correction, this a high-performance fan from SPAL with an inline fuse is rated at 40 A on 10 AWG. \$\endgroup\$ Aug 8, 2020 at 3:24
  • \$\begingroup\$ @jsotola Thank you, great idea! \$\endgroup\$ Aug 8, 2020 at 3:25
  • \$\begingroup\$ Can you tell us what the problem is? \$\endgroup\$
    – mhaselup
    Aug 8, 2020 at 3:29

2 Answers 2


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 "missing" current required to provide the 40 Amps for the fan.

Where you physically connect the fan should not alter the operation.

  • \$\begingroup\$ As Peter states the battery and the fan appear as a single load on the alternator. If the fan is switched on by the thermostat it will draw it's rated current, the battery will also draw current if it is charging at that point. I can't see an issue with a alternator rated at 140 A periodically charging the battery and driving the fan. \$\endgroup\$
    – mhaselup
    Aug 8, 2020 at 3:37
  • \$\begingroup\$ Appreciate your answer. Wouldn't battery technically get a better charge while sitting between the alternator and the fan? All of the generated power has to touch and flow through the battery in that scenario. Right now, the charge \$\endgroup\$ Aug 8, 2020 at 3:55
  • 1
    \$\begingroup\$ No - the power goinng to the fan does not "flow through the battery" regardless of the physical arrangement o fthe wiring - everything is wired in parallel. \$\endgroup\$ Aug 8, 2020 at 4:30
  • \$\begingroup\$ I didn't understand why you used the term "intercept" and now I do. The battery and fan are connected in parallel, they are essentially independent and unaware of each other if you like. When then fan switches on a draws current there would be less current available to charge the battery. \$\endgroup\$
    – mhaselup
    Aug 8, 2020 at 4:32
  • \$\begingroup\$ @AlekseyKorzun: regardless of how you connect things, the new fan WILL reduce the current available to charge the battery. \$\endgroup\$ Aug 8, 2020 at 16:33


simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) What you've got. (b) Maybe better.

In your circuit (a) both loads are drawing current through the alternator lead which has a resistance R1. As the current drawn increases the voltage drop across R1 increases and the junction-box voltage will decrease by that much. The other factor is that the alternator output may droop at the higher current causing a decrease in voltage at ALT. Either of these may be enough to prevent the battery charging correctly.

By connecting the high-current fan directly to the alternator (b) you can avoid the problem of voltage drop through the alternator to battery cable.

You should be able to get a feel for what's happening by monitoring the voltage at various points using a DC voltmeter.

The load isn't "intercepting" the current. It's just drawing additional current.


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