Avoiding battery charge interception with high-current devices

Question

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!

Answer 1

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.

Answer 2

^{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.