Assuming the lights and stereo are off, how many amps can you expect an idling vehicle to pull? In this particular case I'm wondering about an F350, with flexfuel engine?

  • Not a whole bunch, but it really depends upon the vehicle. If no accessories are on, there's far more juice availability from the alternator than there is draw from the engine. Once the battery is fully charged after startup, the regulator should shut down power production until its needed. If it doesn't, it would fry the battery. You could always disconnect the alternator and put an amp analyzer on the main power lead going to the battery. It would definitively tell you what the draw would be for any given vehicle. Dec 8, 2019 at 22:08
  • Disconnect the alterhator completely before running the engine otherwise it is possible to damage it.
    – Solar Mike
    Dec 9, 2019 at 4:49
  • Use an inductive clamp meter?
    – Dan Z
    Jan 8, 2020 at 6:54
  • Charging the ignition coil for like, um, the Sparks... will certainly be significant. The computer is always on too though probably not much power comparatively. Jan 9, 2020 at 0:39

2 Answers 2


Still depends on many things : state of battery charge, other items such as heater blower, not an easy answer...

Just enough to run the minimum may be 5 amps if the battery is fully charged but more if it needs charging.

  • 2
    I've read that the fuel pump draws 4 - 6 amps, and the radiator fans can draw about 35 amps if they are running together. I'm not sure what else is running during idle.
    – Frank
    Dec 8, 2019 at 23:39
  • Fans only run if they need to... temperature controlled.
    – Solar Mike
    Dec 9, 2019 at 4:48
  • Yeah, but in this case i'm looking at running a vehicle for several hours at a time.
    – Frank
    Dec 9, 2019 at 14:57
  • Then the fans, and other things, will cycle as necessary so increasing the load on the alternator as required.
    – Solar Mike
    Dec 9, 2019 at 15:45
  • I'm not sure but i think most cars will not run or run well on battery voltage alone. Alternator charges battery at 14 or so and all other components are designed around that. Jan 9, 2020 at 0:43

Typical Current Loads for Automotive Systems, Lighting, and Accessories:

  • Engine Idling (no lights or accessories on) - 35 to 50 amps. This will vary depending on the number of cylinders (more cylinders draw more power for the fuel injectors and coils), the type of fuel injectors (some draw higher amp loads than others), the type of ignition system (single-coil or multi-coil), the amp draw of the PCM, and the fuel pump (the amp draw will be higher with higher pressure systems).


  • Engine Off (nothing on) - 40 to 50 milliamps (power drain by modules in sleep mode, antitheft system, and keyless entry)
  • Ignition Coil (single oil-filled coil older vehicle) - 3 to 4 amps.
  • Ignition Coil (single DIS coil newer vehicle) - 5 to 6 amps at peak output.
  • Ignition Coil (coil-on-plug) - 6 amps per coil at peak output.
  • Ignition System (primary circuit) - 6 to 20 amps.
  • Fuel Injectors - 4 to 6 amps peak, 1 amp hold
  • Electric Fuel Pump (depends on pressure and flow) - 4 to 12 amps
  • Electric Cooling Fan (depends on the size and how it turns on) - 6 to 30 amps
  • Small bulbs (LED, usually in a new car for dashboard) - 0.04 to 0.06 amps per bulb
  • 500 Watt Sound System (OFF)- 20-200mA (depending on the deep sleep state)
  • Electric Power Steering - 2 to 40 amps depending on load

Assuming it is Gasoline With 50mA idle for security module + Ignition Coil 5A peak + Ignition system 10A peak, with a Fuel injector 5A peak, Electric fuel pump Avg 8A peak, Electric cooling fan OFF, the small bulb on the dashboard ON 0.05A, Sound system off drawing 100mA, without any electric power steering, and without AC ON, and any heater OFF will be peaked about 28.2A on idle.

But because it is a peak amperage draw, it will not always draw 28.2A, if you test it with multitester or DC Clamp current meter it will just reading as low as 8-15A because it is averaging your current, because your current is not straight, more likely to be pulsed current draw.

Source: AA1Car.com

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