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. – Pᴀᴜʟsᴛᴇʀ2 Dec 8 '19 at 22:08
  • Disconnect the alterhator completely before running the engine otherwise it is possible to damage it. – Solar Mike Dec 9 '19 at 4:49
  • Use an inductive clamp meter? – Dan Z Jan 8 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. – Andyz Smith Jan 9 at 0:39

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.

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  • 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 '19 at 23:39
  • Fans only run if they need to... temperature controlled. – Solar Mike Dec 9 '19 at 4:48
  • Yeah, but in this case i'm looking at running a vehicle for several hours at a time. – Frank Dec 9 '19 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 '19 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. – Andyz Smith Jan 9 at 0:43

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