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So, this is in general terms... But say you have a flat battery and you wanted to start a car without another card battery?

Car batteries are 12V but I'm not sure how much current they are.

Do they need about 3000W for starting? (About 250 amps?)

Does the amperage drop when running?

Does the amperage change (when running) depending on lights/wipers/sound system usage?

If I wanted to jump start a car, I'd need at least a source of 3000W, correct?

If I wanted to charge a car with a source less than 3000W (Say, 300W [12V, 25A] supply) it would need to charge for a few hours, correct?

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  • Do they need about 3000W for starting? (About 250 amps?)

    250 A is on the low side according to the Bosch Automotive Handbook (8th edition), which indicates that the primary current of the engine starter can be as high as 1000 A for passenger cars.

    Quote from the book (emphasis my own):

    The relay current (approx. 30 A for passenger cars to approx. 70 A for commercial vehicles) generates power in the relay. This pushes the pinion toward the engine-flywheel ring gear and activates the starter primary current (200 - 1,000 A for passenger cars, approx. 2,000 A for commercial vehicles).

  • Does the amperage drop when running?

    Yes, another quote from the Bosch book indicates a minimum current draw of around 170 A on a passenger vehicle:

    The total output power of all instaled electrical equipment, which draws electrical energy from the battery for several minutes at a time, often exceeds 2 kW.


  • Does the amperage change (when running) depending on lights/wipers/sound system usage?

    Definitely.

  • If I wanted to charge a car with a source less than 3000W (Say, 300W [12V, 25A] supply) it would need to charge for a few hours, correct?

    What you want to replenish is the charge lost in the starting device. A starting device which delivers an average 300 A for 5 seconds loses 1500 coulombs.

    My EE is a little rusty, but how long that takes should be a function of the charge lost, source power rating and supply voltage:

    Power = V * I
    Power = V * Q / t
    t = Voltage * Q / Power
    

    So if you had a constant 300 W, 12 V supply it would take you

    t = 12 * 1500 / 300 = 60 s
    
  • Right. So, on average, it needs about 3.6kW to start? And then when running, can supply about 360W? Is that how you understand it? – BBking Aug 5 '15 at 8:22
  • @BBking : Updated the answer – Zaid Aug 5 '15 at 9:03
  • But I thought most alternators only provide up to 120 amps at about 14.5 volts. Or are my numbers completely off? – Robert S. Barnes Mar 23 '16 at 19:59
  • @RobertS.Barnes I assumed 12 V when I computed 170 A. You are right that it should be evaluated at charging voltage. – Zaid Mar 24 '16 at 9:52
  • @Zaid does that change your answer – Robert S. Barnes Mar 24 '16 at 11:02

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