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Recently, I came across someone needing a jumpstart who asked me to help them. I agreed to help and went about starting to jump their car. I don't know a lot about cars, but I know the basic steps to jumpstart a car. After we let the battery charge for a few minutes (I thought), the car still failed to start. By this time, some family of the driver in need had arrived and told me that, 1. I didn't have the cables connected to her car sufficiently, and 2. that if everything is connected correctly, it should start immediately (I assume he meant from the power of my battery). He then reworked the connections to her battery while I (at his suggestion) started to rev my engine (after he connected the cables again). After a few seconds of revving, the other car started just fine.

My question is, is it actually possible to jumpstart a car using the power from a charged battery directly without letting the dead battery charge, or was it a combination of other factors that allowed her car to start at just the right moment?

Probably relevant: I drive a '92 Chevy Silverado and I was jumpstarting a small car (I don't remember the model, but I think it was foreign). The cables I have are fairly bulky for jumper cables. I don't know the exact gauge, but they are definitely thicker than standard cables.

  • Did you rev your engine while trying to start the car? – Mast May 11 '17 at 18:19
  • I can't remember the exact timing, but I think I stopped revving before it actually started. I did not rev my engine before the family members showed up. – Jay May 11 '17 at 18:24
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    I've jump started a number of cars that required 2-10 minutes of charge-up time before they would start, even while revving. It depends upon how dead the battery is, how many amps of power the charging car can provide, how many amps the dead car will require to start, etc. – Code4aliving May 11 '17 at 18:56
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This depends on the quality of your cables. I have heavy-duty American Wire Gauge 0 cables. Despite their length (5 meters per one cable), they are capable of providing immediately enough power to the recipient car.

On the other hand, jumper cables may be as thin as American Wire Gauge 7. Needless to say, such cables are of no other use than to slowly charge the recipient car. Assuming the recipient car battery can still accept charge, you will be able to start the recipient car but not immediately.

There are also differences between the clamps. The clamps in my cables are made from pure copper, some others may have steel clamps merely coated with copper. Such steel clamps will obviously add to the resistance.

You didn't say what kind of cables you have, but I strongly suspect that the ability to start wasn't caused by rerouting the cables.

Revving your engine shouldn't be necessary because your battery (assuming it's good) should have enough power to immediately start the recipient car. Also, modern alternators will create quite a lot of current even at idle.

And although this may be something you might already know, I'm going to repeat it:

  • Have the donor car engine running
  • Connect the cables positive first, negative then
  • The negative connection to the recipient car is the last connection made and it is made to a metal grounded part, not to the negative battery terminal (so that you can remove the connection without risking hydrogen explosion caused by a spark near the battery)
  • Start the recipient car
  • Remove cables in inverse order
  • If you suspect the recipient car has bad battery (not just flat, but actually dead), NEVER try to jump start! A jump start may work but then when you disconnect the cables, you will have a voltage spike due to dead battery causing all of the car electronics to break
  • If the donor car is a hybrid, it most likely doesn't have a 12V starter battery but merely a boot-up battery, so the safest option is to have the hybrid system on and slowly charge the recipient car battery, and then disconnect the cables and only afterwards start the recipient car
  • I didn't buy the cables, I got them from my grandpa (I think), so I don't know any of their specs, but they do look approximately like the pictures of gauge 0 cables that I found. – Jay May 12 '17 at 12:26
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What probably happened was that your clamps weren't making good electrical contact with the battery posts. Basically, the last connection you make is to an unpainted metal part on the recipient car (a bolt on the engine is my favourite): if you don't see little sparks while making that connection, there's a good chance at least one of your clamps isn't on properly.

Just remember, there's often corrosion on battery posts. Just because your clamps are on tight doesn't mean they are making a good ELECTRICAL contact.

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