There are about three scenarios where someone needs a jump:
1st. Battery is on its way out. Need 400 amps at ~10V to start and can only supply 300. Donor car, running, with a decent battery and the alternator, can supply the other 100. Waiting before cranking might put a surface charge on but the car needs those cables. I rev my connected donor car right before the victim tries to start as everybody needs as much voltage as possible, since it's going to sag when the amps get guzzled. Boosts my alt output from 40 to 100 amps, and since the guts are spinning faster more diodes get a piece of the inrush current. I also feel that my lousy 8- or 10-gauge cables protect my car, as the various little bits of resistance in the alligator clips wouldn't look as much like a dead short to the system in a worst case scenario with the victim car dragging mine down.
2nd. Left the lights on. Idle the good car ~5 minutes to charge the dead one well enough to crank. Cables are just an insurance policy. Donor car doesn't have to be running during cranking, but I'd have it doing so. Victim car should get most of its power from its own battery, which has a more solid connection than the jumper cable teeth.
3rd. Mechanical issues. Dragging starter, high resistance in corroded cables, bad grounds etc. We as good samaritans don't know how bad off someone's car is. Someone living paycheck to paycheck might get dozens of jump starts from various strangers before they get into a shop.
1st. Battery is on its way out. Need 400 amps at ~10V to start and can only supply 300. Donor car, running, with a decent battery and the alternator, can supply the other 100. Waiting before cranking might put a surface charge on but the car needs those cables. I rev my connected donor car right before the victim tries to start as everybody needs as much voltage as possible, since it's going to sag when the amps get guzzled. Boosts my alt output from 40 to 100 amps, and since the guts are spinning faster more diodes get a piece of the inrush current. I also feel that my lousy 8- or 10-gauge cables protect my car, as the various little bits of resistance in the alligator clips wouldn't look as much like a dead short to the system in a worst case scenario with the victim car dragging mine down.
2nd. Left the lights on. Idle the good car ~5 minutes to charge the dead one well enough to crank. Cables are just an insurance policy. Donor car doesn't have to be running during cranking, but I'd have it doing so. Victim car should get most of its power from its own battery, which has a more solid connection than the jumper cable teeth.
3rd. Mechanical issues. Dragging starter, high resistance in corroded cables, bad grounds etc. We as good samaritans don't know how bad off someone's car is. Someone living paycheck to paycheck might get dozens of jump starts from various strangers before they get into a shop.
