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Theory of Jumping a car

When Jumping cars; does accelerating the engine increase power to the other battery and make the jump time faster?And does the size of the engine make a difference to the speed of the jump? thankyou Car Talk

Yes, but maybe a lot less than you might expect. In the old days when we had generators not alternators, and electrical systems that were not as powerful as most are today, it could save you a lot of time. With modern systems there is less need to.

Current flows when a difference of potential exists (and our goal is to get current to flow). The sight increase in rpms from the donor car increases this difference of potential and makes it easier for current to flow (easier and more) to the receptor cars battery.

There are limits to how much a second vehicles charging system can benifit a receptors cars totally discharged battery, just like there are limits to how much a dedicated AC charger can benifit a totally discharged battery, some batteries are so far gone a lighting bolt from Zeus wont bring them back to life,

If the dead car needs a lot of power to start, I would run the engine at a fast idle speed (1000-1200 RPM) to get some extra power from the alternator. Racing the engine much higher won’t help.

I actually jump most cars with my engine off. My battery alone will ususally do the job and doing this eliminates any potential damage to my alternator. If that fails, I’ll start my car and run at fast idle speed.

Engine size doesn’t matter.

In the “old days” before there were idle compensating electronic devices, one would have to step on the gas pedal to increase engine RPMs in order to counteract the extra load placed on the engine when jumping a discharged battery.

Alternator voltage output is related to its RPM. A voltage regulator insures that the output voltage does not go above a safe level for the application. The alternator must turn at a minimum speed to begin producing voltage. The alternator is designed to begin outputting at slightly lower than engine idle speed. This guarantees that the service battery will be replenished while the car is idling in traffic for example.

The load presented to the engine by the alternator is directly proportional to the electrical load on the alternator. In other words, the more current drawn from the alternator, the greater its resistance to rotation and the more load it presents to the engine.

In a modern alternator, the difference in voltage output from idle to 3000 rpm is fairly small due to the regulation controls.

Having grown up through the years of rudimentary electro-mechanical controls in automobiles, I believe the idea of racing the engine during boosting is left over from those days where the additional load placed on the alternator caused the engine to stumble or quit. You had to step on the gas pedal to keep the donor engine running and the alternator speed above the minimum rate for outputting energy. Today, you have a computer controlled idle that automatically reacts to loads and keeps the engine running properly without operator intervention.

You do a charging test off idle so why would you not do a jump start off idle?

Voltage doesn’t change much between idle and 3000 rpm or more when the alternator is in a no load or minimum load condition, but available current does. If the demand for current is not met by the alternator, then the alternators output voltage will drop.

If you are jumping a seriously dead battery and you can’t wait a couple of minutes to put a surface charge on the dead battery, then a higher rpm will help spin the starter faster, but there is a danger here.

If the dead battery has a shorted cell and you raise your rpm, your alternator is going to increase its current output. If the rpm is high enough for a long enough period of time, you will burn out the diodes in your alternator. I recommend that you keep your engine speed low enough so that your alternator output is limited to what its diodes can handle.

The single most important thing about jump starting another car is to get the power from the live car to the dead car’s starter rather than just to the battery.

This may not be as simple as you think. If you try to jump a totally dead battery all of the live car’s electrical energy will go directly into charging the dead battery, and precious little will actually make it to the starter. That’s because a dead battery is essentially a dead short, so all the current will go there. Not much will get to the starter until the dead battery has at least some small charge.

What all this means is that to jump start a dead car the best thing you can do is to simply let the live car charge the dead car’s battery for a few minutes, like maybe 4 or 5, then try to start the dead car’s engine.

Good point Keith! If the idle current from the alternator coupled with the good battery in the donor car are not enough, it’s risking both cars alternators to boost it even further.

All the posts so far are correct. What isn’t being factored in is the cables used to do the jump start. Cheapie cables simply can’t carry much current from the donor car to the dead car. With high quality cables you can successfully start most any car with a minimum of idling time or reving up the donor car.

A long time to do a jump is often due to the cheap cables being used.

Yes it looks like everyone hit this topic out of the park. The comment about batt jumper cables is spot on as well.

People who are NOT mechanics or not gearheads will buy the first set of jumper cables they see…usually in some sort of road hazard kit…and while the cables LOOK nice and new and robust…many x it is just the wire casing that makes you think you have a robust set of wires. When if you were to strip the casing off you would notice a wire not much larger than old speaker wire…I mean C’mon… I have tried to repair a cable set one time and was SHOCKED to find the small gauge wire in the jumper cable set…I mean it was SILLY!

I own some SERIOUS sets of jumper cables…I think I bought ONE pair way back when…and sort of “Aquired” all the rest…Funny how this happens… I find them in trunks of cars at the junkyard, on the sides of the road…almost everywhere it seems. The best set I have thus far was a set I found on the side of the PA TPK…Some Truck Rescue driver must have left them there in the dark or they fell off his truck. They are approx 18-20Ft long and are made of HEAVY gauge braided wire…THey Just oooze quality when you pick them up…From the Rubber casing quality to the clamps…they are SUPER nice. I wish I could have returned them to someone, but… God I must have 8-10 sets of cables now…I discard the cheapies now…

What matters a lot more is the gauge of the jumper cables…With flimsy jumpers, allowing the donor car to charge the dead car for a few minutes can help greatly…

ahhh good tip caddyman; thankyou