GEORGE7041, and severach,
GEORGE you wrote:
I occasionally jump trucks at work with my own vehicle, the Sierra, and have no hesitation starting the truck up with the Sierra engine running. I don't think there is an issue here for the following reasons.
1) The Sierra system voltage might drop,at the most, to 10v when the truck engine is being cranked. There is only so much current that the jumper cables will allow and the Sierra battery is a huge electrical "reservoir".
2)10v vs 14v is a factor of only 1.4. The Sierra is only idling
and the idle current it generates at 10v will still be less than its maximum rated current at around 13v(145 amps, BTW), running at speed.
3) This higher current while the truck engine is being cranked only lasts for 10 to 15 secs, it is not as if you are subjecting the alternator to high currents for hours at a time.
Regarding your:
2)10v vs 14v is a factor of only 1.4. The Sierra is only idling
and the idle current it generates at 10v will still be less than its maximum rated current at around 13v(145 amps, BTW), running at speed.
The voltage regulator that drive the alternator only senses the system voltage and the ambient temperature. It uses the ambient temperature to determine the voltage the battery should be charged up to. If the output is loaded so it falls below the desired voltage the regulator will drive to the best of its ability. This drive current is limited by the internal resistance of the armature of you alternator. If the RPM of the alternator is high enough the alternator will put out enough current to bring the system voltage back up to the desired voltage. When the system voltage reaches the desired voltage the regulator regulates the drive to the alternator armature to keep the system voltage at that voltage. The system voltage does not have to fall off by much for the regulator to drive the armature of the alternator as hard as it can to bring it back up. Because the regulator is solid state a drop off of only a few mill volts (a few 1/1000 of a volt) will cause the regulator to drive the alternator as hard as it has to to bring the voltage back up. Now if the RPM's are not high enough, then the alternator can not produce enough power and will not bring the system back up.
However, a starter motor can draw the maximum power from a battery when the resistance of the starter = the internal resistance of the battery. At that time the battery output Voltage drops to 1/2 of the non loaded voltage. Normally when you crank you engine on a very cold day the engine is not cranking fast enough to allow the alternator to produce enough current to damage the out-put diodes.
Now if the good vehicles engine is running the alternator is turning much faster than when the engine is slow cranking during a hard start. If the bad vehicle is a hard start the battery of the good vehicle can be pulled low enough to allow the alternator of the good vehicle to put out too much current. The output windings of a alternator are three phase Delta winding connected to a 6 diode full wave three phase bridge rectifier.
Regarding your:
3) This higher current while the truck engine is being cranked only lasts for 10 to 15 secs, it is not as if you are subjecting the alternator to high currents for hours at a time.
These diodes are solid state and it only takes a fraction of a second of putting too much current through them to permanently damage them. They usually go leaky and allow a reverse flow through the winding that will act as a constant drain on the battery. I have seen this several times and it is always after someone has jumped started a bad vehicle with the good vehicle engine running. Ask any good alternator repair shop.
To severach, You wrote:
Alternators are current limited so can run any size load without damage. I always start cars with the other engine running and the alternators are never damaged.
There is NO current limit, and there is NO current sensing on the alternator. Some high end units like those in a Corvette have a temperature limit, but NO current limit.
I have a degree in electronics, and have looked at the schematics of how these linear regulators are designed. I understand them down to each component, and what each component does and how it interacts with the entire regulator. I have designed similar regulators for cars, and motorcycles.
If you run the good vehicle while you are cranking the bad vehicles engine you are rolling the dice about damaging the alternator of the good vehicle. The only thing that limits the output current is how poor the connections are, and how high you idle is, and how low the cranking vehicle pulls the battery on the good vehicle. If the bad vehicle pulls enough current the good vehicle battery will drop low enough to allow the alternator to put out enough current to damage the output diodes if the RPM's are high enough.