Dave, the speed of a motor is determined by the voltage applied to it. That’s why your 12v system cranks the engine over faster. Your reference to the garden hose relationship is used as a simple explanation of the relationships of pressure to the current flow thru a restriction. In an actual electrical circuit heat dissipation plays a very important part. Normally when you heat a conductor the resistance goes up unless it can dissipate the heat at the same rate it is generated, This is where the 100% duty cycle becomes important. If you like the water analogy, think of a bucket with a hole in it, the hole where it comes in has to be the same size as the one that goes out or it overflows. In a series circuit the voltage is divided proportionally between the individual resistances. The major components here are the starter, the cable and the battery. If any one of them is over heated the resistance increases in sort of a snowball effect, or chain reaction, i.e., more heat/more resistance/more resistance/more heat and on. What you have to keep in mind is initially your going to hit the starter with the full current and voltage available, and if the cable is undersized it will begin to heat up and require more of the voltage to push the current thru the now increasing resistance. If your really interested in more on your gonna need to start digging into it your self. Ya might start at sites like this;
http://www.stealth316.com/2-wire-resistance.htm I’ve put way to much time in it already and am about burned out myself. I’m neither an electrical engineer nor a terribly good electronics technician; I am a fairly good mechanic though and I was just tryin’ to apply some common sense to the subject.
Jim, like Jerry said earlier, I’m not aiming to get into a whizzin’ contest with anybody here, and I’m not trying to offend you, but I would have to disagree with you on the jumpers. In my opinion, unless the smaller jumpers were drastically undersized they would initially deliver the same current. Most engines start after a few revolutions or within a few seconds. If the jumpers would not start the vehicle it was most likely because an excessively long period of cranking and they were getting hot, i.e., the resistance was increasing and the voltage available at the starter was now being dropped in the cable.
And once again I have to refer back to what the Delco-Service bulletins caution, “Never operate the cranking motor more than 30 seconds at a time without pausing for a few minutes to allow the cranking motor to cool.” The same thing applies to the cables, unless you have a cable large enough to handle 400plus amps at a 100% duty cycle.
Fin;
Denny Graham
Sandwich, IL
