----- Original Message -----=20 From: "Mark Hanchey" To: Sent: Wednesday, May 16, 2012 6:45 PM Subject: Re: [OT] jumpstarting takes a few minutes of "charging"? > On 5/15/2012 2:54 PM, Eric Kort wrote: >> So, this morning I jump started our van (kids wanted to finish >> listening to a story on the CD player when we got home last night, and >> a person who shall rename nameless forgot to go back out later and >> take the keys out of the ignition which was then left in "Acc" >> position all night). This brought to mind a question I have long had >> about jump starting. The battery in my car starts my car immediately. >> But when I connect that same battery to another car (the one I am >> jump starting), that car will not start immediately...I need to wait >> several minutes with the cables connected until it will start. >> >> My question: why is that? I will share some hypotheses I have, mostly >> for their entertainment value: >> >> 1. The resistance of the cables is too high to deliver the needed >> current to start the dead vehicle. (It seems if that were true, >> someone would be out there selling uber-cables that don't have this >> problem, but I don't recall seeing such cables.) > It isn't so much the resistance of the cables as it is the poor > connections at each end. Measure the surface area contacting both ends > and compare it to the size of the cable, if it isn't at least the same > amount of surface area then it doesn't matter how large the cable in > between , it is like connecting a 100 ohm resistor to a 2 inch thick > copper cable, the limit will still be the resistor and increasing cable > size isn't going to change that. This is not true. The resistance of the cable is in series with that of the= =20 connections and thus it is the total of the two that controls the current=20 flow. -- Bob Ammerman RAm Systems --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .