In my experience, jumper cables and their connections often have too high resistance to actually start a car. (Professional models excepted - I'm talking about the cheap ones I always end up using.) Provided the dead battery has some capacity to hold charge, then connecting the cables and revving the engine of the good car will pump some charge back into the dead battery. This is normally enough to get things turning over. A few months ago I tried to start a car with a completely dead (sulphated) battery that wouldn't accept charge and my cables/connectors just weren't good enough. Connection to the engine block rather than the battery is often easier, reduces resistance very slightly and move the sparks further away from the battery - although why a flat battery would be gassing is questionable. I guess it's more important at the "good" car end of things RP On 16 May 2012 07:21, Chris Pearson w= rote: > "And while we are on it, why does the owner's manual suggest connecting t= he > negative end of the cable to the engine block of the dead vehicle...?" > > I've always been told that is done to avoid a spark occurring near the > battery and it's potentially explosive charge/discharge gasses. > > Excerpt from the wiki: "The amount of overcharging is usually very small > and generates little hydrogen, which dissipates quickly. However, when > 'jumping' a car battery, the high current can cause the rapid release of > large volumes of hydrogen, which can be ignited explosively by a nearby > spark, for example, when disconnecting a jumper cable." > > - ChrisP > > > On Tue, May 15, 2012 at 11:54 AM, Eric Kort wrot= e: > >> 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). =A0This brought to mind a question I have long had >> about jump starting. =A0The battery in my car starts my car immediately. >> =A0But 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? =A0I 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. =A0(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.) >> >> 2. The dead battery is a low impedance current sink while it is >> charging, sucking away the available current from the higher impedance >> starter until it is charged enough to start the car on its own. >> >> And while we are on it, why does the owner's manual suggest connecting >> the negative end of the cable to the engine block of the dead vehicle >> (doesn't the engine have continuity with the negative terminal on the >> battery--making it simpler from a procedure point of view to just >> connect the battery terminals on both vehicles)? >> >> Thanks, >> Eric >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .