Risk of jump starting car?

If done correctly (no shorting of terminals, no reversed polarity, etc.) I don't really see how it would damage anything..

Damage can be caused by the power surges caused by jumper cables sparking or the alternator surging to fill the gap left by the newly disconnected battery/vehicle. Turn the headlights on or wait 5 min or so before disconnecting the charging battery/vehicle. The headlights places a resistive load on the surge and will minimise the spike. Most modern cars are protected form these surges now but how would you know.

Both. Using jumper cables irrespectively can produce voltage irregularities and micro-sparks (ones you can't see) causing voltage spikes up to 80-100VDC with AC harmonics upwards. This is only a fraction of a millisecond, but it can wipe-out computers in the ignition system, car audio, alarm system, GPS, UHF radios and many more. Have the ignition turn off. Thieves are known to disable alarm systems in this particular fashion.

Sum it up: Turn ignition off on both vehicles before connecting the leads, this should protect the computer from a surge. Avoid sparks, you shouldn't have any problems. AutoZone, Kragen sells fool proof jumper cables for ten dollars after rebate.

Lots of urban legend surrounding this jumper cable danger thing. From a scientific viewpoint, the only way jumpers could produce high voltage spikes would be if if an inductive load were present in the system. With the ignition off and the alternator isolated by its diodes, I don't see any such danger. OTOH, disconnecting jumpers while still cranking would be foolish.

Regards, Al.

Remove the battery from the dead vehicle and use a battery charger.

Which is exactly what happens when you use seldom-used, tarnished alligator clips on seldom-cleaned, gummy and corroded battery terminals. A good connection for the jump is essential.

Rob

Al Haunts has properly defined why jump starting - even when a sparking cable is removed - does not cause damage. However the destructive transient for auto systems is called load dump

- as defined by both SAE and ISO specifications. If battery cable is loose, and if disconnecting a jumper shakes that loose connection, then a load dump transient is possible.

Auto electr> Lots of urban legend surrounding this jumper cable danger thing. From

The only other risk I can imagine is the risk of a freshly discharged battery exploding. Discharging the battery creates hydrogen gas and a nice spark could ignite it. Of course the dissipation of that gas is fast and not a high possibility but the chance is still present.

-Bruce

That would be charging of the battery produces hydrogen gas.

That would be charging of the battery produces hydrogen gas.

You're right.. I stand corrected..

The largest risk is that the discharged battery may explode. My Bravada recently had a battery explosion, not even jump starting it. I hate to think of the consequences if I had the hood open, and someone was standing nearby. The truck had been starting slow, so I decided to take it to the shop for a battery test. Turned the key, sloooow crank, the BOOM, I thought a bomb had gone off. The entire top of the battery was on the other side of the engine bay, acid everywhere. The battery had a cracked plate, just below the normal acid level. The crack caused the slow cranking, and caused the battery to overheat while charging, thus boiling off acid. When the acid got below the crack, it got exposed hydrogen gas, and the spark caused the next time it statred, blew it up. When I think of all the times I have leaned over a battery while starting a car, it makes my knees weak.

If you had periodically inspected the battery visually do you think that would have prevented the accident or was the crack not particularly visible? It makes my knees weak too!

Hardly!

The reason for making the connection to the engine block is to move any spark generated when connecting away from the battery.

After reading this I had my battery case lid put back on. The BMW has a plastic box that encloses the battery. Thinking it's not a problem, I threw the lid out, exposing the top portion of the battery. I'm thinking of tying the lid down with seat belts, for goodness sake. Glad you told your story!

Here's a radiator cap explosion story: I stroll past a guy fumbling with his radiator cap on my way to an aftermarket auto parts center (AutoZone). Right when I'm at the front of his car... kaboom, an explosion from the cap. My shirt, arms splash with green coolant, my eyes get sprinkled. I thought to myself, I'm the unluckiest idoit in the world (including the guy.) Feeeewww, what a day...make note to self....never go to these aftermarket parts center with god knows what kind of people work on their cars.

The crack was in one of the plates, or in one of the straps connecting the plates. Ther is no inspection, visual or otherwise, short of draining the battery and scoping the inside, that would have caught this problem.