A spark can be caused by a difference in electrical potential between the two objects or faulty contact (i.e fiddling with the positive clamps.).

The difference in potential is the same if you connect the positive terminal of the weaker battery first or last.

If the weak battery was at 5V and the good battery is at 12V, that's 7V of difference. Depending on humidity and charged particles in the air or if there is a heavy concentration of gas, 7V may or may not cause a spark.

 

The point is to make the final connection away from the battery. If there is a spark then it's far away and prevents explosion.

 

You shouldn't expect a spark from the negative clamps, unless there's a fault in the car's electrical system or you connected it to a power source. The positive clamps are the most likely source of sparking.

It's been awhile since I've memorized my Hydrogen limits, but I believe a Hydrogen concentration of 3% is when it can become flammable. 3% is a lot, especially when talking about 12-14VDC battery systems.

 

Not true.

The spark happens wherever you complete the circuit. When you connect the positive leads, provided the vehicles are not touching (and they shouldn't be) the grounds of the two vehicles are not connected together and no circuit is made. Therefore there is no spark.

With a modern negative ground vehicle, you cannot make the positive connection other than at the battery. Therefore you do this first. So far there is no circuit, so no spark can occur.

Then you connect the negative cable to the negative terminal of the battery on one car. Still the circuit is not complete so no spark can occur.

Only when you make the final connection is the circuit complete. The risk is that as you move the clamp to the terminal, the voltage will be sufficient to cause a spark to jump the gap. If this is done near the battery, and hydrogen gas is present in the proper amount (and it can be) the there will be an explosion.

This will, at a minimum give you a fright. It could cause the battery to spray acid in all directions with obvious consequences for anyone nearby. So you need to make this connection in another part of the circuit, which could be at the starter motor terminal or any exposed metal component. Then the spark, if it occurs, happens away from the battery and the chance of an explosion is reduced.

You might do it incorrectly 1000 times with no problem, but the one time you do have a problem, the results could be devastating.

So follow the proper procedure.

 

I see what you mean Virtual and Rosswond. I'll edit my post #13 to reflect the issues with sparking.

 

if you are the car with the good battery, never EVER, leave your car running, turn your car off before the other vehicle tries to start the engine. there could be a power surge and it MAY fry your computer. you don't want that.

 

Exactly right but I don't need to tell you that. You know it already.

 

Let me see...connect 2 fully charged fit batteries together in parallel...it can generate up to 160 amps... Connect 1 full battery and one half fit battery, it'll generate 120 amps.

The fit as a 80 amp fuse...if one battery is full, it doesn't take much to blow up the fuse. If you connect negative to negative instead of one of the negative to ground, then you have a complete closed parallel circuit and the amp rating is the sum of the two batteries. If you disconnect one negative terminal and run it to the ground, it changes the whole amp rating and runs redirects some to the ground.


I'm hoping someone doesn't try hooking up in series generating 24V...cuz that would suck.

 

here are the reasons i will never jump start ANYthing with any of my hondas

1: the alternator for the fit is $358.95 and i work at the dealer, so i'd not pay that much but i see people's alternators ALL THE TIME and i ask how long it's given them trouble and the answer is usually the same "since i jumped a friend" no matter how careful you are there is always the chance you'd going to screw up the way the alternator charges,
2: i bought me a jump box about 7 years ago for $69 dollars (now their $39) and it's always jumped anything i've needed it to

seriously get yourself one of these they are worth every penny they are small enough to fit in your spare tire and it'll always ready, i just check mine everyother month or so and make sure it's charged up

 

I actually saw someone connect a battery charger the wrong way around once, on an S class Merc. I stopped him just in time.

So yes, there are people out there that would do this.

 

i'm confused to the people who are so paranoid about jumping a car. my fiancee's old car used to have battery and alternator problems and I used to jump her car all the time with my cars (Civic, RSX and S2k). Never had any battery or alternator problems on any of those cars. On the Civic and the RSX, I also had a crap ton of audio equipment in there.