Jump starting a car

[CL8]

http://www.automotiveforums.com/vbul...starting+a+car

In this thread I see a lot of discussion on how to connect jumper cables and why to connect the last clamp to a metal part of the car away from the battery to prevent sparking and an explosion.

But does anyone know why the negative clamp should be connected LAST as opposed to the positive one? Is there a real difference as far as safety goes?

Also some say connect both clamps on the good car battery first, others say start with the bad car battery.

Still others say first connect the two positive connectors, then the negative connectors
(all seem to say negative on the bad battery last, to a metal part away from the battery)

Does it really matter except to be sure the last clamp is connected away from the battery on the dead car chassis?

Any knowledge on this is appreciated,
Cl8

[shorod]

Just about everything metal under the hood is connected to ground. If you connected the last negative cable prior to the positive cable, now everything metal under the hood has the potential to cause a large short, sparks, and lots of electrical system issues on the "good" car if the final positive cable were to contact something under the hood, or spring off the battery post. By starting with the positive cables, if you don't first get a good connection, the clamp will hopefully pop off before the grounds of the cars are connected.

-Rod

[tomj76]

There's a post in the referenced thread that clearly explains it.

The last connection completes the electrical circuit. The only connection (assuming the cars are not touching!!!) that can produce any significant electrical current is the last connection. There are four total. When the last connection is made, the circuit is completed, and current will flow from the good battery to the discharged one.

When the last connection is made, as the clamp is brought into contact there is a "bouncing effect" that is unavoidable. The clamp touches the metal intermittently, making and breaking the circuit many times. The bouncing is due to vibration of the car and the shaking of the operator's hands. As this happens, sparks are created. The more current flowing while making the connection, the more sparking will occur.

A discharged battery often creates hydrogen gas. This is because the sulfuric acid (H2SO4) in the battery is broken down into hydrogen gas (H2) and sulfure dioxide (SO2) and oxygen (O2). Some of the hydrogen leaks out of the battery into the engine compartment. The highest concentration will be near the battery. Sparks occuring on the battery posts are more likely to ignite hydrogen gas since that's where most of it will be found. If the hydrogen ignites and travels back into the battery, the battery could explode. The sulfuric acid in the battery will be spayed on only near the battery. Sulfuric acid is highly corrosive and will damage skin and eyes if it is not washed off immediately.

Making the connection at point that is far from the battery and down inside the engine compartement (i.e. the engine block) is less likely to be near any signficant concentration of hydrogen. This is why the negative is connected last, since it is the only connection that can be made in this manner. (The polarity is reversed in vehicles that use negative voltage from the battery)

Some people try to eleminate the risk of explosion by making the ground connection on the vehicle with the good battery last. This is not recommended because the engine is often left running while making the connections. This puts the operator at risk due to the moving parts and hot surfaces found in the engine.

Finally, car makers often warn against jump starting vehicles due to the possibility for damage to the electronic engine controls on either of the vehicles involved in the jump start.

[tomj76]

As far as the order of the other connections, I've always used the Positve bad, positive good, negative good, negative bad sequence. The reason is that it helps to make sure that under any unexpected circuimstances that any connection making sparks will be away from the discharged battery.

For example, suppose that the cars were touching and you connected both postive and negative to the good battery, or only the postive to the good battery first. Since the cars are touching, the negatives are already connected. When the positive connection is made to the bad battery there would be a spark.

[CL8]

Thanks Tom and Shorod for your good explanations.

Cl8

[CL8]

Today I helped a student locate the battery in her Jaguar, not sure what model, it's a 2002, I believe it may have been manufactured in Canada.
The battery is in the trunk. We looked to see if there were auxiliary posts under the hood to connect jumper cables to. We couldn't find any.
Would this mean you would connect the positive cable to the battery in the trunk and the negative to a metal part in the trunk if it needed a boost, or were we just looking in the wrong place for auxiliary posts?

thanks, Cl8

[shorod]

Sounds like the Jaguar that shares the platform with the Lincoln LS. There are no aux posts under the hood. In this application, you would use a clean metal part in the area of the trunk. Lots of painted metal in the area, so if it really came down to it, you might go straight to the long, bare portion of the battery cable, as far from the battery post as possible. In this type of application, a switched portable jumpstart pack would be an ideal solution. You can connect both cables, then flip a switch to close the high-current relay within the jump start pack, avoiding the risk of a spark at the battery.

-Rod

[CL8]

Thanks Shorod, I'll let her know that.

[MagicRat]

Originally Posted by CL8

Today I helped a student locate the battery in her Jaguar, not sure what model, it's a 2002, I believe it may have been manufactured in Canada.

Good for you, meeting a Jag-owning damsel who needs your assistance!!

BTW all Jaguars, even the Ford-based ones are made in Britain.

[CL8]

Originally Posted by MagicRat

Good for you, meeting a Jag-owning damsel who needs your assistance!!

BTW all Jaguars, even the Ford-based ones are made in Britain.

This is a lady is originally from Britain. She got a speeding ticket, so came to our driving school for a defensive driving course to keep her record clean.

We went out to her car to teach blind spots and check what is under the hood. When we saw no battery I suggested it might be behind the back seat,
She got her phone out and called her mechanic just to be sure, and he said it was in the trunk!

(we looked through the owners manual and found no info about the location of the battery)

Are all Jaguars built that way?

[shorod]

Do you recall if it was an S-type Jag? If so, that would be the one that shares the platform with the Lincoln LS, which also has the battery in the trunk. Legend has it those cars have the battery in the trunk to help with the near neutral weight balance. They put the battery in the trunk and made the front hood and fenders aluminum. I'm not sure if other Jag's have the battery in the trunk as well.

-Rod

[CL8]

I plan on calling my student to see if her mechanic answered her questions about jump starts. I will ask her the model then.

[CL8]

Originally Posted by shorod

Do you recall if it was an S-type Jag? If so, that would be the one that shares the platform with the Lincoln LS, which also has the battery in the trunk. Legend has it those cars have the battery in the trunk to help with the near neutral weight balance. They put the battery in the trunk and made the front hood and fenders aluminum. I'm not sure if other Jag's have the battery in the trunk as well.

-Rod

It is a Jaguar vanden Plus , not sure if that means "s-type"

[shorod]

Nope, the Vanden Plas was not based on the same car as the LS.

-Rod

[CL8]

The battery pack of the second generation Toyota Prius consists of 28 Panasonic prismatic nickel metal hydride modules—each containing six 1.2 volt cells—connected in series to produce a nominal voltage of 201.6 volts. The total number of cells is 168, compared with 228 cells packaged in 38 modules in the first generation Prius. The pack is positioned behind the back seat.

According to this info, I guess it would be unwise to jump start a conventional car battery with a hybrid car battery, this says over 200 volts,
other hybrid batteries I read have over 300 volts

What kind of damage would attempting to jump your car battery with a hybrid do to either vehicles electrical system?

Thanks,
cl8