Jump starting a car

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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

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

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.

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.

Thanks Tom and Shorod for your good explanations.

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

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

Thanks Shorod, I'll let her know that.

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.

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:lol: 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?

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

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

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"

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

-Rod

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

I would assume that the 12v car would be badly damaged by the excess voltage. Some hybrids carry a 12v battery for accessories that can be used for jump starting.

I expect the high voltage is all contained to the driveline system, and not used for the accessories such as the radio, lights, 12V accessory port for you GPS, power windows, power locks, etc. I remember reading about training sessions being offered for rescue personnel so they would know where the high voltage wiring resides in the cars. It can be quite a shock (pun intended) to cut in to one of those wires with the Jaws of Life.

-Rod

Never, ever, ever, ever connect two batteries that are not the same design voltage.

For example, a three cell lead-acid battery puts out ~6 volts. Do not try to jump a vehicle using a 6 volt, three cell battery with a "standard" six cell, 12 volt battery. You very likely will have a battery explosion on your hands. To make matters worse, it might be the good battery that explodes.

A high voltage battery like those used in a hybrid is a very dangerous potential energy source. If you were to connect one to a standard 12 volt battery you will probably see peak currents high enough to vaporize the metal in the jumper cables. Many an electrician has been killed or severely maimed by when working on a live junction box and making a mistake that causes the explosive vaporization of metal. Not only are they severly burned by the hot metal vapor, but this becomes a conducting path for electricity that further burns their body. Dispite the fact that I am a professional electrical engineer, I would not mess around with a high voltage, high energy hybrid battery without understanding all of the safety issues. This is one thing that is best left to trained people.

To help understand the amount of energy contained in these devices...

A standard car battery has at least 200 cold cranking amps. A cold cranking amp for a 12 volt battery is the current that the battery can produce for thirty seconds with at least 7.2 volts of output. Then battery power is then at least 7.2 * 200 = 1440 watts = 1440 Joules/sec. Now since the battery only does this for 30 seconds, the total energy available is 43,200 Joules. A gallon of gasoline releases about 100,000 BTUs when it combusts. This is around one million Joules of energy. Therefor a typical 12 volt battery holds the equivalent energy of 5.5 oz of gasoline. Burn that amount of fuel in less than a second, and you can begin to understand the explosive potential of even standard batteries.

Now, consider twenty, 12 volt batteries, each with 200 CCA, stacked in series to deliver 240 volts. This battery has 864,000 Joules of stored energy. This energy is equivalent to about 7/8 gallons of gasoline, or a little less than 1/2 a stick of dynamite.

Shorting batteries creates opportunities to release their stored energy very quickly. Jump starting is a process that carries a strong risk of shorting the battery. The potential for injury is extreem when dealing with hybrid electric batteries.

I read recently of a person jumping a dead car battery with the auxiliary battery (12 volt) and there was sparking on the dead car.

Can using the auxiliary battery of a hybrid be just as dangerous?