Jump starting...why (-)

TempestSabre

When jump starting why do you connect the (-) on the dead car to bear metal on the car and not the battery?

seamus

Apparently there's a risk of explosion if you do (I just googled it) :eek:

Actually, on further reading, it provides a ground for the current, should something go wrong.

TempestSabre

seamus wrote:

Apparently there's a risk of explosion if you do (I just googled it) :eek:

Actually, on further reading, it provides a ground for the current, should something go wrong.

I knew about the explosion bit. But the cars on 4 rubber tyres. How is that ground? Is it just somewhere else for the current to run to if theres a surge/spike or something.

gearbox

seamus wrote:

Apparently there's a risk of explosion if you do (I just googled it) :eek:
QUOTE]

jesus, ive never put it on the metal of the car always on the battery!

TempestSabre

gearbox wrote:

seamus wrote:

Apparently there's a risk of explosion if you do (I just googled it) :eek:
QUOTE]

jesus, ive never put it on the metal of the car always on the battery!

Everyone I know does it that. I stopped because someone said it can damage a cars electrics. But there has to be some sort of reason for it to go BANG!

Cake Fiend

TempestSabre wrote:

But there has to be some sort of reason for it to go BANG!

Hydrogen gas (released by the electrolyte) + spark = BANG!
Not very likely to happen, but possible.

greglo23

car batteries give off hydrogen gas which is highly flammable. when you connect the jump leads to the battery it can cause a spark which can ignite the gas so its recommended to connect the earth connection far from the battery. basically its not necessary to do this if you take normal safety precautions. be careful of modern cars though as it is possible to blow the computer if the leads are connected wrong or the voltage from the car being used as the power source is higher than it should be. some cars require the battery to be removed,charged and refitted to avoid this.

TempestSabre

Umm on further reading, the last jumper connection (-) can spark, simply because of its proximity to the (+) jumper. The idea of putting it on the car seems to be to maximise the distance between the jumpers, when you make the final connection. Thus less likely of a spark, which can ignite the hydrogen gas and then the battery explodes. Since you'd be standing beside it that would not be nice. Putting the final jumper on the car somewhere also puts you further from the battery.

So they should really spell that out. Also not to hold both jumpers (+) and (-) minus in either hand, as you might inadvertantly put them too close, and they definately spark. Do them seperately and keep them well apart.

I know this is all kinda obvious, but I'd never really thought about the specifics before.

Con_316

You know, putting the lead straight onto "bare metal" on the car like the bonnet, as i saw someone do the other day is not such a good idea, that could get you a nasty shock if something went wrong with the currrent, putting the "-" (negative) onto an engine lifting "eye" or in other words, the thing a mechanic will hook onto to lift out the engine is a better idea

AMurphy

Con_316 wrote:

You know, putting the lead straight onto "bare metal" on the car like the bonnet, as i saw someone do the other day is not such a good idea, that could get you a nasty shock if something went wrong with the currrent, putting the "-" (negative) onto an engine lifting "eye" or in other words, the thing a mechanic will hook onto to lift out the engine is a better idea

Hooking the -ve onto the bonnet, door, boot lid, etc, is a good waste of time and voltage, not to mention damaging the paint, scratches etc.

Nothing will "go wrong" with the current, just that all the joints and connections between the starter motor -ve and the sheet metal are not designed to handle high starting currents (300 amps), and offer a high resistance path, so of your average 12V available, 3~5 will be lost in the jhoints and connections, leaving 7 across the starter motor.... not a good situation.

To get maximum "bang" for your "jump", connect the +ver to the battery/cable terminal and the -ve to the engine metal, such as the lifting hook, some solid piece of connected metal, not the water pump shaft, accelerator cable bracket, clutch lever, etc.
apart from explosion of hte gasses, another good reason to not connect teh clamps to the battery is proximity, the handles can touch each other and raise sparks from there.

Jump stared a friends van last night and the main problem was the blasted theft alarm, which was non-functional until the battery wend flat and woke up on attaching the jumper cables..... course being non-functional, nobody had a fob for it, it took a while before the owner figured out the over-ride method, once that was accomplished, we were outta there in about 2 minutes.

TempestSabre

I mean by bare metal the engine block somewhere.

Whats the corrected procedure for changing a battery? In terms of what you should and shouldn't do.

AMurphy

Disconnect the -ve first (or whichever is the ground, jic you have a +ve Gnd), Then the +ve.

Attach the +ve first, then the -ve,

Now why?. If you disconnect the +ve first, and the spanner hits bar metal, the Gnd being connected = sparks.
If you disconnect the -ve first, if the spanner hits the frame, being of the same potential, no sparks.
Now with that disconnected, if you then hit spanner off the frame while disconnecting the the +ve, the -ve beign disconnected = No sparks, cause there is no complete circuit.

and I'll leave ye to figure out the reverse process.

Berkel

Jump starting the battery uses only battery terminals can fry your ECU if something goes wrong - if my memory serves me correctly this was a big problem on some Ford cars (Galaxy I think)...

ECU's aren't cheap to replace either....

stratos

Just to add to the above, The surge problem which affects newer cars.
This can be avoided by using a high current user, such as the heater blower on the good car whilst attaching cables.
When the dead car is started turn it's blower on whilst disconnecting cables.
The high loads keep each battery's voltage pulled down and avoids spikes.
There is no problem connecting the leads directly to the battery.
The only risk is high current short circuit if the connectors touch.
If this happens there is almost unlimited short circuit current Flashy Bangy, people have been badly burned from this flying molten copper heat etc be warned.
If your going to attach the negative lead to the car it must only be connected to the engine block lifting eye etc.
Also make sure the 2 cars are not touching in any way like an open door touching the body of the other car.

Jump starting cars although it's done a lot must be treated with respect.
You are dealing with a high current device with no short circuit protection.
I have seen molten copper from a short circuited battery burn a hole through a windscreen.