How to jump start a car

When you make the last connection, there will be some sparking.

If you make this connection near the battery, there's a danger it would ignite any Hydrogen that's been generated in the battery.

If you've ever seen the result of a battery explosion, you'd know why this is very bad.

i can answer that for you or at least clear it up for you.....
The battery is to provide the starter motor with power to start the car ....
The starter motor is bolted on to the motor ...
The link between the battery and starter is via 2 cables, neg and pos and part of the neg link is the body or chassis, the battery will have a cable (neg) from the battery to the body or some even to the motor as well, all will have a cable from the body to the motor ....
So the reason behind clamping the neg to the body or chassis is to take out the need for the current to pass through at least 1 cable as every cable it has to pass through it loses power ...
So in reading the above the ultimate way would be to connect both the positive and neg jump cables direct to the starter bypassing all cable losses but for saftey reasons if you just connect the neg to a bare metal part on the block and the pos to the pos on the battery at least you will get the neg losses down and help it to start ...
But ....... if you want to just charge the battery then connect it direct to the terminals of the battery as it will run into the battery without going through cables (reverse)
Cheers
Joe

If the 200 to 400 amps needed to start a car/4WD had to come through the tiny contact area provided by Jumperlead clamps, they would glow red and weld in place.

Arc welding can be done with 50 amps.

The jumper leads feed current that puts some charge into the flat battery, then it can run the Starter Motor using short heavy wiring.

ive heard of it but always have connected + to + and - to - No pupies died no one starved in rawanda and freo never one a grand final because of doing it old school

Yep you have Rod and it relates to the possibility of a spark/explosion near the battery as others have said. Interesting to note the RACQ comment re spiking - DON"T disconnect the leads while either car is running (woops, that's what I've done in the past, without problem thankfully).

'Operating a vehicle with certain battery faults or disconnecting jumper leads while either engine is running can cause voltage spikes that can damage sensitive electronic components and the vehicle’s electrical system'.

RACQ advice

Have a read of your vehicle's manual - it will have a photo of where to put the negative lead - in the one's I've read, its been directly to the motor, which makes sense to me.

I get some change back into the dead battery for 5 minutes before attempting to crank, so there could easily be hydrogen lying around.

Thats the recommended way, and as Joe and some others have said it is to reduce resistance making the starter turn a little easier and reducing sparks.

However I never thought that it was right advice and we connect direct to
terminals (usually clamps on terminals) from running car, after checking
batteries with voltmeter.

Vital to listen to donor car as you complete connection, if its speed drops
to much you should disconnect a lead right away before possible short etc
can cause damage or generate H gas.

I think there are more potential problems, particularly with people who never lift a bonnet, of connecting to wrong part of engine or even to an aircon pipe etc.

The risk of hydrogen explosion in the above jump start senario is very low.
Usually after someone starts a car they reach in and remove leads, and you should remove black first and you don't want people reaching further into the running engine bay than you have to.

Also by connecting to battery as direct as possible you are more likely
to discover issues with high resistance connections ( about 1/2 of reasons for flat batteries) and also you limit spikes into wiring.

I have done many, many jump starts on flat batteries in a past working life.

I would always connect vehicles + to +, - to vehicle engine block ALWAYS with donor battery/ vehicle engine OFF and only after ensuring the flat vehicle had ALL electrics turned off, even shut door to turn off light.

After a few minutes connected I would start the donor car THEN the flat car. Whilst the flat car was running I would do my paperwork, talk to the owner to try to ascertain why the battery was flat to allow at least 5 minutes running then turn off BOTH vehicles then disconnect cables.

The flat vehicle will now re-start on the residual charge put into it, if it doesn't then that battery is clagged and we fit a new one.

If you don't turn off both vehicles before disconnection the flat vehicles alternator which was happily idling away having a fully charged donor battery in circuit is suddenly left with a flat battery and kicks itself into full charge mode possibly frying electrics.

This is what I understand happens and I never 'lost' a car by following this procedure.

Cheers Neil