How to: Jump Start A Car

[304runner]

OK, first off the battery jump-starting off a running car MYTH* is probably the biggest crock I've heard.

Love the haters who are gonna bring the hate:boxing:, But i assure you this should even make sense to the Queen.

Its the users laps in concentration that causes a jump starting issue, not because your car has a computer.


I will explain.


Electrical sh!t ONLY pulls as much amperage as resistance allows..

The resistance is exactly that, it is resisting the flow of electrons ( voltage)


VOLTAGE on the other hand, can be "forced" thru something and kill it..
(think lightning or plugging something that's 120 volts into a 240 volt outlet.. lol)

Amperage is a function of resistance and voltage tho.
it takes 1 amp to push 1 volt thru 1 ohm of resistance, by "law"

Well, for a concise example here, if running the car with an alternator that outputs 120 amps nets the regulated 14.4VDC is running.. cut it off, the voltage may drop, but you still have a battery capable of generating 600+ CCA. ( cold-cranking amps)


So, how is there more power available with the car running?
if only by voltage there's more "power" then.. yes.. but it will still be regulated.

With the car running, you'll only see 2 more volts, but the amperage is STILL there whether the car is running or not.

The battery, whether the car is running or not, is still capable of immense current, and INFINITE current for a fraction of a second when shorted. talking upwards of 10,000 amps+.


ALL electrical (automotive/bike) systems are regulated at a peak working voltage of 14.40 volts DC ( direct current)

the target charging voltage set is 13.86VDC. the normal working voltage for an electrical system is 12.6 VDC.

Now, you or someone you know popped an alternator, so, how the #### am I right?

Well, I won't say it can NEVER happen.. it can and will.. BUT there's a reason behind
why charging systems fail after a jump-start off a car.

TYPICALLY, it comes from surge/spike current from dragging the jumper cable clips across the terminals of the battery creating sparks. Those sparks in turn, create a spike in the system that sensitive electrical **** frowns upon.


The other main reason is accidentally shorting the cables out or reversing the polarity of the cables.

This will pop something 99% of the time. :undead:

if you're lucky NOT to melt an entire harness then chances are you'll only need to replace a regulator/rectifier.

But alas there is a correct way to connect jumper cables. It’s not that one way is wrong and another is right, but one is safer.

1) Connect the positive (normally red) jumper cable to the (+) lead of the discharged battery.

2) Connect the other end of the positive jumper cable to the (+) lead of the charged battery.

3) Connect the negative (normally black) jumper cable to the (-) of the charged battery

4) Connect the other end of the negative jumper cable to the (-) of the discharged battery or metal on the discharged vehicle.

Sounds really trivial, but there is logic to this which can minimize the risk of incident from jump starting a vehicle. For one, the frame of either vehicle is going to be directly connected to the negative terminal of its battery.

By connecting the negative leads of the two batteries together you’ve extended the ground to both cars.
If you touch the positive jumper cable to anything metal on EITHER car with the grounds already connected you are completing the circuit and will get a HUGE spark.

If you connect the positive terminals together first then you don’t have any chance of a shock if those touch any bare metal during the jumping procedure.

I hope this helps.

 

[bcw61]

it takes 1 amp to push 1 volt thru 1 ohm of resistance, by "law"

The battery, whether the car is running or not, is still capable of immense current, and INFINITE current for a fraction of a second when shorted. talking upwards of 10,000 amps+.

ALL electrical (automotive/bike) systems are regulated at a peak working voltage of 14.40 volts DC ( direct current)

the target charging voltage set is 13.86VDC. the normal working voltage for an electrical system is 12.6 VDC.

4) Connect the other end of the negative jumper cable to the (-) of the discharged battery............

Just to clarify a few points................

Firstly, if you are referring to Ohms Law, you have your volts & amps mixed up. Basic Ohms Law states: 1 Volt potential applied across 1 Ohm resistance will cause 1 Ampere of current to flow.

Secondly, a lead acid battery is not capable of "INFINITE current for a fraction of a second when shorted". A little thing called the internal resistance of the battery and the resistance of the load applied will preclude that. On the other hand NiCad batteries (referring to wet cell type here) have a much lower internal resistance, and are capable of a much higher current draw, however they too are not capable of instantaneous "Infinite" current supply.

Thirdly, not all automotive electrical systems are nominally 12 Volt. 6 Volt systems are used (particularly in older VW systems), and truck/large vehicle systems are often 24 Volt. This definitely needs to be considered when carrying out jump starting.

Lastly (and most importantly), you should NEVER connect the last -ve terminal (assuming a negative earth system) directly to the battery during jump starting. It should ALWAYS be connected to a large earth point - usually on the engine block somewhere. The reason for this is that lead acid batteries can produce flammable gas emissions (hydrogen) when being charged and under heavy discharge. Disconnecting a live jump start cable terminal from the battery -ve terminal (causing possible sparking) could have disastrous results. By having the last jumper cable -ve terminal connected to an earth away from the battery, disconnecting this point first (always disconnect the leads in exact reverse of connecting), will cause any spark to be generated away from the battery.

FYI, most auto clubs (RACV, NRMA, RAA, etc) have excellent guides on how to carry out jump starting. I would strongly recommend a read first, if not sure of the procedure.

 

[VYMAD]

Eerr, point of thread?
It tells us in our owner's manual what the correct jumpstarting procedure is......

Shows that you seem very knowledgable on this subject, though.

 

[bcw61]

Eerr, point of thread?
It tells us in our owner's manual what the correct jumpstarting procedure is.......

Assuming your comments are directed at the OP, I suppose that one reason for this thread is that this forum may be read by folk that may not have access to their vehicle owners manual (lost, etc), or their owners manual may not have the jumpstarting procedure in it.

 

[304runner]

After coming across this thread " http://forums.justcommodores.com.au/vr-vs-holden-commodore-1993-1997/172717-jump-start.html " I thought it would be best to help out the people in doubt or those who are just flat out unsure. If everyone had an owner's manual then this forum wouldn't be littered with jump start threads.

 

[VYMAD]

Yeah, sorry mate for jumping down your throat. Been a bad week.

Actually it's quite a good thread and should help a lot of people out.There would be quite a lot out there with no idea of the correct procedure. Soz...

 

[304runner]

Yeah, sorry mate for jumping down your throat. Been a bad week.

Actually it's quite a good thread and should help a lot of people out.There would be quite a lot out there with no idea of the correct procedure. Soz...

Cheers All good

 

[vs-6]

i think the reason people leave the car running is as the alternator is then charging 2 batteries thus putting charge in the flat battery which you might find works better than an initial crank