On-Line auto electrical info

**drivesafe** · 11th March 2017, 11:46 AM

THE CORRECT PROCEDURE FOR JUMP STARTING

First off, jumper leads do not need to made with huge cables. 6B&S ( 13.5mm2 ) is the most convenient cable size but cabling as thin as 8B&S ( 7.8mm2 ) will easily jump start a vehicle with a flat battery.

Also, you do NOT need suppression type jumper leads. These are nothing more than a salesman’s con job.

Whether the crippled vehicle’s battery is flat or faulty, it will still hold a surface charge, even if that is for just a few minutes. When jump starting a vehicle, you don’t need it to hold that surface charge for more than a few seconds.

The correct procedure for jump starting a vehicle is as follows.

Bring the donor vehicle as close as practical to the crippled vehicle.

Leave the motor running in the donor vehicle at all times.

If you turn the motor off in the donor vehicle and connect the jumper leads, while you will not flatten the donor vehicle's battery, you can cause a sufficient drop in the voltage of the donor vehicle’s battery that the motor can not be started without disconnecting one of the leads.

Make sure the PARK BRAKE is applied in both vehicles.

Turn the ignition off and remove the kew from the ignition of the crippled vehicle. This is not done to protect the vehicle’s electronics, this is done to remove any remaining current draw from the crippled vehicle’s battery.

Many vehicles can draw quite high currents with the ignition key still in the switch ( up to 40 amps ).

Note, in many new vehicle, not until you remove the ignition key will all current loads be turned off and in some vehicle, like new Land Rovers, the computers will remain active for up to 3 minutes, while they go through shut-down routines, after the key is removed.

With Keyless Entry type vehicles, make sure the vehicle’s ignition is off and take the key away from the vehicle.

Next, connect the positive lead to the crippled vehicle’s positive ( + ) battery terminal, then connect the other end of the positive lead to the donor vehicle’s positive ( + ) battery terminal.

The POSITIVE Lead is connected first as a safety precaution. If your to drop the Positive lead while trying to connect it to the donor vehicle, you will nor cause a short.

If you were to connect the NEGATIVE Lead first and dropped the POSITIVE lead as you were trying to connect to the donor vehicle’s battery, if this lead can in contact with any metal parts on the donor vehicle, there could be a considerable short.

Next, connect one end of negative lead to the donor vehicle battery’s negative ( - ) terminal.

You MUST NOT connect to the crippled vehicle battery's negative ( - ) terminal.

This again is for safety reasons. If you have been cranking the motor trying to start is with a flat battery, or if the battery dropped a cell while you were driving and you stopped the motor, say to fuel up and then went to try to start the motor again.

In these situations, the battery will most likely have been gassing. When you connect up the last lead, it will complete the circuit and can regularly cause a spark as you make the last contact.

If you connected to the crippled vehicle’s negative ( - ) terminal on the battery, there is a chance you could ignite the gas coming from the battery, causing the battery to explode.

PLEASE NOTE, if a battery does explode, it will NOT vaporise the vehicle, it will simply spray electrolyte around the engine bay, and a quick wash down with a hose will remove the electrolyte.

The danger posed if you connect to the cranking battery’s negative terminal is that if the contact causes a spark and ignites the gas, you are more than likely going to be over the top of the battery at the time and could easily end up with a face full of acid ( electrolyte ).

By making the last connection somewhere other than at the battery, while it far less likely to ignite gasses at the battery, if it does cause the battery to explode, you are in a much safer position.

Now find a suitable earthing point in the crippled vehicle’s engine bay ( if the cranking battery is located there ). This earthing point should be a bolt on the motor or something of that nature.

Do not use any body parts as the earthing point, because they are usually not thick enough to carry the startup current load.

With all connection made, leave the vehicles connected with the donor vehicle’s motor running at a high idle, for at least 2 minutes.

Once some time has elapsed, try starting the crippled vehicles motor.

If it does not start straightaway, turn the crippled vehicle’s ignition switch off and remove the key and give the crippled battery more charging time.

If the motor starts, remove the negative lead from both vehicles first, then remove the positive lead.

DO NOT switch the crippled vehicle’s motor off to test the crippled vehicles battery. If you are in the middle of nowhere and your motor is now running, don’t tempt fate.

**Homestar** · 11th March 2017, 12:14 PM

Originally Posted by drivesafe

OK folks, I’ll start a Digital Multi Meter ( DMM ) tutorial in the next week.

But first, make sure you have a DMM handy.

This is just a suggestion, if you need to buy one, there are some very cheap and useful DMMs but if you think you might get some use and want a little better than basic, Altronics have a DMM for $45 ( Q1134 ) and this one has a handy advantage over most.

Most DMMs have a maximum current limit of 10 amps, but this one can handle up to 20 amps and this is a more useful limit when measuring circuit currents around a vehicle.

Anyway, I will start putting something together and see how we go.

Just a thought but maybe include an explanation of voltage and current in this from a very basic viewpoint - one of my jobs at work is Generator Training for our staff and one of the things I see all the time is misconceptions of what current is. You get a lot of people thinking that a 10 amp power point produces 10 amps, etc and not understanding it's the load that dictates current flow. I also think this lack of understanding is why most people don't understand how batteries charge and think a 100 amp alternator will put 100 amps into their battery. Things like that...

And on batteries - that would be a good chapter - explaining them in more detail, diffrent types, common misconceptions around use, abuse and charging from various devices.

DiscoMick · 11th March 2017, 02:01 PM

Thanks.
Is it true if both vehicles are connected while both engines are stopped, and then the donor vehicle is started, starting it can send a high current surge into the flat battery damaging it?
Just something I've heard said, but never tested.

**alien** · 11th March 2017, 02:11 PM

Originally Posted by AK83

Or more importantly why a company(such as Toyota!! on their '04 Rav models) don't use a relay to power the headlights!

The switch is one of those turn-ey type indicator stalks, and the contacts inside eventually burn out, which results in the extra expense of having to purchase an indicator stalk when one really only wants a headlight switch .. which should have been a relay anyhow!

May have to wire up a couple of relays into the lighting system to prevent future occurrence.

Yet again Land Rover leading the way, every Defender is wired this way.

**Homestar** · 11th March 2017, 03:21 PM

Originally Posted by DiscoMick

Thanks.
Is it true if both vehicles are connected while both engines are stopped, and then the donor vehicle is started, starting it can send a high current surge into the flat battery damaging it?
Just something I've heard said, but never tested.

Assuming you mean when jump starting?

Answer is no. When the donor car is connected current will be flowing to the flatter battery to start with, when the donor vehicle is started, the alternator will also assist and supply what it can to the flatter battery - the current flow is dictated by the flat battery and how large the jumper cables are. The flat battery will draw as much current as it can - or what it is limited to by the cables and the other system. I'm my experience, the flat battery will only pull a 100 amps or so for a short time, then as it starts to take a charge, the current drops as the charge increases.

This is just the thing I was talking about in my last post - people not understanding current flow and why and when it does what. The donor vehicle can't send a surge of power - the current required is dictated by the flat battery and the limitations of the leads and system (battery and alternator) of the donor vehicle.

just scare tactics. There's certainly a right and wrong way to jump start a vehicle but killing the flat battery like this is BS - remember the flat battery could be U/S anyway depending on how and why it went flat so maybe that's where the myth comes from.

**drivesafe** · 11th March 2017, 04:01 PM

Originally Posted by DiscoMick

Thanks.
Is it true if both vehicles are connected while both engines are stopped, and then the donor vehicle is started, starting it can send a high current surge into the flat battery damaging it?
Just something I've heard said, but never tested.

Hi Mick and no a high “CURRENT” spike will not harm anything, including batteries, flat or otherwise, and high current "surges" occurs in all vehicle during normal use.

I think what you might be relating to is something Homestar raised.

Could you get a high “VOLTAGE” spike in that sort of situation.

First off the likelihood of voltage spikes being generated is always a possibility, and not just when jump starting. There are a large number of devices in vehicles that generate voltage spikes hundreds if not thousands of times, every time you drive a vehicle, but all vehicles have a massive voltage spike suppressor.

It’s called your cranking battery.

Even a stuffed cranking battery is still a very effective voltage spike suppressor.

While cranking batteries suppress major spikes, all automotive vehicle electronic devices come with their own builtin spike suppression and it is almost impossible to damage any vehicle’s electronics with high voltage spikes.

There are two exceptions, the first is when someone uses an electric welder on a vehicle with out setting the vehicle up properly.

The other, is if you attempt to replace a stuffed battery with a good one while the motor is running.

I have come across this situation twice, where someone had a stuffed cranking battery, got a jump start and then drove to a mates place, where he had a battery sitting around doing nothing.

When they got to the mates place, they were not sure if the replacement was any good, and to save having to possibly jump start the vehicle again, they simply left the motor running while they disconnected the stuffed cranking battery and by the time they had the replacement fitted, they had fried some of the vehicles electronics.

With no cranking battery to suppress the output of an alternate, an alternator can produce voltage spikes of 60+v and vehicle electronics are just not designed to tolerate such high spikes.

**Don 130** · 11th March 2017, 09:28 PM

Tim could we add correct procedure when welding on a vehicle. I Always disconnect the battery, but am often told by 'experts' (self proclaimed), that there's no need to.
Don.

**weeds** · 12th March 2017, 07:16 AM

Originally Posted by Don 130

Tim could we add correct procedure when welding on a vehicle. I Always disconnect the battery, but am often told by 'experts' (self proclaimed), that there's no need to.
Don.

Yes disconnecting is the best option without a doubt.....

Although for exhaust work I have never disconnected the earth is place right next to where you are welding , ever seen an exhaust place disconnect a battery.

**pop058** · 12th March 2017, 10:10 AM

Originally Posted by weeds

Yes disconnecting is the best option without a doubt.....

Although for exhaust work I have never disconnected the earth is place right next to where you are welding , ever seen an exhaust place disconnect a battery.

Had some exhaust work done awhile ago (300TDi D1) and they put a "suppressor" across the battery.

**donh54** · 12th March 2017, 10:35 AM

Originally Posted by pop058

Had some exhaust work done awhile ago (300TDi D1) and they put a "suppressor" across the battery.

Our local exhaust guy did the same. He told me that it isn't really necessary if the welder earth lead is close to the spot you're welding, and the welding current doesn't pass through anything other than the exhaust. He does it more for customer peace of mind - the customers in his shop can see the work area from the waiting room, and he has a lot of questions about that very subject.

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THE CORRECT PROCEDURE FOR JUMP STARTING

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