Thread: can you weld on a vehicle while battery still connected

Hi Brett,

Great analogy but I will change it a little and it will be a snap to understand.

The pressure is the voltage, the litres per minute is the current and the pipe diameter is the resistance.

In water terms ...... As you increase pressure and keep the pipe diameter the same you will increase the amount of water delivered.

In electrical terms ..... As you increase voltage whilst keeping the resistance the same more current will flow. Say a 12 volt supply and a 15 volt supply at 1 ohm resistance. The first will draw 12 amps whilst the latter will draw 15 amps.

Back to water. As you can see in logical terms also that increasing the pressure of water being forced into a pipe will increase it's velocity (flow over time) thus increasing the amount delivered to the outlet in the same time period.

You could expand this analogy further by then considering the battery to be a water tank, the amount of water is the charge, the more chrge you use, the less pressure you have, i.e the battery loses voltage as it goes flat.

John

Originally Posted by FenceFurniture

Hi Welding, if I could divert you attention for just a moment. This is nothing directly to do with welding, but a general understanding of electricity, and you sound like just the chap.

Power = Volts x Amps. I'm trying to make a correlation between the different components here. In high school (some 4 decades ago) my nigh on genius pal helped me to understand electricity with a garden hose analogy. He said think of the power as the total volume of water running through the hose (let's say litres per minute). The resistance was (or pressure increase/decrease I guess) was determined by the hose diameter (and with electricity the wire length as well). The current would be the pressure of the water, I think.

That leaves voltage, or potential difference for me to wrap my head around.My best estimate here is to substitute (say) a saline solution for the water. A stronger solution would equate to a higher voltage. This works in my head for the following reason:

Total salt flow per minute = solution strength x litres per minute of flow
which is the same as
Power = V x A

How am I going here? This has had me perplexed for decades (coz I didn't quite get Voltage even then!)

Regards, Brett

That's great John, clear as a bell now. Thanks very much.

So, if I had a battery that is 3Ah, 18v, feeding a 500w drill then I could work out how many holes it will drill at full power, given that each hole took 5 seconds to drill.

3Ah = 10800 amp seconds
18v
500w
5 seconds per hole

Amps per hole = 500/18 = 28 (rounded up)
Amp seconds per hole = 140
holes per charge = 10800/140 = 77 holes

I tink dat's roight!

Regards, Brett

Hi Brett,

Not quite. Ampere hour is a measure of the ability of the battery to discharge over a 20 hour rate. A 3 Ah battery can effectively supply 150 milli amps per hour for 20 hours before it's voltage starts to dip dramatically and it is considered to be flat. A 3Ah battery is very small in capacity, about half of the capacity in Ah of what you typically find in a domestic alarm panel but they are 12 volt.

If you are running a 500 watt load off this 18 volt battery you would be pulling 500W/18V amps, this is 62.5 amps from the battery. I dare say you would be lucky to drill one or two holes. Remember that the quoted charge dissipation rate is 150 thousandths of an amp (150 mA) per hour for 20 hours and suddenly it is being asked to pump 416 times more power at 62.5 amps which is way over the top.

A suitable battery would be as follows

62.5 amps x 20 hours = 1250 amp hour. One hole per 5 seconds is 20 hours divided by 5 seconds is 12 per min x 60 min x 20 hours = 14400 holes per charge. Your average car battery is 12 volts at 45 Ah so an 18Volt 1250 Ah battery is something in the range of a very large forklift array.

Moral of the story, use an extension lead and a 240 Volt 500 watt drill or else a generator.

Regards
John

Ok, thanks John

There is a failure in understanding here.

Firstly forget that the chasis or any other part of the vehicle or wiring is an earth or common of any form......once we start welding it is simply a conductor with no particular status.

All the electrical wiring and all the metal body simply presents a complex network of paralall paths.

because the system runs at 12 or 24 volts everything is a low resistance, every lamp, motor, solenoid relay or even the battery its self can present a low resistance path between these paths

If there are several paralell paths (in any curcuit), current will flow in all of them.....it matters little how much lower or higher the resistance of each path is...SOME current will flow in each.

The voltage across each path will be the same.....the relative resistance in each individual path will form a voltage divider and that will determine the voltage that will occuer at a given point.

so if there is 70 volts (or a couple of hundred volts TIG) across the combined paralell paths and your ECU happens to be in one of those paths......regardles of the current flowing in that path it might cop a dose of excess volts.

Mostly it is voltage not current that will snot our electronics

The above complex system is realy beyond practical analisis, especially the simple reasoning above.

There are two main reasons we disconnect the battery in a modern car.
1. to remove the common link between the whole positive distribution system and the chasis/earth system.....the battery may act as a low impedance link or a high impedance link...ya just don't know..it depends on which paralell path it ends up in.

2. to de energise all the electronics..... because if they are not powered up they have a far better chance of survival.

Welding on a motor vehicle is very much a lottery.....ya simply can not properly analise what is going to happen.

Disconnecting the batter is just a simple, easy and reasonable precaution that may help in some situations.

What will help is to always place the welding earth as close to the weld as possible and to make as good a contact as possible...... but isn't that a fundamental rule of welding..........and be incredibly carefull

personaly If I have a choice I will remove the item rather than weld on the vehicle.

If it was my car and it had funky electroncs and I had no choice but to weld on the vehicle.....I'd yank the ECU and unplug the radio and anything else from the harness.

This issue is bigger than just disconnecting the battery.

Now think about this one.....we are welding on a set of brush bars on a 4WD, the battery is removed, the radio is still connected as normal.
The brush bars are connected solidly to the chasis.....the earth clamp is near by as it should be
.
Being a little careless and half blind because of the mask....we accdentaly strike arc on the guard or the sill or something else connected to the body.....the boddy is mounted on rubber blocks and has a token effort earth strap bonding the chasis to the body.
One of the paralel paths

Now the aerial is mounted and earthed to the guard... one of the paralell paths is..from the body, thu the aerial earth connection along the coax to the radio, in one end of the PCB and out the other ( no back strap or chasis earth connection for the radio) along the negative supply wire of the radio and into the negative electrical common for the dash....and back to the battery...AHH but what else in the dash is connected to the negative common....so what could we possibly snot here.

Yeh welding on vehicles is something I would rather not do...and if I had to I'd want to be incredibly carefull.

What is an ECU or dash cluster worth for your car.



cheers

Hi Soundy,

I just wanted to clear something up.

The length of the welding arc is a function of it's electrical potential so the 60 - 70 volts is not in the chasis but between the chasis and the tip of the electrode. This is also the open circuit voltage, once you strike the arc and start current flowing, the arc voltage drops to around 20 - 25 volts but again this is the potential difference between the chasis and the tip of the welding electrode.



John

Yes but those are " nominal" voltages. These steady state voltages are probably the least harmfull.

There may be very much higher voltage spikes at the time of strike and lift off...particularly if it is an old style iron cored welder....if the arc is spluttering or discontinuous these spikes may be repeated many times in a second.

In the case of TIG....the high frequency high voltage component may disapear or be substantialy once the arc is established.....But the instant of strike up or lift off those voltage will be there and that is plenty

It only takes a single instantanious incident to snot a semiconductor, something that takes a fraction of a second.

Some semiconductors, Cmos devices take very little to damage, and some power diodes have very poor reverse voltage tolerance.

Blowing the diodes in the alternator is not an uncommon welding damage in vehicles......

Remember too that large amounts of our logic and microprocessor semiconductors operate on 5 volts sometimes less......the the normal power supply route may be well protected by voltage regulators and surge supression, but the sensor inputs and the earth plane of this equipment may not be.

Hell quite a lot of automotive and marine equipment does not even have rudimentary reverse voltage protection, you can comprehensivly snot these items with 12 volts the wrong way arround.



cheers

Oh... just think on this too.

We have an earth clamp on one end of a chasis...we strike arc on the other end of the chasis..at the time of striking arc....what is the voltage in the middle of the chasis.

The earth path, any earth path has a finite resistance, and there fore presents a voltage divider.

If something is "earthed" to the chasis nearer the striking point...that earth point may have say 50 volts of the available 70 volts compared to the end of the chasis with the earth clamp connected...this does not account for spikes.

Say we have a brake sensor on the rear brakes that derives its earth from the rear of the chasis and the brake CPU is at the front of the vehicle and derives its earth near the earth clamp....there is now 50 volts between the sensor earth and the cpu earth this means there may be 50 volts across the the interface in the cpu and the sensor live terminals.....SNOTTED
cheers

Hi Soundy,

I have read your post and digested the information however have you considered that the resistance of steel is between 1 x 10^-7 ohms per meter and 10 x 10^-7. If we take the mid point of the general 5 meter car and assume you are welding at 80 amps the voltage present at the mid point would be 0 volts plus (2.5 x 10^-7 x 80 amps) which is 0.00002 Volts. At the other end of the resistive scale it would be 0.0002 Volts

You need to add the voltage to the zero volt reference as the earth clamp is bonded to the car. The potential (70 volt bit known as an electrode) is located away from the car at a distance (several millimetres) as it is being used to weld.

You also cannot call this a voltage divider. It is a circuit comprised of several in series components, welder itself, the cable to the rod, the rod, the arc, the chasis, the return earth cable and each component has a resistance. Using ohms law V = I x R and considering you have the same current passing through all of these parts as they are in series, you can multiply the different resistances of all of the these parts by the current and added up they will be equal to the total voltage available by the power source under load.

Please don't take my word for it. Bolt both the positive and negative outlets of your welder together with a bridge (steel or copper bar of suitable diameter to conduct the current) and run at full amps and measure the voltage and current across the bridge. This will give you a reading that can be used to calculate the internal resistnce of the welder.

Then repeat again with the chasis of a car. You will find the same reading. The reason is that the .00002 volts or 0.0002 Volts depending on the steel in your chasis will not be measurable by your multimeter.

John

Originally Posted by soundman

Oh... just think on this too.

We have an earth clamp on one end of a chasis...we strike arc on the other end of the chasis..at the time of striking arc....what is the voltage in the middle of the chasis.

The earth path, any earth path has a finite resistance, and there fore presents a voltage divider.

If something is "earthed" to the chasis nearer the striking point...that earth point may have say 50 volts of the available 70 volts compared to the end of the chasis with the earth clamp connected...this does not account for spikes.

Say we have a brake sensor on the rear brakes that derives its earth from the rear of the chasis and the brake CPU is at the front of the vehicle and derives its earth near the earth clamp....there is now 50 volts between the sensor earth and the cpu earth this means there may be 50 volts across the the interface in the cpu and the sensor live terminals.....SNOTTED
cheers

Originally Posted by welding

I have read your post and digested the information however have you considered that the resistance of steel is between 1 x 10^-7 ohms per meter and 10 x 10^-7. If we take the mid point of the general 5 meter car and assume you are welding at 80 amps the voltage present at the mid point would be 0 volts plus (2.5 x 10^-7 x 80 amps) which is 0.00002 Volts. At the other end of the resistive scale it would be 0.0002 Volts

John,

I'm being pedantic here. I think you are confusing resistance with resistivity. Resistivity is a property of the material, whereas resistance is a property of the resistively of the material and its geometric shape.

Have a look at Electrical resistivity and conductivity - Wikipedia, the free encyclopedia

Ya still fail to understand.... the current the welder is set to and the suposed current flowing after the arc is struk is irrelivent.

0 volts at one end of the chasis 50 volts at the other end of the chasis....what is the voltage in the middle... not steady state....Instantanious.

If what you are proposing is the whole story it should be imposible to harm any electrical or electronics in a car with a welder.

It is well established that it is possible to, damage batteries, alternators, and modern electronics in motor vehicles with a welder.

Situations are never ideal.
Cars are made up of many joined parts.
are the joins in the chasis, welded riveted or bolted, how continuous are those joins.......is what you are welding on electricaly well bonded to the chasis.

when the end of that welding rod is grounded to strike arc...does it make a perfectly smooth step from the open circuit voltage to the welding voltage, what about when you lift off...how much back EMF does that big choke (the welder) generate.

remember even a straight bit of wire has some inductance..so does a chasis.


In the majority of cases where there is a good solid low impedance earth path......... there will probaly be no problem....... but in a situation where there is some sort of non ideal situation......SNOTTED.

How about a real situation.
Aluminium boat, bare...sitting on the damp grass......earth clamp on the stern...... bloke tig welding not far from the earth clamp..me holding a patch in place with an insulated screwdriver with a gloved hand.......unfortunately I am sitting on the damp grass and I lean against the boat.....I get a tingle thu my shirt.......nothing major....but an indication of earth potential rise even though there is a short low impedance earth path.

cheers

You are right.

I did a quick calc before the post and using 0.9mm steel thickness with 5 metres length and roughly 5 metres circumference the multiplier was 1.111 etc so I just used the resistivity value as the resistance. My error for not wording correctly.

John

Originally Posted by chrisp

John,

I'm being pedantic here. I think you are confusing resistance with resistivity. Resistivity is a property of the material, whereas resistance is a property of the resistively of the material and its geometric shape.

Have a look at Electrical resistivity and conductivity - Wikipedia, the free encyclopedia

Eddy currents and skin effect (not your skin but the skin of the ally), induced in the aluminium due to current flow as your friend was welding with AC current. It would not have occured with DC.

John

Originally Posted by soundman

How about a real situation.
Aluminium boat, bare...sitting on the damp grass......earth clamp on the stern...... bloke tig welding not far from the earth clamp..me holding a patch in place with an insulated screwdriver with a gloved hand.......unfortunately I am sitting on the damp grass and I lean against the boat.....I get a tingle thu my shirt.......nothing major....but an indication of earth potential rise even though there is a short low impedance earth path.

cheers