Any point in disconnecting the battery when welding on a vehicle?

Thread Starter

strantor

Joined Oct 3, 2010
6,338
This came up on another (non-electric/electronic) forum and the majority of replies were saying that you need to disconnect the battery. I put in my 2 cents below, but I'm wondering if there's something I'm missing, and why it's such a common recommendation. My reply:



If your tractor has a computer then it would be a good idea to disconnect it. I don't think it would hurt to disconnect the battery too, but neither do I understand how it could help. Is it to prevent damage to the battery itself? If so, it's worth pointing out that the battery is the most robust electrical component on the machine; even more stout than the wires that connect to it. Or are we thinking that the battery completes a circuit, so removing it is like opening a disconnect and preventing current from flowing in any circuit?

There are only two ways I can conceive that welding could damage electronics:

1. By raising the electrical potential of one ground higher than that of another ground, causing current to flow through electrical circuits in ways that it isn't intended to flow. And I think that would happen whether or not the battery was connected. The best way to prevent that is to place your ground clamp as close as possible to the area being welded.

2. By inducing high frequency EMI into wiring harnesses. This also is possible with or without the battery connected. The best way to prevent this is to keep ground lead and welding lead as far as possible from any machine wiring, and if you must cross a wiring harness with a welding lead, do it at 90 degrees, never running alongside a harness.
 

t_glover

Joined Mar 16, 2021
43
I worked in truck and equipment dealers for 24 years. There was always at least a couple full time welders. They never unhooked batteries when welding on a truck or a machine. Never had battery or electrical problem from welding. Not until electronic controls became the norm.
Link-Belt machinery in the mid 1990's had a sticker on a couple spots on the machine that warned to disconnect the electronics before doing any welding repairs on this machine. Or something like that, it's been a few years.
The welder was welding ice lugs on the track pads of a log loader. 1"x1"x4" square steel bars. He was moving the welding ground to each pad as he welded the lugs on. When he went to rotate the track after welding the first few lugs on none of the hydraulic functions would work. He failed to unplug the ECU. A replacement ECU fixed the none functioning hydraulics. The new ECU was pricey. I'm pretty sure the welder was "spoken with".
I took the circuit board out of the ECU and several traces on the board were badly burnt. The burnt traces came from a row of heat sink mounted transistors along the side of the board.
 

AmeliaGrey

Joined Dec 30, 2022
19
Disconnecting the battery when welding on a vehicle is a recommended safety precaution. This is because welding creates a lot of electrical current and voltage, which can cause electrical components in the vehicle to malfunction or become damaged. Disconnecting the battery helps to prevent this by isolating the electrical system of the vehicle from the welding equipment. Additionally, welding on a vehicle that has a live electrical system can also be dangerous for the person doing the welding, as it can create an electrical shock hazard. If you are not familiar with welding and the safety procedures related to it, it's best to consult a professional.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,338
Disconnecting the battery when welding on a vehicle is a recommended safety precaution.
Right. I'm aware of that as I did Google before asking and saw that it is recommended far and wide, albeit with little explanation of "why" ever given. Pretty much they all just say this:

This is because welding creates a lot of electrical current and voltage, which can cause electrical components in the vehicle to malfunction or become damaged.
Understood. But this is true (in my head at least) whether the battery exists in the circuit or not.
Disconnecting the battery helps to prevent this by isolating the electrical system of the vehicle from the welding equipment.
How does it help exactly though? How does it isolate anything? The vehicle uses chassis as a current carrying conductor. It's like you're welding on the actual neutral wire of your house. If you wanted to isolate the entire electrical circuit, wouldn't you need to disconnect all the grounds in addition to the battery?
Additionally, welding on a vehicle that has a live electrical system can also be dangerous for the person doing the welding, as it can create an electrical shock hazard.
That's even less plausible than to me than protecting the electrical circuits by removing the battery.
If you are not familiar with welding and the safety procedures related to it, it's best to consult a professional.
I am familiar with welding and safety procedures. I am posing a technical question to professionals.
 
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Thread Starter

strantor

Joined Oct 3, 2010
6,338
I worked in truck and equipment dealers for 24 years. There was always at least a couple full time welders. They never unhooked batteries when welding on a truck or a machine. Never had battery or electrical problem from welding. Not until electronic controls became the norm.
Link-Belt machinery in the mid 1990's had a sticker on a couple spots on the machine that warned to disconnect the electronics before doing any welding repairs on this machine. Or something like that, it's been a few years.
The welder was welding ice lugs on the track pads of a log loader. 1"x1"x4" square steel bars. He was moving the welding ground to each pad as he welded the lugs on. When he went to rotate the track after welding the first few lugs on none of the hydraulic functions would work. He failed to unplug the ECU. A replacement ECU fixed the none functioning hydraulics. The new ECU was pricey. I'm pretty sure the welder was "spoken with".
I took the circuit board out of the ECU and several traces on the board were badly burnt. The burnt traces came from a row of heat sink mounted transistors along the side of the board.
Right, Disconnecting the electronics I totally understand, and would do the same. But the battery I don't get.
 

crutschow

Joined Mar 14, 2008
31,128
Right, Disconnecting the electronics I totally understand, and would do the same. But the battery I don't get.
The battery completes the circuit to ground (the battery has essentially zero resistance), so any difference in voltage between the battery ground and the electronics ground will possibly be seen by the electronics.
If the battery ground is removed that path is removed.

But there still could be a ground voltage difference between other grounds and the electronic ground, so disconnecting the electronics is the safest (and recommended) thing to do.
 

ronsimpson

Joined Oct 7, 2019
2,540
In my wife's suburban, when the battery is disconnected, the heat/AC computer forgets and must be recalibrated. The radio forgets, and needs reprogrammed. In another car the seat and mirror position is set by who is driving and is lost.

I hate it when the battery is disconnected.

As to the voltage difference along the frame, If you connect the welder, cold wire, to the front of the car and welded on the hitch, I can see there being a problem. Who welds like that? Think about welding a new muffler. The current is in the exhaust pipes and not in the ground system.

If you don't disconnect the battery and something dies you are in trouble. If you do disconnect the battery and something dies, same story.
 

t_glover

Joined Mar 16, 2021
43
I have a device in my tool box that plugs into one of the power points AKA cigarette lighter in a vehicle. It has a nine volt battery connected to it. It backs up the power when the battery is disconnected. You just have to make sure the power point is not switched.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,338
The battery completes the circuit to ground (the battery has essentially zero resistance), so any difference in voltage between the battery ground and the electronics ground will possibly be seen by the electronics.
If the battery ground is removed that path is removed.
The path from the +12V rail to ground still exists though, through any/all circuits we are concerned about damaging. I'm starting to think that, If anything, removing the battery is similar to removing a huge capacitor that could have otherwise protected those circuits.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,338
Maybe the best way to prevent damage is (in addition to Disconnecting modules), disconnect the battery and short the battery leads together (after dissipating any residual charge through a resistor).
 

MisterBill2

Joined Jan 23, 2018
13,804
IF the welding current affects the voltage at tghe point where the battery negative lead attaches to the body metal, then the battery positive terminal voltage will also be affected. That might be enough to cause some damage. Disconnecting the battery "just in case" is cheap insurance. Now you know the mechanism of a possible damaging effect of welding current on items seemingly not connected. Certainly it might be uncommon, but any electronic device is going to cost a lot more than the labor time to disconnect a battery terminal.
The fact is that a car or truck is built from a lot of pieces and a couple hundred noisy amps can develop a damaging voltage across some of those body joints, because the resistance is not zero.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,338
IF the welding current affects the voltage at tghe point where the battery negative lead attaches to the body metal, then the battery positive terminal voltage will also be affected. That might be enough to cause some damage. Disconnecting the battery "just in case" is cheap insurance. Now you know the mechanism of a possible damaging effect of welding current on items seemingly not connected. Certainly it might be uncommon, but any electronic device is going to cost a lot more than the labor time to disconnect a battery terminal.
The fact is that a car or truck is built from a lot of pieces and a couple hundred noisy amps can develop a damaging voltage across some of those body joints, because the resistance is not zero.
That's all valid if you assume "Disconnecting the battery can't hurt" as I initially did. But do you think there is no case to be made for the battery acting as a large capacitor and being more beneficial left in the circuit than removed?

How about Disconnecting the battery and connecting the leads together? In your view is that better or worse than simply Disconnecting them?
 

MisterBill2

Joined Jan 23, 2018
13,804
That's all valid if you assume "Disconnecting the battery can't hurt" as I initially did. But do you think there is no case to be made for the battery acting as a large capacitor and being more beneficial left in the circuit than removed?

How about Disconnecting the battery and connecting the leads together? In your view is that better or worse than simply Disconnecting them?
I don't see any way the battery acting like a big capacitor could be a benefit.
Momentary shorting the leads might possibly do some good, but it would certainly reset memories on some devices. So just disconnecting one lead is adequate. Disconnecting the ground is slightly safer because if your tool hits ground while unbolting there are no sparks.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,338
I don't see any way the battery acting like a big capacitor could be a benefit.
In that it could absorb voltage spikes on the +12V rail caused by EMI. And by placing a stiff positive bias on all circuits to prevent EMI or unequal ground potential from resulting in reverse polarity voltage being applied to circuits. That seems safer to me than just leaving things "floating" in a known chaotic electrical environment.

But even safer still I suspect, might be to disconnect the battery and connect the leads together, leaving them connected together for the duration of the welding. As I see it, this would not allow any spikes whatsoever on the +12V rail. Any current induced on those wires would be drained immediately to ground.

In all these cases though, circuits on "ignition" rail (only on with key turned) would still be susceptible to EMI. So maybe the key should be turned on so that but remains tied to +12V and whatever method of protection you decide to employ is extended to these circuits as well.

Momentary shorting the leads might possibly do some good, but it would certainly reset memories on some devices.
Why momentary shorting? What does that accomplish? Yes, memories will be reset, but like you said, "cheap insurance."
 

MisterBill2

Joined Jan 23, 2018
13,804
Momentarily shorting the leads discharges capacitors at various places. The alterative, usually suggested, is to switch on the headlights momentarily, which achieves the same goal but without flexing the battery cables. Probably a smarter option. Wit the cars built in the last 20 years or so, there is a lot hidden from us.
 

shortbus

Joined Sep 30, 2009
9,762
@strantor Ron in post #7 has the correct answer. If you keep the welder ground near the actual welding point your good. The others are giving some kind of "theoretical voodoo" answers. Think about it, the welding current is totally different than any system in the vehicle, the welding current goes right to the ground clamp in the quickest way possible. It's not going to take a tour of the cars electrical system. I've worked on and welded on cars my whole adult life, never killed one yet when welding. Now if you weld one of the vehicles wires then you will have a problem.
 

cmartinez

Joined Jan 17, 2007
7,881
@strantor Ron in post #7 has the correct answer. If you keep the welder ground near the actual welding point your good. The others are giving some kind of "theoretical voodoo" answers. Think about it, the welding current is totally different than any system in the vehicle, the welding current goes right to the ground clamp in the quickest way possible. It's not going to take a tour of the cars electrical system. I've worked on and welded on cars my whole adult life, never killed one yet when welding. Now if you weld one of the vehicles wires then you will have a problem.
That's similar to a piece of advice I got a while ago from a car workshop. When jump starting your car from another car while its engine is running, ALWAYS connect both grounds to chassis and never directly to the batteries' negative poles. Otherwise the inductive kickback current might travel directly into the cars computers and fry their circuits. Connecting the cables to both chassis dissipates said currents and minimizes risks.
 
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Thread Starter

strantor

Joined Oct 3, 2010
6,338
@strantor Ron in post #7 has the correct answer. If you keep the welder ground near the actual welding point your good.
I agree with what he said and I mentioned it myself in the op:
There are only two ways I can conceive that welding could damage electronics:
1. By raising the electrical potential of one ground higher than that of another ground, [...] The best way to prevent that is to place your ground clamp as close as possible to the area being welded.
But there is still the matter of EMI. Depending on the type of welder it could be inducing high frequency into wires if welding leads aren't routed sensibly. My hypothesis about leaving battery connected or shorting the leads together is meant more to address that.
 

Thread Starter

strantor

Joined Oct 3, 2010
6,338
That's similar to a piece of advice I got a while ago from a car workshop. When jump starting your car from another car while its engine is running, ALWAYS connect both grounds to chassis and never directly to the batteries negative poles. Otherwise the inductive kickback current might travel directly into the cars computers and fry their circuits. Connecting the cables to chassis dissipates said currents and minimizes risks.
Inductive kickback? Can you expound on that? What is the inductive component? Which car is at risk? The running one or the dead one or both?

The inductive kickback explanation is the first and only explanation I've ever heard for this, but "connect to the frame not the battery" is another thing in the list of automotive advice I've heard, never understood, never followed, and never had an issue. Admittedly there is another list of automotive advice I've heard, didn’t understand, didn't follow, DID have an issue, and understanding arrived shortly thereafter.
 

cmartinez

Joined Jan 17, 2007
7,881
Inductive kickback? Can you expound on that? What is the inductive component? Which car is at risk? The running one or the dead one or both?
Inductive kickback coming from the starter solenoid and the starter motor itself being disengaged. The car at risk is the one aiding the other car's starter. That is, the one being used as the power source.
 
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