Do not disconnect battery?

Recently I was reading a post on another forum where the question was asked about disconnecting the battery before welding on a car. One response stated that some new cars tell you not to disconnect the battery as it will goof up some calibration and need calibrated again. I did not believe it but did then come across some manufacturer documentation which stated something like this. I forget now where I found the documentation (dang it) but have been puzzling over this in my mind.

I would like to suspect that what was meant was that disconnecting the battery would cause the car to 'relearn' the fuel trims, etc. I cannot fathom the scenario where a manufacturer would build a car where a battery replacement would lead to having to have the car serviced. Has anyone run across this?

I always disconnect the battery when welding. And definitely on new cars with dozens of modules. Ok your 'keep alive memory' gets wiped out, fuel trims have to relearn etc... So what
If you find that documentation please post it. I'd like to read it.

OK, I'll have to make an admission to reply to this....so don't start throwing stones...(please). Besides being a franchise owner (non-auto related) and retired military I also put in some hours part-time as one of those chain store parts counter guys. I'd rather be in a garage, but I don't have that kind of time so I get my auto passion fix being one of those parts guys a dozen or so hours a week. There, it's out...now you know.

I can tell you having changed countless batteries there are some cars that have a special note that pops up on our computer screen that we can't change the battery because a relearn has to be done. See them about once a month on average. Off the top of my head can't think of specific vehicles, but I think one of them is an Audi or a BMW...could be remembering wrong.

I can tell you having changed countless batteries there are some cars that have a special note that pops up on our computer screen that we can't change the battery because a relearn has to be done.

You're right. Forgot to mention that :whistle:
In case of an AGM battery and cars with start/stop system you'll have to perform a relearn. There's an IBS (Intelligent battery sensor)
But you can always use one of these then:

I've not seen any info suggesting NOT to disconnect a battery, but that doesn't mean it isn't out there. Of course, there are those wack cases out there of someone changing a battery and ending up with a dead module afterwards. But, those are the exceptions, and not the norm.

I've also been told to always disconnect the battery before welding, and I do. Though, I gotta admit, I've forgotten a couple times. :blush: Got away with it, luckily!

Tyler wrote:

Though, I gotta admit, I've forgotten a couple times. :blush: Got away with it, luckily!

Same over here :S

The disconnecting the battery when welding welding advice has been around since the 1960s PNP Germanium transistors that were in common use then were much more sensitive to reverse voltage than the NPN Silicone transistor we see today.

I'm reminded of warning I read a few years back about an E class Mercedes. As I recall, a battery disconnect would result in the power windows and head restraints(possibly) needing to be recalibrated. That was a while ago, so I may be misremembering that.

Also, for what it's worth, I don't always disconnect the battery before welding. I don't personally know of anyone who has damaged a module by not disconnecting the battery before welding. I'm sure it's happened, but I haven't run into it yet. Fingers crossed!

Andy.MacFadyen wrote: The disconnecting the battery when welding welding advice has been around since the 1960s PNP Germanium transistors that were in common use then were much more sensitive to reverse voltage than the NPN Silicone transistor we see today.

You're awesome, Andy! This makes more sense than anything else I've heard previously.

Also, for what it's worth, I don't always disconnect the battery before welding. I don't personally know of anyone who has damaged a module by not disconnecting the battery before welding. I'm sure it's happened, but I haven't run into it yet. Fingers crossed!

See, I've never heard of it, either. The argument I always get is that the high current flow generated by the welder can feed into sensitive PCM circuits and damage them. But, doesn't the starter draw a ton of current during cranking, too? :huh: Maybe I'm misunderstanding the reasons...

Interesting topic

I found this article so I'll post it here:

"Sharing a very good article which I’d come across browsing recently while preparing to undertake the DG platform welding/ strengthening work, it gives the ideal explanation for the need of disconnecting the Batteries of the machinery to safeguard its Engine control unit & other electronics; I’ve compressed/ modified the article slightly to accommodate it here, hope it does helps anyone reading it.

#Speculation runs rife regarding whether the negative terminal of a vehicle’s battery should be disconnected before welding on that vehicle. The reasoning is that it helps to prevent damage to the vehicle’s ECU and other electronics. Many people believe the negative terminal should be disconnected but cannot explain why it should be done. Other people believe disconnecting the battery terminal has no impact on the ECU since the ECU is still connected on both terminals and the affect of disconnecting the battery only isolates the battery and nothing else. They believe the idea to disconnect is simply an old wives’ tale with no valid reasons to substantiate it. So who is right?

Electrical Principles- To help explain what’s happening we need to understand a couple of principles: Ohm’s Law and Kirchhoff’s Circuit Laws. I’m going to explain them very simply as they relate to this problem.

Ohm’s Law: This is usually presented as the formula V= iR where V is voltage, i is current and R is resistance. It tells us that voltage is required for current to flow. If there is current flowing through a conductor then there must be a voltage across that conductor.

Kirchhoff’s Circuit Laws: This law states that the sum of the voltages around a continuous electrical loop must be zero and the sum of the currents into and out of an electrical node is zero. Regardless of the current flowing, regardless of parallel or alternative loops, the voltages around every unique loop must sum to zero and the currents through each node must sum to zero.

Welding Circuit- Below is a circuit diagram that depicts welding a vehicle. On the left is welding with the negative battery terminal connected and on the right is welding with the negative battery terminal disconnected. The resistor represents the car body. Assuming that the car battery voltage is 12V and the welder voltage is 100V. The actual voltages are irrelevant, the point is to see whether welding could damage the ECU and whether disconnecting the negative battery terminal would reduce that risk. All voltages are measured with respect to the battery’s negative terminal. The box labeled “ECU” is the engine control unit, the device that is to be protected.





V1 is the voltage across the ECU with the negative battery terminal connected whilst welding. V2 is the voltage across the ECU with the negative battery terminal disconnected whilst welding. Kirchhoff’s laws and Ohm’s laws resolve the voltage at each point. The left side has a closed loop through the ECU where Kirchhoff’s voltage law easily verifies the voltage across the ECU. The right side has an open loop where Kirchhoff’s current law dictates that no current flows through the ECU and Ohm’s law determines that the voltage across the ECU is zero. Similarly Ohm’s law could be used in isolation with the open circuit interpreted as an infinite resistance.

So from the diagram we see that: V1 = 112V & V2 = 0V

It’s seen that in the left circuit the ECU is suffering from over-voltage and could be damaged. It is designed for 12V yet has 112V across it. The right circuit, with the negative battery terminal disconnected, there is no voltage across the ECU and therefore it does not experience any over-voltage.

The ECU damage is most likely when first striking the arc and not during steady DC current flow. When you first strike the welder it yields a step function which would generate a reaction different to what the steady state DC resistance would predict. A step function is made up of many high frequency components. Not only are there many high frequencies present, but the voltage would be high when first striking the arc. Before welding starts the voltage floats up to the open circuit voltage which is typically between 30 to 100V. When first striking the arc the full open circuit voltage is applied between the welding rod and the earth clamp.

One might argue that adequate earthing will maintain the body at zero potential throughout. Ohm’s law tells us that if current is flowing there must be voltage regardless of your earthing. Further, earthing does not control the resistance through the body and thus does not necessarily prevent potentially large voltage drops.

From this evaluation it is confirmed that welding a vehicle/ DG platform does risk damage to the ECU and disconnecting the battery’s negative terminal will mitigate that risk.

Why Does This Happen?- The problem lies with the ECU and the welder sharing a single conductor – the vehicle’s body. For the welder to push current through the body, it must impose a voltage across it regardless of how good your earthing is (Ohm’s law). With the battery connected there is a closed electrical loop where the voltage across the vehicle’s body could add to the battery’s voltage and cause an over-voltage condition at the ECU. With the battery’s negative terminal disconnected, the loop is broken, both terminals of the ECU will float to the same voltage and no over-voltage will occur regardless of the voltage across the vehicle’s body."

In SA we had a nissan pick up that if you disconnect the battery on some you towed it to the agent to start it again so we took out starter motor live and I have heard that the Audi Q7 battery must not be disconnected ??

In the old days on cars without and oxygen sensor disconnecting the battery could cause major headaches if adaptions had been set in the ECU.

Great topic . I disagree with that article and will explain later. As a side note I am a leave the battery connected welder...

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Ben wrote: Great topic . I disagree with that article and will explain later. As a side note I am a leave the battery connected welder...

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Cool I like people who disagree Interested in what you think about it.

How much later?

your jamming all kinds of amperage into the ground side of the circuit. I work on Land Rovers and Jags. I would disconnect the battery if I have a welder within 4 ft of one of these cars.

Would any of you guys who have good luck welding with the battery connected consider coming out to northern Illinois and buying a lottery ticket for me. Ill pay, I just want you lucky people to purchase it for me.

skip wrote: your jamming all kinds of amperage into the ground side of the circuit. I work on Land Rovers and Jags. I would disconnect the battery if I have a welder within 4 ft of one of these cars.

Would any of you guys who have good luck welding with the battery connected consider coming out to northern Illinois and buying a lottery ticket for me. Ill pay, I just want you lucky people to purchase it for me.

lol! If you're paying for gas, I'll come hook up your ground lead.

I don't blame anyone for wanting to err on the side of caution, especially when working on world class luxury vehicles.

Aren't you guys thoroughly reading your Hayne's and Chilton's manuals:

"Step 1) Disconnect the negative battery terminal."

Noah wrote: Aren't you guys thoroughly reading your Hayne's and Chilton's manuals:

"Step 1) Disconnect the negative battery terminal."

:lol: Don't forget the universal Step #2: Remove radiator cap.

Just thinking out loud... How about an amp probe on the negative battery cable while welding on the body? Or the exhaust? :huh: I'm willing to volunteer the Scion as a guinea pig if anyone thinks this would be a useful test.

Tyler wrote:
Just thinking out loud... How about an amp probe on the negative battery cable while welding on the body? Or the exhaust? :huh: I'm willing to volunteer the Scion as a guinea pig if anyone thinks this would be a useful test.

That would be awesome! Since this is a subject with a lot of opinions why not test it in reality
An amp clamp and maybe a voltage probe to check for spikes?

Dylan wrote: That would be awesome! Since this is a subject with a lot of opinions why not test it in reality
An amp clamp and maybe a voltage probe to check for spikes?

Good idea. Exhaust is the most common reason for welding around here, but we also use it for extracting broken exhaust studs/bolts. I'm thinking of trying both while I'm there, see if it makes any difference.

The xA has, like, three modules, so at least it'll be cheap(ish) to fix if I let the smoke out. :lol: