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Jul 15, 2009 14:25:36 GMT
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Call me contoversial but... I have always had a problem with this theory that you are supposed to disconnect the battery before doing mig welding on a car. Sounds like utter bollards to me. What possible interest to any electrical component on the car would a few amps being passed along the bodywork be? Mig welder puts out say 150 amps absolute tops , more like 50 on thin bodywork. Voltage? Maybe 30 or 40 volts? Startermotor draws up to 500 amps, and that current goes through the body on most vehicles. Ignition system puts out what, 20,000 volts, and does that fry the alternator? No. I have never bothered to disconnect anything when welding on a car, and during 20 years of welding, have never had any kind of problem. So Whos with me on this, urban myth, which should be filed along with 'your handbrake cable has stretched, or 'your car is pulling to the left, must be the tracking', and 'that noise when your foot is OFF the clutch is the release bearing'  |
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69 Plymouth Fury Convertible 75 Range Rover 2 door 82 Range Rover 4 door
84 Range Rover 4 door 78 Datsun 120Y 2 door 78 Datsun 120Y Coupe
78 Datsun 620 Pickup 81 Datsun Urvan E23 86 Datsun Vanette van
98 Electric Citroen Berlingo 00 Electric Peugeot Partner 02 Electric Citroen Berlingo 76 Honda C50
04 Berlingo Multispace petrol 07 Land Rover 130 15 Nissan E-NV200 15 Fiat Ducato
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Jul 15, 2009 14:29:12 GMT
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I'm with you in the never seen it cause a problem camp.. however could in some random chain of events something pop or fry.. probably just not likely. Better safe than sorry is the correct response.. but I'm lazy and lost my radio code  |
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Daily: Spazda Mx5
'A52's Fastest steak eater 2010'
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Jul 15, 2009 14:43:05 GMT
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Its not the battery your protecting, its the alternator.
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Volvo back as my main squeeze, more boost and some interior goodies on the way.
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Nathan
Club Retro Rides Member
Posts: 5,628
Club RR Member Number: 1
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Jul 15, 2009 14:43:13 GMT
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I remove the battery as they can explode if hot elements touch them, but thats not what your asking.
Removal of the clamp is done to not knacker the battery, however like yourself I never have seen or had this done to me (pretty much the same as dicsonnecting the alternator due to it papping them).
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Last Edit: Jul 15, 2009 14:43:52 GMT by Nathan
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Jul 15, 2009 14:46:22 GMT
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Ive seen me mate (a pro car builder) clamping one of them alterantor protector thingies on cars before he welds em and he shares his workshop with a guy who rebuilds alternators and starter motors, I'm pretty sure if anyone is gonna know it'll be them two and he still wont weld a car without it on there or the electrics disconnected, i guess it only has to happen once on a customers car and all your profit could be gone.
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Volvo back as my main squeeze, more boost and some interior goodies on the way.
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MrSpeedy
East Midlands
www.vintagediesels.co.uk
Posts: 4,786 
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Jul 15, 2009 14:52:27 GMT
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The idea is to protect the alternator from any stray currents running through it and frying the diodes/rectifier pack.
Well, that's my understanding of it anyways, and i still won't weld a car/van/boat/woteva after 20 years, with it connected !
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Seth
South East
MorrisOxford TriumphMirald HillmanMinx BorgwardIsabellaCombi
Posts: 15,523
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Jul 15, 2009 14:58:30 GMT
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Didn't Mr Bo11ox recently fry something electrical while welding or am I making that up?
I always disconnect the battery but then have occasionally forgotten and haven't yet hurt anything.
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Follow your dreams or you might as well be a vegetable. |
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Jul 15, 2009 15:04:06 GMT
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I've done some REALLY STUPID things in the past! Stuff were I thought "this won't require me to disconnect the battery" later on that day I had to by a new battery cause I fryed it! Not welding related, but now I disconnect it no matter what I'm doing.
For the sake of 2 mins with a 10mm spanner it's worth disconnecting it, no matter what your doing ;D
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Sierra - here we go again! He has an illness, it's not his fault.
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Jul 15, 2009 15:14:36 GMT
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If things like alternators really WERE prone to damage from stray welding current, disconnecting the battery isn't going to do much anyway. I want someone to draw me a diagram of how on earth anything could get damaged from mig welding currents, when there are far greater currents within the alternator and other parts of the car on a daily basis.
I think more cars get upset disconnecting and reconnecting the battery (abs, airbags, ECU etc) than welding on them!
Also think of the currents and voltages involved when people put hydraulics on a car. 36 / 48 volts and thousands of amps.
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Last Edit: Jul 15, 2009 15:22:17 GMT by datman
69 Plymouth Fury Convertible 75 Range Rover 2 door 82 Range Rover 4 door
84 Range Rover 4 door 78 Datsun 120Y 2 door 78 Datsun 120Y Coupe
78 Datsun 620 Pickup 81 Datsun Urvan E23 86 Datsun Vanette van
98 Electric Citroen Berlingo 00 Electric Peugeot Partner 02 Electric Citroen Berlingo 76 Honda C50
04 Berlingo Multispace petrol 07 Land Rover 130 15 Nissan E-NV200 15 Fiat Ducato
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MrSpeedy
East Midlands
www.vintagediesels.co.uk
Posts: 4,786
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Jul 15, 2009 15:24:22 GMT
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I think it's more to do with the possible REVERSE currents tbh. I guess things like electronic instruments and stuff could also be at risk ? Yes, i'd have thought ecu's/abs/airbags etc would be at risk, but i didn't think about them as i don't have any ! LOL |
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Last Edit: Jul 15, 2009 15:26:36 GMT by MrSpeedy
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Jul 15, 2009 15:24:27 GMT
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In my opinion (as an Automotive electronic engineer!), you're probably better off leaving the battery on than removing it. At least any stray current that finds its way into the system has somewhere to go (Dumped in the battery) and the voltage will stay at 12 or so volts, whereas without the battery acting as a current sink, the alternator would bear the brunt of it.
Unless you are welding within a few inches of earthing points, I wouldn't worry either way, especially not about the alternator. It contains diodes, which can be killed by voltage spikes, but an ECU would be killed long before!
One possibility is the resistance of the shell causing a difference in potential between the welding point and the ground clamp. Depending on the topology of it, this could increase or decrease the effective voltage seen by the alternator. You'd have to be welding somewhere between the engine earth strap and the battery earth for this to happen though, and on about 90% of cars these are either on the same bolt or through the same wire.
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Last Edit: Jul 15, 2009 15:27:44 GMT by cobblers
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Jul 15, 2009 15:53:19 GMT
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Well i sent me buddy a question about it, he loves a good diagram and technical explanation so if he knows i expect i'll get some good info.
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Volvo back as my main squeeze, more boost and some interior goodies on the way.
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Jul 15, 2009 16:15:22 GMT
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I remove the battery as they can explode if hot elements touch them, but thats not what your asking. Removal of the clamp is done to not knacker the battery, however like yourself I never have seen or had this done to me (pretty much the same as dicsonnecting the alternator due to it papping them). its the gas escaping from the battery that explodes. where i used to work we had batterys that had been in massive fires come back that still worked fine and were run through a massive test to check them for warranty. with regards to running to many amps through them, a short burst shouldnt hurt. a long load or charge from it. id also heard that it protects the alternator. although a friend tryed weldling his car with an arc welder and managed to permanantly engage the starter  |
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Jul 15, 2009 16:21:27 GMT
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Ok heres the story from a man who welds cars for a living, building road and race cars, he says
"alternators can have the regs blown up by welding at high ampage and diodes are prone to popping
more so on older cars that don't have complicated electrics
and an arc welder will do more damage"
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Volvo back as my main squeeze, more boost and some interior goodies on the way.
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Jul 15, 2009 16:23:26 GMT
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not the battery that is ruined its the electrical components weve had apprentices that have wrecked ecu's airbag moduals ect
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Jul 15, 2009 16:30:44 GMT
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Doesn't electricity take the path of least resistance,and wouldn't that be the welders earth clamp?
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Jul 15, 2009 16:58:28 GMT
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Doesn't electricity take the path of least resistance,and wouldn't that be the welders earth clamp? yes |
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Jul 15, 2009 17:04:29 GMT
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Doesn't electricity take the path of least resistance,and wouldn't that be the welders earth clamp? yes It has to then go through the shell, and while the majority takes the path of least resistance, it doesn't take just this path. Imagine a circuit with 5 resistors in parallel, 4 are 20 ohm, and one is 10. Overall this is 3.3Ohms. Imagine 1 amp flows from end to end, this is split up over the resistors proportionally (0.167A per 20 ohm resistor, 0.33A per 10ohm resistor) This sort of thing works over a flat plate of metal too - Directly across the shortest path between the two contacts is the lowest resistance, but the whole plate is used as a conductor. |
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Jul 15, 2009 18:31:49 GMT
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I've heard of black boxes (bike) being fried, and a friend had a battery explode (spark ignited gas sat in the vent tube), but never experienced it - but then I've always removed the battery and put it out of reach. Same with grinding. No sparks near batts.
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'66 Amazon <-> '94 LS400 <-> '86 Suzuki 1135 EFE
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Jul 15, 2009 18:34:24 GMT
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I've heard of black boxes (bike) being fried, and a friend had a battery explode (spark ignited gas sat in the vent tube), but never experienced it - but then I've always removed the battery and put it out of reach. Same with grinding. No sparks near batts. Batteries do genuinely make bad gas - A lad at work didn't believe me, so I put a latex glove over the vent on one while it was charging. Only had about a gobful of air in, then held a match to it. The bang surprised even me, and I was fully expecting quite a boom! EDIT: I took the glove off the battery first! |
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Last Edit: Jul 15, 2009 18:39:21 GMT by cobblers
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