Disconnect battery before welding. URBAN MYTH?

Its not the battery your protecting, its the alternator.

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.

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.

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.

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.

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

not the battery that is ruined its the electrical components weve had apprentices that have wrecked ecu's airbag moduals ect

yes

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.