[phkahler]

Load dump is not about connecting two batteries in series to achieve 24V. It's about (accidentally?) disconnecting the battery while charging from the alternator - which is an inductive source. When the load taking the current is disconnected that current is going to continue flowing through the vehicle electrical system which will cause a voltage spike until the current goes down.

On a related note, I heard this 2nd hand from a guy who was talking to CAT about a module they wanted to have 80V over voltage protection for a crazy amount of time (minutes). My friend asked what scenario required that and it was something like this: Guy at a construction site can't start his bulldozer. He doesn't have jumper cables, but he's got an arc welder... I don't know if that actually happened but it would not surprise me.

[CamperBob2]

Yes, that was a weird mistake for someone in the author's position to make.

That said, I may not understand what a 'load dump' condition is, myself -- I always thought it referred to the common case where a high current load is switched off abruptly, causing the voltage to spike until the alternator's regulator circuit can respond by reducing the field coil current.

Disconnecting the battery would not be expected to cause such a condition, since the charging current is not all that high most of the time compared to other loads. I wouldn't expect much of anything to happen when disconnecting the battery from a running car. It should just keep running.

[neuralRiot]

In old cars it was a quick test to see if the alternator was working so some people still think that this is a good idea, IT IS NOT! newer alternators are controlled via LIN bus by the BCM so to generate more current when needed not just using battery voltage as a reference, if you disconnect the battery the voltage can go up to 30v even a bad battery or one with very low charge can make the charging system either shut off or go nuts.

[icefo]

The battery acts as a big capacitor to smooth the voltage and depending at which speed the alternator is turning I'd expect the voltage to vary wildly. This may make weird things happen especially on newer cars

[upofadown]

Just to expand on this a bit. The load dump is not directly caused by the inductance of any component. That would only produce a very short voltage spike that would be easy to suppress. A load dump can involve greatly increased voltage for up to half a second.

Alternators tend to produce a constant current in proportion to the field winding current (the variable the voltage regulator controls). When you disconnect the battery the alternator attempts to maintain that current by increasing the voltage. The voltage regulator has to reduce the current in the field to close to zero to stop this voltage surge. The field has some magnetic energy (inductance) that the voltage regulator has no good way to get rid of quickly. So the output voltage zooms up to maybe 10 times the normal voltage for an extended period and any electronics in the car might be very sad.

[stergios]

Mig welders are used as temporary power supplies all the time. They allow for variable DC voltage (some offer AC), current, wave forms, and the bigger one have close to 100% duty cycle. Here's a guy using a mig welder to power a 15 ton radial arm dill. https://www.youtube.com/watch?v=2zYrabDGIEg