Ri of those is typically 1 Ohm or thereabouts, which at 500 A develops 500W in the battery itself and a large multiple of that in your 'load' (in this case your soldering iron :) ).

When working with large battery arrays I use tools that are taped in all the way except for the business end, just in case. All you need to do is drop a wrench in the wrong spot and it's party time.

Not 1Ohm, in my case the array had an internal resistance of 15 milliOhms, which is way more fun. Though I imagine the BMS and fuse were relatively effective resistors in that situation too.

Agreed about taped tools, I figured that one out right after replacing the fuse :)

Oh that's really danger territory.

A BMS typically has a small shunt that helps to figure out the state of charge as well as a large transistor in series with the current to allow switching the battery in and out of circuit.

Yes, I had an additional fuse in series, what I was referring to is that the BMS's own internal resistance is larger than that of the battery pack. Indeed in an overcurrent situation you can't rely on the MOSFET not to fail short.

I've seen fets in BMS in Bosch packs cracked in half, top blown off. Quite impressive.