McGyver Soldering Iron...

[jrsh92]

I will agree with what most people here have said, and not be worried about the risk of electrocution from this trick.
High current through your body is technically what makes a shock dangerous and maybe fatal. However, high current does not just happen because a car battery has high current capacity... if there's only a 12 volt charge across the battery, the current through your body if you were to touch one pole with each hand will only be 12 volts divided by the resistance of your body. No matter which way you look at it, if you became part of this circuit (another way of saying "touched the clips the wrong way") there would not be enough voltage to put current through you. The amount of current flowing through that soldering tip doesn't actually matter... nor does the fact that the battery could probably put out even more current still. The voltage combined with current is enough to melt solder, but the voltage just isn't enough to create much current through a human being.
Think of it this way: a 9 volt battery won't kill you-- I think everyone can agree on that here. Note that a 9 volt battery isn't safe because it can't put out the 30 ma or so required to be dangerous. If you put a piece of solder between the terminals of a 9 volt, it would likely spark and melt just like this... but it's safe because 9 volts simply is not enough voltage for the battery to put any current into a high resistance object like your body!
The 12 volts of a car battery isn't much more than the 9 volt you'd use in a smoke detector. Instinctively, it seems scary because the amount of power and current it's capable of can create huge sparks and melt solder, but 12 volts of electricity can't make much current flow through your body-- even if the battery is capable of putting lots of amps into a piece of solder!

 

[jrsh92]

To give you a better idea of why a 12 volt car battery is unlikely to give a fatal (or even very painful) electric shock... I've seen some estimates saying that a person with dry skin is close to 100,000 ohms while wet skin can be as low as 1,000 ohms. What this means is that in the worst case scenario a car battery could put as much as 12 ma through your body. This wouldn't be pleasant and it could make you hurt yourself if your arm jerked or you jumped back and ran into something-- this is one of the more common ways minor shocks hurt people. So to start out it's probably not going to kill you and let's be realistic here... hopefully you're not soldering soaking wet! So you're going to experience something between 0.1 and 0.2 ma from a car battery. You'd feel that, well, about as much as you feel the shock from touching both ends of a 9v. Almost none at all. There is danger from a car battery if you weren't quite dry and you touched those clips, but that danger is more from you jumping and crashing into something when you get shocked than it is a risk of death by electrocution!