Why does a car battery make such violent sparks, yet it it's safe for me to touch both terminals? YouTube videos have assured me that it's safe to touch the terminals of a 12V AGM car battery with my bare hands. They even claimed it'd only be a tingle if I connected wires to the terminals and touched them to my tongue.
However, when replacing my 12V AGM car battery recently, there was a fairly loud, bright and violent spark when the negative terminal temporarily re-made contact with the car's negative terminal connector.
How harmful to my hand could it have been if my hand was holding that terminal while it sparked, and what specifically determines the heat/danger associated with such sparks?
 A: It is only safe with respect to electric shock, because the battery voltage is too low to cause an electric shock under most conditions.
The potential harm to your hands is a burn injury due to $I^{2}R$ heating and the resulting high temperatures that can occur if, for example, the terminals are shorted by a conductive object you may be holding or in on your hand. A burn injury can also occur to your tongue if, somehow, your tongue shorted the terminals (though at the moment I can't imagine how).

I'm unclear about how long it would take for the conductive object to
heat? If there was short circuit with a spanner, how long until that
becomes hot enough to burn me? If a metal ring on my finger completed
a short circuit, would I instinctively pull my hand away long before
it got hot enough to burn me?

A wrench (American for spanner) is pretty massive so it has a high heat capacity. It would take a while for its temperature to get high enough to cause a burn. Regarding the ring on your finger, you might be able to pull your hand away before it gets burned. But if the heat transfer rate to your skin is high enough, you may get burned before you sense the high temperature. That’s what happens if you accidentally touch an electric stove top. You get burned before you are able to sense the heat.
Hope this helps.
A: A car battery can drive a lot of current through a low resistance connection, and that can make sparks, set things on fire, etc. The resistance of human skin is relatively high, so the car battery doesn't have the voltage required to drive a hazardous current through your hands. But don't wear a ring when working with a car battery.
