When I was young I got a good burn on my skin pinching a tiny bit of my finger between wires hooked up to a 6v remote control car battery.

Smelled awful and had a brown patch a few mm wide on my finger for a while.

This happened more or less instantaneously.

When I was a kid, I stuck a 9V battery across my braces (top to bottom teeth). Just about knocked me out.

I was thinking on a longer distance, like between left and right hand, or between a hand and a leg (which I still think is near impossible to cause any harm with 12 volts). But surely it can happen on a distance of less than few cm, when the + is in a very close proximity to the - (touch here to get your fingertip heated!).

Doesn't need to be a terminal, touching a broken wire with sweaty hands would be enough to induce flash evaporation of fluids on your skin.

not with 12V. 12V batteries are pretty safe for human skin contact. If you put your wet tongue across a 12V battery it will hurt, but not more than that (it's a good way to test 9V batteries actually)

Well, if you put your tongue on it, you'll obviously have a lot more distance to cover and a lot more fluid to handle the current.

Batteries in general are fairly safe because their voltage breaks down at high loads. The chemistry can only react so fast internally and is controlled by various factors.

Contrast this with capacitors, where there is always a general safety warning when handling them in live circuits. They have no limit based on chemistry.

You don't want to test a capacitor with your tongue. A capacitor has no other current limit than it's internal resistance, with a modern sub10mOhm ISR a capacitor can, for the briefest of moments, discharge at up to 14 Kilowatts, quite the upgrade from the 14 measly watts that your spit will handle. A 100 Millifarad Low ISR Cap can sustain a kilowatt of output for almost 0.1 milliseconds, at which point you'll likely regret the choice of mishandling a cap. (It's only 14 Joules but these 14 Joules will be moving very fast)

only on HN..