ELI5: Why is a 12V battery in a car so dangerous to short circuit but a normal 12V battery is harmless?

Adding to other answers:

Batteries have something called “internal resistance”. This acts like a small resister that the current has to overcome to flow, in addition to the circuit.

In a AA battery this is somewhere in the region of 0.5 ohms plus/minus a bit. In a car battery it’ll be in the region of 0.02 ohms. Car batteries need a low internal resistance so they can put out a large current (~200 amps) when starting the car. Hook a good conductor up to the positive and negative terminals and the car battery will churn out ~25x (maybe more) current than a bunch of AA batteries lined up to get 12V. This much higher current causes much higher heat and energy discharge that can result in dangerous overheating of the battery and/or cables.

A car battery can’t put 200 amps through you, because your resistance is waaaay over a few ohms. Despite the huge sparks you’ll see if you hook a car battery up to a wire you can touch the terminals without being in danger - if your hand is wet you might feel a tingle if you lay your hand across both the positive and negative terminals at once. Same as if you line up 9 1.5V AA batteries.

/r/explainlikeimfive Thread