The credit belongs to Bill this time.
The circuit to which you connect a battery obeys Ohm's Law:
I = E / R
where I=current, E=voltage, and R=resistance.
The voltage, E, is the EMF supplied by the battery, in this case 12 volts.
The current is then inversely proportional to the resistance of the circuit, i.e. the lower the resistance the higher the current.
Note that the current is determined by the resistance of the whole
circuit, which includes not only the resistance of the load and connecting wires but also the internal resistance of the battery.
A typical transistor-radio battery might have an internal resistance of, say, 3 ohms. With 3 ohms accounted for inside the battery itself, the maximum current you can draw is limited to:
I = E / R = 12 / 3 = 4A.
If you connect the radio or a bulb to the battery, then the resistance of that load is added to that 3 ohms and the current will be much less.
But if you put a dead short across the battery terminals, the current is still limited by that 3-ohm internal resistance to a maximum of 4 amps.
A lead-acid battery, on the other hand, has a chemical composition which gives it a much lower internal resistance. maybe in the region of 0.01 ohm.
If you connect a bulb to the battery, the current is still restricted by the complete resistance of the circuit, including that of the bulb.
But if you short the car battery, you have only the internal resistance of 0.01 ohm to limit the current:
I = E / R = 12 / 0.01 = 1200A
So you have two batteries, each supplying the same EMF (voltage), but the difference in internal resistance means that one is capable of supplying much more power than the other:
P = I x E = 4A x 12V = 48W
P = I x E = 1200A x 12V = 14,400W
In practice, there will be some resistance in whatever object is shorting the battery, so the maximum current will be a little less, but you get the general idea.
That 14kW or so of heat will be partly dissipated by whatever is causing the short (e.g. a wrench, or a bracelet on your wrist!) and partly inside the battery itself.
If the battery gets hot enough, the case can crack and result in the spillage of acid.
[This message has been edited by pauluk (edited 11-04-2003).]