The internal resistance (IR) of a battery is defined as the opposition to the flow of current within the battery.
There are two basic components that impact the internal resistance of a battery; they are electronic resistance and ionic resistance. The electronic resistance plus the ionic resistance will be referred as the total effective resistance.
Real batteries are constructed from materials which possess non–zero resistivities. It follows that real battery is not just a pure voltage source but has an internal resistance. Incidentally, a pure voltage source is usually referred to as an emf (which stands for electromotive force). Of course, emf is measured in units of volts. A battery can be modeled as an emf connected in series with a resistor (which represents its internal resistance). A standard dry cell is usually rated at 1.5V and 100mA. Thus, nothing really catastrophic is going to happen if we short–circuit a dry cell.
On the other hand, a car battery is usually rated at 12V and something like 200A. It is clear that a car battery must have a much lower internal resistance than a dry cell. It follows that if we short–circuit a car battery the result would be fairly catastrophic.
The internal resistance of a cell depends upon the following factors :