Battery internal resistance forms a divider with external load.
So to calculate internal resistance, take those steps :

• measure the battery voltage without load (Vnoload)
• measure again with a resistor of RloadΩ attached (the lower ohm value, the better, but watch outfor power ratings !), you have a difference with previous measurement (Vdrop)

The battery internal resistance is then :

Rbattery = Vdrop*Rload/(Vdrop+Vnoload)

Real world example on AA :

Vnoload = 1.421v
Rload = 460Ω
Vdrop = 0.002v

Thus : Rbattery = (0.002 x 460)/(0.002 + 1.421) = 0.65Ω
Since my metter has a milliVolts resolution, we just have a range between 0.001 and 0.003 for Vdrop.
So Rbattery can be anything between 0.33Ω and 1Ω.

Short-circuit current for this AA battery would be somewhere between 1.421/0.33 = 4.33A and 1.421/1 = 1.42A
Mesasurement made, it is around 4.2A. Watch out for your metter current rating if you do this. Some batteries have a much lower internal resistance.