internal resistance of accumulator (luminous 12V 100Ah)
To get the internal resistance of a battery, measure its voltage at two different currents.  The resistance is then the change in voltage divided by the change in current between the two operating points.

However, the internal resistance of a battery is not a fixed quantity.  It varies greatly by state of charge.  It also varies somewhat by operating point.  For example, using 1 A and 2 A discharge may result in a different internal resistance value than using 1 A and 5 A discharge, or 1 A and 3 A charge, or ...

At best, you can get a rough idea.