There is two ways to measure the internal impedance of a cell (battery), one using DC current , the other AC current .
The DC way can be done in charge or discharge mode , with 2 constant current of C/20 and C/2 rate( this can be changed ) , measure the cell voltage, difference between the two readings , divided by the current variation will give internal resistance , when using DC method , it is preferable to do it when the cell ( or battery) is fully charged , doing at a quick rate so to minimize internal heating occurring from current flowing ( remember W=I*XR ) .
The AC current is easier on the cell since it is performed with a very small constant AC ( 5 -20 ma )current at 1 Khz the power is limited to about 2 microwatts (yes ! ! micro) impedance is obtained by divided the ac voltage dropped across the cell by the current flowing through it ( very small current has not detrimental effect on cell ) .
The AC method is preferred since it can be done at any state of charge of the cell and left on indefinitely without doing any damage , however measuring ac at those very low level is difficult to say the least .
The impedance of battery or cell can be measured using a HP4328A milliohmmeter , this instrument is the standard one used by most battery manufacturer ( check their spec sheet) have recently seem several units sold on E-Bay from $100.00 to $ 800.00.( Newer Agilent digital version can be obtained for over $5,000 new , 2-3,000 on e-bay HP4338B)
Have been using one for over 25 years and would never consider working on battery be without it .
It is the first measurement that I make to determine the cell state of health .Regardless of cell chemistries ,volt, amp/hour rating , the impedance is the determining factor how fast you can move the energy in/out of the power pack .
Impedance becomes of much less importance when charging and discharging occured over long period of time ( anything less than C/20) such as 20 hours or more .