Wet cell lead acid battery impedance testing does require measurements using low frequency AC (sine waves ... not square waves) voltages due to the fact that a lead acid battery is basically a very large capacitor storage device and hence their impedance value has an AC component to it. Currents drawn from batteries during testing must be set properly high enough (not micro-currents) by the testing loads used.
Here's a link to probably way too much information on lead acid battery testing as published by the manufacturer of Megger testing equipment - which includes battery testing products. Reading through their document confirmed my suspicions - lead acid battery internal impedance readings do indeed increase as the batteries age. Now ... if only we could easily monitor this increase as our RV battery(ies) age so we can know when to get rid of them before learning the hard way on a camping trip:
http://www.surgetek.co.za/items/btg100.pdf
