Measuring internal resistance of a battery while it's being charged
The internal resistance of a battery can be determined with AC techniques. One method is to inject a current into the battery using a transformer and a decoupling capacitor to isolate the battery voltage from the transformer.  Then the AC voltage across the battery is measured.  The resistance is calculated by dividing this voltage by the driving current. A description of this method is given at http://www.cdtechno.com/pdf/ref/41_7271_0512.pdf. This is done with the battery being charged.