I just got a battery conductance meter..to test Internal resistance of batteries, based on a AC kelvin bridge 100mA 1khz sender and 1khz voltmeter, measure range 0.1mOhm to 20Ohms.
measued my surrette S600 battery (6 volt 600ampHr) get a reading of 6.2m Ohms
100AH 12 volt AGM get reading of 24m Ohm.
other than data from vendors, is there a rule of thumb for lead acid batteries amp-hour and its Internal resistance ??