I just had the same issue with a interstate batt in my S600.
I made a adapter to fit the lighter socket and hooked my digital voltmeter up.
charging voltage was 14Volts. With engine off the voltage would drop at once to 12.1 Volt.. If I ran the heater on REST for about 8 min. the low battery symbol would apear on the heater readout. The voltage would drop to 11.55 volts.. I went to Interstate dealer and they put a fancy electronic tester on it and said the Battery is OK... but the voltage is about 1 volt low..They of course told me it just needed charging..BS
I argued long enuf and they did give me a new battery as it was only 8 months old.
All the problems went away.. Voltage now stays above 12.3 Volts with heater REST mode on for up to 25 min.