better driveability after battery change

I wouldn't discount the possibility that the alternator was being overworked and/or the ground connection was poor. The purpose of the adaptability of the ECU is, of course, to attempt to compensate for variations in sensor wear, fuel octane rating, altitude and temperature. maybe something else I can't think of.

Carl

The ECU is continuously doing "adaptive learning" to maintain the correct air/fuel ratios and timing with minimal knock. It also learns the values for the idle controller to maintain the proper idle speed. All cars do this, not just Subaru. When you disconnect the battery the learned parameters are cleared and reset to their default values, so the process has to begin all over again. You may notice the car runs differently for a while until the ECU has re-learned. This is a dynamic process, which allows the ECU to compensate for aging components and changes in the environment (temperature, barometric pressure etc.). It is also possible that if your old battery was weak it was placing a noticeable load on the alternator.

I'm starting to believe this a little more. The voltage regulation was not as good with my old battery. When the engine was at idle, the headlights were a little dimmer than at any other RPM. I could notice this at stop lights. When I took off again the lights would get brighter as soon as the RPMs went up. It also happened when simply reving the engine while stopped. I could even see my dome light flicker if I had the hazard lights on!

... the mood of your significant other. If the passenger airbag sensor detects too much fidgeting it turns airbag off and reprograms dual zone control to blow cooler air and turns on seat cooling, er, the latter was Mercedes option. My fault.