I think your assumption is spot on. I had exactly the same problem with my E320CDi, the car is 6 yrs old and I knew the battery was starting to fail. The only problem I had with the car was the cruise would not set after initial start-up. After finishing that first journey the cruise would set after restart.

After I replaced the battery this problem disappeared. I can only assume that the battery being slightly undercurrent caused the ECU to close the CC down. Once the battery had regained charge it would work.

There may be some reset procedures for the electric windows, sunroof and ESR etc when you reconnect the battery, your handbook will confirm.

Hope this helps

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00 320CDi Avantgarde Estate