I promised to get back once I purchased a new Di2 battery. Installed the new battery, but the problem persisted. So it was not the battery. I then started to isolate every part of the Di2 system (rear derailleur, front derailleur, left shifter, right shifter, etc) by de-connecting the cable and then monitoring whether the battery was still draining within 24 hours. The method succeeded, and I found that the batteryholder was defect. Replaced the batteryholder, and the problem was solved. Shimano kindly offered to pay for the replacement battery holder.