1989 420SEL Dead Battery

[Cigar Havana]

1989 420SEL with 300k km has dead battery, after sitting off for 2 days.

Charged the battery with charger for 20 hours. Battery reads 13.4v with key off. Put an amp meter around the neg. lead at the battery, shows 1 amp drain, when key off. With engine running, shows 14.1V at the battery, 15 amp on the neg. battery lead. Alternator belt is not loose.

Battery was new in Dec. 2013, alternator (bosch re-built)was replaced 6,000 km ago. The car did sit throughout the winter Nov. 2014 to April 2015 in an unheated garage, never turned on. The battery was disconnected for the period, put on a timer, on an automatic intelligent battery charger, and turned on for 30 min. every 24 hours during the winter. This is in Toronto Canada so there were some nights as cold as -30c. When the battery was reconnected and the car started immediately and the charger showed only 1.7 amp to charge the battery.

Where should I start the diagnosis?