Morning start problems after use of preheating

Hello guys (and girls), can anyone explain this? Do and Fr last week: 1x daily use of heating à ca. 35min with subsequent 2x 3 minute drive (acc. to my wife – yes – I know, you shouldn’t do it). Weekend: Vehicle is not used. Monday: 7 am start of heating via FB. 7:50 am car reached. Heating is off (ok). Start attempt. Nix happens. Message Battery voltage too low. Rest is clear: It runs in Kita / I will pick you up to work. Further start attempts fail. Normally SH does not start if juice is not enough to start the engine. Has definitely already worked for me. Inspection only 6 days ago. All i.o. incl. battery. Workshop on the phone at a loss – should not have happened. I should hang battery on the charger and watch in the future. Everything is no problem, only worries me that I could not be immune in the future…. Thanks for any tips. kleenerfaxe