Like a modern mobile phone, a modern electronic car NEVER goes completely to sleep, UNLESS the battery has to be replaced. Even my 20 yr W210 and W202 are like this. The implication of this is that
there is ALWAYS a small current drain on the battery, even when the ignition is switched off and the car garaged.
I drive my W210 only about one every three weeks. To get around a battery drainage problem, I trickle charge the battery about once in every 4 to 6 days, with a modern toroidal ring battery charger.
(You may say, I kid my car, that it is being driven regularly, about once per week.)
As some of us may remember, I went to Europe for 3 months (Aug Sept Oct 2018).
In this case, I fully charged both batteries, then immediately disconnected them, so that they had reasonable charge on my return, and to save them from total discharge and remain flat, which can severely shorten their life.
I was warned before I left, that some of the electronics systems in both cars were not going to work, unless the cars' electronics were re booted, like a computer. THAT indeed, proved to be the case. A re booting procedure can be found on the Internet, and was followed.
With careful battery charging maintenance, I am able to get around 7 years life out of a standard medium quality battery, even with cars that are driven a lot less than the manufacturer envisaged.
for the past 5 years. currant battery is less than 2 years old. There is a currant draw with everything off over night. Heater fan motor was running once with everything off ??? I will have the battery tested this morning ....but where to then ?
