I've maintain a number of vehicles that are seldom used...
The longest I had a car parked and it started was 13 months for a 72 Plymouth Valiant... it wasn't planned that way... it just was over a year before I got back to where it is stored.
My brother and another friend of his have late model Chryslers and on several ocasions, when the cars were still new, the batteries would not hold enough of a charge in as little as 3 weeks storage to start.
Dealer said this is normal and they even sold Tenders at the parts counter... he bought the one with the cigarette lighter adaptor...
Fire House also keeps chargers plugged in at all times in the station house... these trucks are on the go a lot... only takes someone leaving on a light or radio to cause a no start condition...
I must own more than 20 chargers with most being tenders... 6v, 8v and 12v