What battery charger for winter storage?

I guess as I get older I get "ornery"!. I hate the fact its hard for me to find a 5 yr battery that will last that long. the " guarantees" for that 5 yr is a joke!???
I have 5 old mopars that I am either working on or driving some during winter. s w Missouri roads are dry and salt free in the country here 99% of the time.
I have an open shed, 2 sides completely open. I never tried those slow chargers. from what I gather here, if I place it on a battery one day a month, they will stay up, or should I just place charger on there for full charge every so often?????
I try to keep bout 3 batteries on hand for the old mopers.... then the cummins has 2 ,, the tractor has one BIG one! need bank loan to keep enough batteries!!!??