Downside to leaving even fully charged batteries aboard over winter storage
period is freezing. If temps get low enough(?) and a battery charge becomes
low enough the electrolyte will freeze. This often leads to a fractured
battery case. When the thaw comes the electrolyte gets everywhere in the bilge
you don't want it to be. This leads to a major clean up in the Spring plus the
time and trouble to replace batteries.

Saw this happen a few times at our marina in Northern Virginia so I always
removed my batteries after hauling the boat. Now that I live on the Gulf Coast
I no longer remove the batteries but I boat pretty much year round here.

IMO, you're talking about really cheap insurance.

Butch