Suggestion.. Get either two bottles of Nail Polish -OR- two small cans of spray paint (or two large cans) one RED one Black
With the batteries PROPERLY connected spray the positive terminal with RED (or brush it on) Overspray onto the battery is a good thing if spraying, Dripping red paint (or just flat brushing it on) the battery likewise.
Use black on the Negative end
now red painted wire to red painted battery terminal. Black to black, Unpainted to Unpainted and your job is always going to be right.
Now,, the winter storage.. Did you use a Battery Minder (TM) or Battery Tnder (TM) or a trickle charger (Junk) to maintain the batteies?
There are several different devices.. When I waS a child our only choice was a trickle charger, these low-amprage chargers had no regulation and depending on teh size of the batteires and the size of the charger either over charged or failed to maintain (IN THEORY you might hit one that worked 100% but odds are against it like 99.9 to 0.1)
Today we have two more editions
One is an automatic slow charger,, It charges 'em up and shuts down
The other is the Battery Minder/tender 2-stage automatic charger (Absorption/float) these continuously monitor and will switch back to ABSORPTION (Charging) if necessary.
READ the instruction manual because what I posted is a generality.. not all products work as well as others.
But do paint both the wires and the battery in the area of the terminal (or and the terminal itself) to insure red to red.