My weird but somehow productive technique is to use a C-Tek smartcharger once a month during storage. With no charger and a Spring boost from car my bike batteries were lasting 2-3 years. With a C-Tek smartcharger plugged in often my batteries were lasting 5 years. In 2012 I bought the Mighty Max for $50 and it was still OK but I decided to replace it last year. My Benz battery using the same procedure also lasted from 2015 to now.
No idea why but i am surely sticking to that procedure and just charge the stored batteries the first of every month.