Several years ago I came across an ad for "Battery Minder" that has a pulse/desulfator in search to try and extend the life of the high price of my boat, 4-wheel & tractor batteries. The larger of them can maintain several at the same time "think electric carts.
After having bought the 1st I've bought two more to keep all my in storage batteries in check.
I can actually say that I get an additional couple years out of them, including the large and expensive tractor battery.
Thats my main go to with my occasional summer maintainer being the Battery Tender that I use on my trolling motor mostly then switch to the Battery Minder for long term storage.