I have an extra battery with a trickle solar charger. If it's never used how long will it last? It's only for backup on my pontoon.
Boats & Motors
Battery lifespan
My last batteries on my Ranger boat were 8 years old when I traded it in. There were 3 trolling batteries and a starting battery. All of them were agm and plugged into a dual pro charger after every use and all winter long. The boat was stored in an insulated garage with intermittent heat.
My deep cycle batteries would last 2 yrs. I do use it as an outboard starting battery but it is only 25 horse so I dont think there is any harm there. Now days I trickle charge it several time every winter and it makes them last about 1 yrs longer, so I get 3 yrs out of them and that does not seem very long at all. A guy that worked at Johnson Controls gave me a deep cycle battery once and it lasted 4 yrs. It was one of the good ones. I wish I would have taken better care of it, by charging it through the winter here and there. I always forget to check the water cause I thought those days were over, but I guess not.