In the old days of lead acid batteries, the case was made form different material than today's batteries. Although some may have a different opinion, todays batteries are fine sitting on concrete. That said, are you pulling out your batteries during off season storage. I haven't done that in many years. It's too mush work and unless they would be subjected to hot temps during storage, they will hold up fine. If your batteries will be subjected to freezing temps, either in your boat or your garage, they must be fully charged since discharged batteries (lead acid) freeze at higher temps than fully charged ones.