Automotive batteries stored on a concrete floor do not discharge any faster than they would on other surfaces,in spite of worry among Americans that concrete harms batteries. Early batteries might have been susceptible to moisture from floors due to leaky, porous cases, but for many years lead–acid car batteries have had impermeable polypropylene cases,and are maintenance-free, so they do not leak battery acid.
I must have missed this one when my mother was telling me not go swimming after eating...