Many people have the impression that when batteries sit on concrete, energy "leaks
out" or they are ruined. The short answer is that letting modern batteries sit on
concrete does not harm or discharge them in any way.
However, this legend is historically based in fact. The first lead-acid batteries consisted of glass cells that were enclosed in tar-lined wooden boxes. A damp
concrete floor could cause the wood to swell, breaking the glass inside.
The Edison cell (i.e. the nickel-iron battery) that preceded the rubber-cased battery was encased in steel. Those that weren't isolated in crates would discharge into
concrete quite easily. Later battery cases used primitive hardened rubber, which was somewhat porous and could contain lots of carbon. A moist concrete floor combined
with the carbon in the battery cases could create electrical current between the cells, discharging them.
None of this is a problem with modern batteries in their hard plastic shells. In fact, concrete is generally an excellent surface on which to place a battery. The
electrolyte in a battery sitting on an extremely cold floor with very hot air around it could stratify, causing damage from sulfation; whereas concrete provides good
thermal mass to buffer any temporarily extreme temperatures in the battery compartment.