Sorry Ghart, have to call BS on this one! I'm cut and pasting right off of the Interstate Battery website.
Will storing my battery on concrete drain the charge?
No. Regarding today's batteries, this is a myth. A battery placed on concrete will not discharge any faster, but a battery will discharge over a period of time wherever it is placed. If the battery has a surface layer of acid or grime which is conductive, the battery will self-discharge more rapidly than if it were clean and dry.
This myth does have some historical basis. Many years ago, wooden battery cases encased a glass jar with the battery in it. Any moisture on the floor could cause the wood to swell and possibly fracture the glass, causing it to leak. Later came the introduction of the "hard rubber" cases, which were somewhat porous. A current could be conducted through this container, which had a high carbon content, if the moist concrete floor permitted the current to find an electrical ground. The wise advise of the old days to "not store batteries on concrete" has apparently been passed down to us today, but it no longer applies.