Batteries lose capacity as their temperature drops. Concrete is a good conductor of cold and aids in the capacity drop when in contact with the battery. As the battery capacity is reduced, sulphation crystals begin to grow and will eventally puncture the seperators shorting the positive and negative plates. The cold concrete floor may accerate this process due the the temperature remaining lower than on a board or a shelf. I'm not convinced of most other theories I've heard on this. Just my 2 cents.
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