Question: “Why do they say not to put a car battery on a concrete floor?”
Kelly La Rue provided a good answer to this, quoting from Snopes, Car Talk, and Todayifoundout, explaining that the hard rubber cases of which batteries used to be made, being somewhat porous, could permit seepage of the electrolyte to the concrete creating a conductive path that would cause the battery to discharge.
Let me add a bit of perspective. The caveat not to put an automobile battery on a concrete floor is a LOT OLDER than the people at Snopes, Car Talk, or Todayifoundout realize. Although inapplicable to today’s polypropylene battery cases, the warning used to be sensible. And, before either polypropylene or hard rubber were used for battery cases, those cases used to be made out of WOOD and used a rubber liner called a jar. Really.
I have Catalog No. 12, for 1922–1923, from the Chicago Automobile Supply Warehouse. Automobile batteries in the 19-teens and 1920’s were rebuildable. And they were expensive. Adjusted for inflation to 2017 dollars, a battery for a Ford T would cost you $274. A battery for a Mitchell Light Six would cost you $848. Thus, rebuilding them made sense.
One could buy replacement lead plates, treated wood separators, positive and negative plate straps (which electrically connected and placed a terminal on plate groups), cell connectors, covers, carboys of battery acid, terminal molds, battery “jars”, and wood battery cases. You could also buy battery paint, molds for terminals (which you would pour from molten lead), and battery steamers. The latter were devices used to disassemble batteries for rebuilding.
The cases were described as “made of either maple or birch lumber, 5/8” or 9/16″ ends with 1/2″ sides. Corners are fitted with dowel pins; bottoms are tongue and grooved with screws on sides.”
These wooden cases would, of course, be porous and could become electrically conductive if the electrolyte leaked through or around the rubber battery jar or through the battery cover which was sealed with tar, or leaked through the vents, or if electrolyte or distilled water were spilled on the battery.
But there was another, and probably much more important, reason why mechanics were warned not to place these wood cased automobile storage batteries directly on concrete floors: Uninsulated concrete wicks moisture up from the earth beneath the floor. It is always somewhat wet. And WOOD MOVES as it absorbs or loses moisture from or to the surrounding environment.
It moves A LOT! If placed directly on a damp concrete floor the bottom of that wood battery case would begin to absorb moisture. It would then begin to expand, but the expansion would be differential and uneven. The bottom side of the case in direct contact with the concrete would expand, in a dimension perpendicular to the grain, more than the interior side of the base. The battery’s base would thus become convex rather than flat, stressing and then breaking the tongue and groove joint with the battery end pieces. The expansion of the base would then push out the sides of the battery case, breaking the joints between the sides and the ends. The battery case would, thus, self-destruct, simply because of differential moisture absorbtion, BECAUSE IT WAS PLACED ON A CONCRETE FLOOR.
Considering how expensive these batteries were at the time wooden battery cases were used, how tedious the rebuilding process was, and how much damage could be caused to the car by leakage of battery acid, it is much more likely that wooden battery case failure, rather than electrolyte seepage and accidental electrical discharge, was the primary reason mechanics were instructed to not place an automobile battery directly on a concrete floor.
As battery cases changed from wood to hard rubber and then to polypropylene the reason for not placing such batteries directly on concrete floors became less important and then eventually disappeared altogether. But the change from wood to rubber and then to polypropylene did not occur instantaneously. Wood battery cases were for a time being sold along side of hard rubber battery cases. And, as these batteries were designed to be rebuildable, and garages had the equipment for doing so, wood battery cases were used for a long time after hard rubber case became standard. Thus, this heuristic, this rule-of-thumb that said, “Don’t put a car battery on a concrete floor.” remained valid. It is always safer to teach an apprentice “Don’t do it.” rather than to try to teach him “Don’t do it…except in these cases. And then it is OK, maybe…if you are sure.” Moreover, once such a heuristic becomes common within a trade, it is repeated even after the original reason for it has disappeared. And, when the rule-of-thumb seeps out into the general population, people who do not have the technical knowledge which would enable them to understand the reasoning behind the rule tend to repeat the rule and turn it into an old wives’ tale.
This, interestingly, is an example of researchers (in this case Snopes, Car Talk, Todayifoundout) stopping prematurely at the first plausible explanation (one that would be found in their own lifetime or one they learned from the nearest prior generation) rather than pursuing their search back beyond that to find the original source or reason. It is a very common research error.