|An Old Wives Tale|
But, I wondered, how could that be? It just doesn’t make sense because concrete is a mix of cement/gravel/sand/rocks. Besides, there’s a hard plastic layer between the floor and the guts of the battery, so how could a battery discharge?
A hundred years or so ago car batteries were lead-acid batteries that had glass cells all encased in a wooden box. If they were left on concrete or cement floors, the moisture from the floor could cause the wooden box to wrap, allowing the glass cells to shift and break. Battery acid would then leak. Moisture and wood were the culprits.
Car batteries evolved into a nickel-iron battery encased in steel with a hard rubber casing which meant the glass cells wouldn’t break, but the batteries still discharged. How was that possible? The rubber casing was porous and often contained carbon. The battery would absorb moisture off of the floor creating an electrical current between the battery cells causing them to discharge.
Modern batteries are encased in a hard plastic shell that eliminates the soaking up of moisture, so concrete floors are no longer cause for a car battery to go dead. In fact, nowadays cement and concrete floors provide a fairly good barrier between the battery and extreme temperature changes that could cause damage to the cells allowing for a discharge leak.
However, car batteries can still lose their charge in a number of ways, so they do have a shelf life.
Dirt and dust can become carbonized, creating electrical conduction which drains the battery. The top should therefore be kept clean.
Self-discharge occurs over time with lead-acid batteries when reactions within the plates happen as the battery ages.
Heat is a battery’s enemy. According to one manufacturer a car battery in a hot climate will last, on average, only two thirds as long as it would in a cold climate. When stored, the warmer the air surrounding the battery the faster the rate of discharge. Batteries should be kept in a cool place.
That's cool, not cold. A battery that is at a low state of charge and exposed to freezing temperatures can freeze causing the case to crack, so they should be kept fully charged.