The chemicals react with the plates inside the battery to produce a flow of electrons. This flow of electrons constitutes an electrical charge. When a lead-acid battery remains unused for four months or more, lead sulfate begins to form on the plates. The sulfate gradually accumulates and crystallizes, clogging the porous plates until the battery no longer holds a charge. This process is known as sulfation.
Extreme temperatures affect both battery performance and engine start-up.
In other words, the time at which it's most difficult to start an engine is precisely when a battery might not be up to the task because it's not producing enough charge. To make things worse, both very hot and very cold temperatures require more amps to turn on an engine. Extremely cold temperatures reduce battery efficiency and may cause the water inside to freeze and expand, cracking the casing. Since concrete is a poor insulator, it can get quite frigid during the winter. A battery stored on a concrete floor could become cold enough to lose its charge.