The fundamental problem faced by lead acid batteries during their “life span” is corrosion of their positive electrode grids.  Basically, all batteries generate electricity by setting up a chemical reaction.  In very simple terms, the positive grids within batteries are designed to collect electrons released during the chemical reaction.  (Incidentally, in early lead acid batteries, these were solid plates rather than grids.  Grids came into use because they require less material to manufacture and because they expose more of their surface area to the chemical process.)  The electrons thus collected are then carried through the grids to the battery’s positive terminal.  The problem is that the grids, which are made of lead, are forced to carry out their task in an incredibly harsh and corrosive environment.  The grid corrosion that results will inevitably become the failure mode for the battery once it reaches the point where it interrupts the critical flow of electrons to the terminal.  To make matters worse, many lead acid batteries, like those in automobiles, are required to perform under extremely high temperature conditions, and heat accelerates the corrosion process.