What is the impact of deep cycling (high depth of discharge) on the lifespan of a lead-acid battery?

Deep cycling, which involves repeatedly discharging a lead-acid battery to a high depth of discharge (DOD), significantly reduces its lifespan. The depth of discharge refers to the percentage of the battery's capacity that is discharged during a cycle. High DOD means a large percentage of the battery's stored energy is used before recharging. Deep cycling accelerates several degradation mechanisms within lead-acid batteries. One primary mechanism is the shedding of active material from the positive electrode (lead dioxide). During discharge, lead sulfate is formed on the electrodes. With each cycle, particularly deep cycles, the formation and dissolution of lead sulfate crystals cause mechanical stress on the electrode material, leading to the shedding of active material. This reduces the battery's capacity over time. Another factor is sulfation, which is the formation of large, irreversible lead sulfate crystals on the electrodes. Incomplete recharging after a deep discharge can lead to the accumulation of these crystals, reducing the active surface area and increasing internal resistance. Stratification of the electrolyte, where the acid concentration is higher at the bottom of the battery than at the top, also becomes more pronounced with deep cycling. This uneven distribution of acid can lead to accelerated corrosion and reduced performance. Due to these degradation mechanisms, lead-acid batteries subjected to frequent deep cycling have a significantly shorter lifespan, measured in the number of charge-discharge cycles they can endure before their capacity drops below a specified threshold (typically 80% of their original capacity), compared to batteries used in shallow cycling applications, where the DOD is much lower.