All rechargeable batteries degrade over time. Lead acid and sealed lead acid batteries are no exception. The question is, what exactly happens that causes lead acid batteries to die? This article assumes you have an understanding of the internal structure and make up of lead acid batteries.
If lead acid batteries are cycled too deeply their plates can deform. Starter batteries are not meant to fall below 70% state of charge and deep cycle units can be at risk if they are regularly discharged to below 50%. In flooded lead acid batteries this can cause plates to touch each other and lead to an electrical short.
A lead acid battery left in storage at moderate temperatures has an estimated self-discharge rate of 5% per month. This rate increases as temperatures rise and as the risk of sulfation goes up. Sulfating: This is a buildup of lead sulfate crystals and it occurs when a lead acid battery is left sitting without a full charge.
In both flooded lead acid and absorbent glass mat batteries the buckling can cause the active paste that is applied to the plates to shed off, reducing the ability of the plates to discharge and recharge. Acid stratification occurs in flooded lead acid batteries which are never fully recharged.
If you are not familiar with lead acid batteries, see our article What is a lead acid battery. Ironically one of the most common reasons for battery failure is not an actual failure of the battery itself, it is people thinking the battery is dead.
Since that is no longer an issue (and never was an issue with lead acid batteries) there is not a need to fully discharge. By discharging a lead acid battery to below the manufacturer’s stated end of life discharge voltage you are allowing the polarity of some of the weaker cells to become reversed.