Battery Failure Mode: Negative Plate Sulfation
When a lead-acid battery is left to self-discharge (in storage or installed but seldomly used) or is exposed to excess and repeated high-rate charging (such as is the case with Start-stop vehicles), a point can be reached where the reaction at the negative plate that should convert the lead back to active material (PbSO4 back to Pb) can not accommodate all of the charging currents. In this case, the excess electrical current escapes and causes hydrolysis, where water is divided into hydrogen and oxygen, which escape as evaporation.
This inefficient charge acceptance occurs almost exclusively at the negative plate, where the surface area of the active material is much lower than that of the positive plate. This negative accumulates lead sulphate (Sulfation) on the negative plate. This sulfation of the negative plate will cause battery performance to decline incrementally and result in premature battery failure.
A battery with highly sulphated negative plates will eventually only accept a surface charge, resulting in a false positive high state of charge readings. In this condition, a battery may appear fully charged but have very low capacity, as expressed in Amp Hour (AH) or Reserve Capacity (RC). This false state of charge reading tricks modern vehicle charging systems into thinking the battery is more charged than it is and leads to 1) batteries always being in a Partial State of Charge (PSOC) condition and 2) increased alternator wear and fuel consumption.
Negative plate sulfation is accelerated by acid stratification. What’s more, modern vehicle batteries experience the Sulfation effects even more dramatically when they suffer from acid stratification while operating in a PSOC condition, seldom receive a full charge, and/or are constantly being deeply cycled or micro-cycled. For this reason and others, average battery life is declining for the first time since the beginning of the 20th century.