Battery Failure Mode: Positive Plate Active Material Softening / Shedding & Corrosion
The discharge and charge process cause first the expansion, then the contraction of the positive (+) active material. Expansion occurs both in the plane (height and width) of the plate as the grid is pushed/stretched by corrosion processes over time and in the thickness of the plate as the active material is forced to expand to accommodate the lead sulphate (“PbSO4”) with each discharge.
The above image represents the chemical process in the battery charge and discharge process. The volume increase of the positive (+) lead dioxide (“PbO2“) plate during transformation to positive (+) lead sulphate (“PbSO4”) can be greater than 90%. The volume increase of the negative (-) lead (“Pb”) plate during transformation to negative (-) lead sulphate (“PbSO4”) can be greater than 160%. Recharging restores most of the lead dioxide in the positive plate to almost its original size, but, step by step, the positive plate will grow. By contrast, the negative plate does not expand over time because lead (“Pb”) is softer than lead dioxide (“PbO2“).
Progressive expansion and contraction of the positive plate as the battery is cycled causes an ever-increasing amount of the active material to be lost (“shedding”) from the grid/plate wires (a process called “corrosion”). This change in the active material mass manifests itself as a loss of battery capacity as expressed in Amp Hour (“AH”) or Reserve Capacity (“RC”).
Positive plate softening (active material appears muddy) will happen before shedding if the battery is regularly undercharged. In the field, a “new” battery that presents itself as being low on capacity can often be conditioned using an external charger and successfully put back into service. However, if we made a tear-down analysis of that battery, we would observe positive plates that appear to be in good shape, but the active material looks to be softening and muddy. In a battery suffering from acid stratification, the muddy appearance may be concentrated on the bottom of the plate. Muddy-positive plates are usually accompanied by negative plates that show signs of Sulfation.
Since grid wires are the current collectors upon which electrical current is delivered to the starter (Cold Cranking Amps or CCA), corrosion decreases the electrical performance of the battery. In a corroded battery, much of the current gets lost to resistance (in the form of heat) as the grid wires become exposed and/or disconnected from the active materials. The mechanical integrity of the plate is broken down as the structural integrity of the active material breaks down into individual crystallites that eventually break their bond with the grid wires and shed from the plate’s active material mass within the grid/plate. The result is grid wires become exposed to accelerated corrosive activity during charge. And over time, these conditions cause the battery to fail.
In an acid stratified battery, shedding and corrosion, and Sulfation happen much faster at the bottom of the plate leading to earlier battery failure. Moreover, modern vehicle batteries that operate in a Partial State of Charge (PSOC) seldom receive a full charge and/or are constantly deeply cycled or micro-cycled combined with acid stratification to accelerate shedding and corrosion. For this reason and others, average battery life is declining for the first time since the beginning of the 20th century.