What are common battery failures?
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 lead dioxide in the positive plate to almost its original size, but the positive plate will grow step by step. 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 did a tear-down analysis of that battery, we would observe positive plates that appear in good shape, but the active material looks 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, corrosion, and sulphation 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.
Battery Dry Out and Thermal Runaway
When a battery is charged, evaporation reduces the volume of electrolyte solution (Water + Sulphuric Acid) inside the battery. It is mostly the water volume that is lost in this process. A vicious cycle is created as lower volumes of electrolyte (now with higher acid-to-water ratios) increase internal resistance causing excessive heating during charge, and that causes a further increase in water loss through evaporation. At some point in this incremental process, the water volume depletes (battery dry-out) to the point where a battery’s growing internal resistance, combined with the corrosion processes described earlier, causes so much heat during charge that a thermal run-away event can occur such as battery fires or melting.
Battery dry-out and the chance for thermal run-away are accelerated by acid stratification. Moreover, modern vehicle batteries that operate in a Partial State of Charge (PSOC) condition, that seldom receive a full charge, and/or are constantly deeply cycled or micro-cycled combine with acid stratification to supercharge battery dry-out conditions and increase the likelihood of thermal run-away. For this reason and others, average battery life is declining for the first time since the beginning of the 20th century.
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) cannot 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 a 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.
If either the negative or positive plate is continually undercharged, a premature decline in capacity will occur because of Sulfation. Undercharging is on the rise across the world in vehicles of every type. Undercharging can be caused by defective charging or persistent Partial State of Charge (PSOC) operation. Defective charging can happen as a result of faulty equipment or as a result of some of the other battery failure modes discussed in this document. PSOC operation is a growing trend due to the growing number of vehicle systems that rely on the battery to function correctly and the deep and micro-cycling that occurs in start-stop vehicles.
In addition, battery failure due to undercharging is accelerated by the effects of acid stratification. For this reason and the others discussed in this document, it is not surprising that average battery life is declining for the first time since the beginning of the 20th century.