Corrosion, shedding, and internal shorts are related failure processes in lead-acid batteries. Positive-grid corrosion gradually weakens the current-collecting structure, while mechanical activity can loosen active material from the plates. Dislodged material may collect at the bottom of the cell and eventually create an unintended conductive path between plates.
These mechanisms cannot be eliminated completely because they are part of how lead-acid batteries age. However, suitable charging, controlled discharge, clean connections, and appropriate mechanical installation can slow them and make developing faults easier to recognize. Internal shorts are particularly difficult because some begin as partial conductive paths that allow a battery to appear normal immediately after charging.
How to Reduce Battery Corrosion, Shedding, and Internal Shorts
The most effective approach is to reduce the electrical, chemical, and mechanical stress placed on the battery. Avoiding repeated deep discharges, preventing prolonged overcharge, using the correct charging profile, and protecting the battery from vibration all help limit the conditions that promote corrosion and shedding.
A maintenance routine should also include visual inspection and periodic testing. Look for electrolyte leakage, corrosion around terminals, damaged cases, loose connections, abnormal heating during charge, and a cell or battery that loses voltage more quickly than comparable units.
Limit Discharge Depth and Control Overcharge
Repeated deep discharge places greater stress on a lead-acid battery than shallow cycling. Limiting discharge depth where the application allows it reduces the amount of chemical and mechanical work required on each cycle and can extend service life. The correct limit depends on the battery design and application, so the manufacturer’s operating guidance should take precedence over a universal state-of-charge value.
Overcharge creates a different type of stress. Prolonged overcharge accelerates positive-grid corrosion and can be especially damaging in sealed lead-acid batteries, including valve-regulated designs. A flooded lead-acid battery has some ability to tolerate overcharge because water can be replaced during maintenance, but this does not make uncontrolled charging desirable. Excessive gassing, water loss, heat, and corrosion can still shorten its life.
Sealed batteries cannot be maintained by routinely replacing lost water. They therefore need the manufacturer’s specified float voltage and charging limits. Float charging should maintain the battery without continuing to force excessive current through it. A charger that is suitable for one lead-acid design may not be suitable for another.
Charging temperature also matters. Battery voltage requirements change with temperature, and charging outside the manufacturer’s specified temperature range can increase the risk of undercharge, overcharge, gassing, or damage. Use the charger’s temperature compensation or control function when provided, and follow the battery manufacturer’s temperature limits rather than applying one voltage or temperature rule to every lead-acid battery.
Specific Gravity, Battery Design, and Charging Precision
Specific gravity (SG) indicates the density of the sulfuric-acid electrolyte relative to water and is useful for assessing the state of charge of serviceable flooded cells. Battery design involves a trade-off between electrolyte concentration, energy, corrosion, and expected service life.
Some long-life lead-acid batteries use a fully charged specific gravity of approximately 1.200. High-performance lead-acid batteries may use approximately 1.265 or higher. The lower value can reduce corrosion, but it also reduces specific battery energy. These figures describe design examples, not a universal target for adjusting or refilling every battery. The battery’s specified electrolyte and charging procedure should always be used.
Lead-acid batteries generally tolerate some variation in charging conditions, although poor charging still causes aging. Lithium iron phosphate (LiFePO4) batteries require more precise control of voltage, current, charge termination, and protection against abnormal operating conditions. They are not simply lead-acid batteries with a different chemistry.
A conventional automotive charging system may not provide the control needed for a LiFePO4 replacement. Alternators and vehicle regulators are normally designed around lead-acid charging behavior and may not provide the required charge limits or termination behavior. A LiFePO4 installation may also require a battery-management system, suitable charging equipment, and protection against charging under prohibited conditions. Compatibility must therefore be assessed for the complete vehicle electrical system, not just the nominal battery voltage.
Terminal Corrosion and Lead Shedding
Terminal corrosion commonly results from electrolyte leakage, acid vapor, contamination, or a poor electrical connection. A loose connection can increase resistance and local heating, while contamination can provide a path for surface leakage. Corrosion products may also interfere with starting or charging current.
Useful preventive measures include:
- Keep the battery case, terminals, and cable ends clean and dry.
- Inspect for cracks, leaks, bulging, or damaged terminal seals.
- Tighten connections according to the battery or vehicle manufacturer’s requirements.
- Correct the source of electrolyte or acid vapor instead of only cleaning the corrosion.
- Use suitable terminal protection where specified, without allowing protective material to insulate the actual contact surfaces.
Lead shedding is different from external terminal corrosion. During cycling, vibration, and other mechanical activity, active material can loosen from the plates. Repeated expansion and contraction of the electrode material can contribute to this process. Poor installation, severe vibration, and aggressive cycling can increase the mechanical stress.
The shed material may accumulate below the plates. As the deposit grows, it can reduce the amount of active material participating in the reaction and therefore reduce effective capacity. If the material bridges the space between plates, it can also contribute to an internal fault or short circuit. A battery can consequently lose capacity before it develops an obvious external symptom.
Soft Shorts and Elevated Self-Discharge
A soft short is a partial internal conductive path between otherwise separated battery components. It does not necessarily produce an immediate, permanent short circuit. Instead, it increases self-discharge, so the battery loses stored energy while sitting unused or may show a voltage that falls unusually quickly after charging.
Soft shorts are difficult to identify because charging can temporarily hide the problem. Immediately after charge, the battery may show a normal voltage and appear to function normally. An unusually warm battery case during charging can be a warning sign, but temperature alone does not prove that a soft short exists.
A practical check is to allow the battery to rest for approximately 6–12 hours after charging, disconnected from loads and chargers where safe and appropriate. Then compare its open-circuit voltage with that of a healthy battery of the same type and state of charge. In a serviceable flooded battery, specific gravity measurements can provide additional information. A battery affected by self-discharge may show lower open-circuit voltage, reduced specific gravity, and reduced measured capacity because some of its stored energy has already been consumed.
Testing should be performed with appropriate battery safety procedures. Do not handle a battery that is hot, leaking, swollen, or visibly damaged as though it were a normal test unit. Disconnecting and testing high-energy battery systems may require qualified service personnel.
A 2010 Battery Council International (BCI) Failure Mode Study reported that shorted batteries accounted for 18 percent of battery failures, compared with 31 percent five years earlier. The reported reduction was associated in the source material with possible improvements in manufacturing methods. This is a dated industry study, not a current universal failure rate; its result should be treated as historical context rather than a prediction for every battery type or application.
Mossing, Lead Drop, and Serious Internal Shorts
Mossing is another form of soft-short mechanism. If plates and separators are slightly misaligned, parts of a plate may become exposed or “naked” at the edges. The exposed region can promote the growth of conductive crystalline material, sometimes described as moss. This conductive growth can increase self-discharge and may eventually form a more effective path between plates.
Mossing is primarily associated with internal construction and separator alignment, so it is not normally corrected by cleaning terminals or changing an external cable. Its effects may first appear as unexplained self-discharge, a lower rested voltage, or a battery that repeatedly loses capacity despite apparently correct charging.
Lead drop is a different mechanical failure. Chunks of lead can break loose from welded bars that connect the plates. Unlike a soft short that develops progressively through wear and cycling, lead drop may occur early in battery life because of a manufacturing defect. A detached piece can create a serious internal short and a permanent voltage drop.
The consequences depend on the location and size of the detached material and on the internal construction of the battery. A lead drop can disable a cell, reduce total battery voltage, or create substantial internal current and heating. In severe circumstances, the resulting fault may contribute to thermal runaway, but not every lead-drop failure produces thermal runaway.
A battery with a permanent voltage drop, abnormal heating, rapid self-discharge, visible damage, or evidence of an internal short should be removed from service and handled according to applicable battery-safety and recycling procedures. Do not attempt to shake, puncture, open, or repeatedly recharge a suspect sealed battery in an effort to restore it. Corrosion and shedding are inevitable aging mechanisms in lead-acid batteries, but controlled operation can delay them and careful testing can identify unsafe failures before they become more severe.
References
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- Battery University | BU-804a: Corrosion, Shedding and Internal Short. (n.d.). https://www.batteryuniversity.com/article/bu-804a-corrosion-shedding-and-internal-short
- BU-804a: Corrosion, Shedding and Internal Short - Battery University. (n.d.). https://batteryuniversity.com/article/bu-804a-corrosion-shedding-and-internal-short
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