How batteries go bad: Understanding battery failure modes

Batteries have become essential components of our infrastructure; they provide uninterrupted power to data centres and facilitate the integration of renewable energy into our power grids. Now that we rely on batteries more than ever, understanding battery failures is not merely an academic exercise; it's essential knowledge for anyone responsible for battery systems.

The financial implications of battery failures are significant. When a battery system fails, organisations face not only the direct replacement costs but also the indirect costs related to system downtime, potential damage to connected equipment and, in some cases, the loss of critical services. A single hour of downtime in a data centre can cost as much as $1 million.

With all this in mind, let's explore the main ways batteries can fail.

Lead-acid battery failure modes

Lead-acid batteries are one of the most common types of stationary battery. While they're reliable and well understood, they can fail in several ways. 

Positive grid corrosion

Positive grid corrosion is a chemical process where the lead alloy that forms the battery's positive grid gradually converts to lead oxide. This process is accelerated by high temperatures, overcharging and excessive cycling. While some degree of grid corrosion is normal and actually designed into batteries, excessive corrosion can significantly shorten battery life, leading to:

• Physical expansion of plates
• Increased internal resistance
• Reduced power capability
• Eventual battery failure

Sulphation

During normal battery discharge, the active materials in a lead-acid battery (lead and lead dioxide) react with sulphuric acid to form lead sulphate. This is a natural and necessary process. However, there's a crucial difference between the soft, normal lead sulphate formed during regular discharge and the problematic crystalline sulphate that can develop under certain conditions. Sulphation is largely preventable and normally reversible, but it can become permanent if batteries remain in a discharged state, when charging is insufficient or when regular maintenance is neglected. It results in:

• Reduced capacity 
• Increased internal resistance
• Physical damage to the plates

Internal shorts

Internal shorts often develop gradually and can be difficult to detect until significant damage has occurred. Unlike external shorts, which are usually obvious, internal shorts work silently within the battery, potentially creating dangerous conditions.

Internal shorts in lead-acid batteries generally fall into two categories: hard shorts and soft shorts. Hard shorts are typically caused by paste lumps resulting from manufacturing defects. Soft shorts are the result of excessively deep discharges where the specific gravity becomes so low that lead begins to dissolve into the electrolyte. This lead gets trapped in the separators, causing the short circuits. Both hard and soft shorts cause:

• Immediate capacity loss
• Excessive heat generation
• Potential thermal runaway
• Fire risks in severe cases
• Release of hazardous gases

Dry-out (VRLA specific)

While valve regulated lead acid (VRLA) batteries were developed to be maintenance free, they face a unique challenge: dry-out. Unlike their flooded counterparts, once a VRLA battery loses its electrolyte, there's no way to replenish it. This makes dry-out one of their most insidious failure modes.

VRLA batteries can lose electrolyte through excessive heat, overcharging, poor ventilation or improper charging voltage. Loss of electrolyte leads to:

• Increased internal impedance
• Reduced capacity
• Higher operating temperature
• Decreased efficiency
• Loss of plate-to-electrolyte contact
• Accelerated ageing
• Potential thermal runaway
• Shortened backup time
• Unreliable performance
• Increased operating costs
• Premature failure

Thermal runaway (VRLA specific)

Thermal runaway is a dramatic and dangerous failure mode that can happen with any battery chemistry. In lead acid batteries it is more common in VRLA than VLA types.  Unlike other failure modes that develop gradually, thermal runaway can escalate rapidly, potentially leading to catastrophic failure within hours. Thermal runaway is a self-reinforcing process where heat and current feed off each other in a dangerous spiral where the battery heats up and passes more current, which generates more heat, so the battery passes even more current, and the cycle continues until failure occurs.

Early detection, based on these criteria, is crucial for preventing catastrophic failures:

• High temperature
• Lower resistance 
• Increased current 
• Gas generation exceeding recombination rate
• Pressure build up, forcing venting
• Electrolyte loss 

Lithium-ion battery failure modes

Lithium-ion batteries have revolutionised energy storage, but they come with their own unique set of failure modes.

SEI layer build-up

The solid electrolyte interface (SEI) layer is essential for the operation of a lithium-ion battery but, during the life of the battery, it increases in thickness over time at a rate that is influenced by multiple factors. The growth of the SEI layer has direct implications for battery impedance:

• Progressive impedance increase
• Higher voltage drop under load
• Reduced maximum current capability
• Increased heating during operation

Lithium plating

Lithium plating is one of the most serious failure modes in lithium-ion batteries, potentially leading to catastrophic failures. Unlike gradual degradation mechanisms, lithium plating can create immediate safety hazards and careful management is required to prevent it. When the battery is functioning normally, lithium ions intercalate (insert themselves) into the anode's graphite structure during charging. However, under certain conditions, ions accumulate on the anode surface and metallic lithium deposits form. Eventually, dendrites grow, and the structure becomes unstable.

Lithium plating creates serious safety concerns:

• Internal short circuits
• Thermal runaway potential
• Cell rupture possibility
• Capacity degradation
• Increased internal resistance
• Accelerated ageing
• Cell imbalance
• Potential fire hazard

Non-uniform ageing

Many battery ageing processes are usually considered to be uniform across the cell, but reality is often more complex. Non-uniform ageing occurs when different parts of the cell age at different rates, creating localised weaknesses that can significantly impact performance and safety. It can result in:

• Reduced overall capacity
• Increased internal resistance
• Uneven current distribution
• Variable voltage response

BMS failures

The battery management system (BMS) is the critical intelligence that keeps lithium-ion batteries operating safely and efficiently. When the BMS fails, it can compromise both battery performance and safety, so understanding these failures is crucial. BMS failures can manifest in various ways, such as hardware, software or calibration problems.

BMS failures can have serious safety implications:

• Overcharge risk
• Over-discharge potential
• Temperature control failure
• Current limit failures
• Lack of critical warnings
• Delayed shutdown response
• Failed emergency disconnection
• Inadequate thermal management
• Cell imbalance
• Thermal runaway 
• Excessive stress on cells
• Accelerated ageing

Battery string issues

In addition to individual battery failures, battery strings are susceptible to another important failure mode: degradation of inter-cell connections if they are not properly maintained. This degradation is due to multiple factors including corrosion, vibration and repeated temperature changes. As the connections degrade, their resistance increases. This may not be noticed when the string is passing only a small float current, but when it is called upon to deliver higher current, the increased resistance of the connections can lead to excessive heating, which, in some cases, creates a fire hazard. 

Inter-cell connections are often the weakest link in a battery string, and their failure can have serious consequences:

• High resistance
• Increased heat
• Reduced capacity
• Voltage imbalances

Testing and prevention: The best defence against battery failure

Understanding failure modes is crucial but preventing failure through proper testing and maintenance is even more important. Proper maintenance and regular testing with the right equipment aren’t just good practice; they are essential for:

• Ensuring system reliability
• Protecting investments
• Maintaining safety
• Meeting obligations
• Optimising performance

We’ll cover testing and maintenance approaches in more detail in a future post.

Conclusion

As we've seen, batteries can fail in numerous ways, from the gradual degradation of positive grids in lead-acid batteries to the potentially dangerous lithium plating in lithium-ion systems. Understanding these failure modes isn't just an academic concern – it's about protecting critical infrastructure, ensuring business continuity and maintaining safety.

Some failure modes, like sulphation or SEI layer build-up, work slowly and steadily, gradually undermining your battery's performance. Others, like thermal runaway or internal shorts, can strike quickly and dramatically. But all share one crucial characteristic: they give warning signs before causing catastrophic failure.

This is why regular testing and maintenance are so important. The cost of implementing proper testing and maintenance programs is minimal compared to the potential consequences of battery failure – consequences that can range from expensive replacements and system downtime to serious safety incidents.

In our next post, we'll explore how to detect warning signs through proper testing and maintenance. Knowing how batteries fail is only the first step – knowing how to prevent failure is even more important.