Lead-acid batteries are a reliable and cost-effective solution for energy storage, widely used in sectors such as automotive and emergency power systems. However, one of the main risks associated with these batteries is thermal runaway—a phenomenon that can compromise both the safety and lifespan of the storage system. In this article, we’ll explore the causes of thermal runaway, its effects, and the best practices to prevent it.

What Is Thermal Runaway?

Thermal runaway is an uncontrolled process in which a battery’s temperature increases progressively due to internal factors, potentially leading to severe failures, electrolyte leakage, or even explosions. The issue occurs when the heat generated inside the battery exceeds its ability to dissipate it, creating a self-reinforcing overheating cycle. This phenomenon is especially critical in sealed lead-acid batteries, where the lack of direct maintenance makes it more difficult to detect early signs of overheating.

Main Causes of Thermal Runaway

Understanding the key causes of thermal runaway is essential for effective prevention:

1. Battery Overcharging

Overcharging occurs when the battery is charged at voltages higher than recommended. The excess energy is converted into heat, destabilizing the battery’s internal chemistry and initiating the overheating process. This happens because, beyond a certain voltage threshold, the water in the electrolyte decomposes into hydrogen and oxygen, increasing internal pressure. In VRLA (Valve Regulated Lead-Acid) batteries, this can lead to swelling and structural damage.

Technical data:

• The optimal charging voltage for a 12V lead-acid battery ranges between 13.8V and 14.4V depending on the technology (gel, AGM, flooded).
• Prolonged charging above 14.8V can cause excessive internal temperature rise and accelerate lead plate corrosion.

2. Internal Short Circuits

An internal short circuit happens when the separator between the positive and negative plates is damaged, allowing direct contact between electrodes. This causes a rapid current flow, generating heat and significantly increasing the risk of thermal runaway. Causes can include manufacturing defects, advanced sulfation, or the accumulation of metallic debris.

Technical data:

• A battery with an internal short circuit typically shows very low resistance (<0.01Ω), causing a rapid increase in current and temperature.
• Internal temperatures can exceed 80°C within minutes in the event of a short circuit.

3. Electrolyte Dry-Out

The electrolyte is essential for ion transfer between the battery plates, ensuring proper system operation. If the electrolyte evaporates due to high temperatures or prolonged use, heat dissipation capacity is reduced, accelerating battery aging and raising the risk of thermal runaway.

Technical data:

• In VRLA batteries, a 20% loss of electrolyte can reduce heat dissipation capacity by 50%.
• Operating temperatures above 40°C accelerate electrolyte evaporation, potentially halving the battery’s lifespan.

How to Prevent Thermal Runaway in Lead-Acid Batteries

To reduce the risk of thermal runaway, several preventive measures can be taken:

1. Charge Management

Always follow the manufacturer’s specifications for charging voltage and duration to avoid overcharging. The use of chargers with automatic voltage and current control helps prevent excessive charging.

2. Regular Inspections

Check batteries regularly for physical damage, electrolyte leaks, or signs of corrosion. Batteries with deformed plates or swelling should be replaced immediately.

3. Temperature Monitoring

Use monitoring systems to detect abnormal temperature increases. Thermal sensors placed on battery terminals can help prevent overheating.

4. Proper Ventilation

Install batteries in well-ventilated areas to support heat dissipation. In UPS systems and backup power units, include cooling fans or air ducts to reduce the risk of thermal buildup.

5. Routine Maintenance

Clean terminals and connections to reduce electrical resistance and prevent overheating. Electrical resistance due to oxidized terminals can generate additional heat, increasing the risk of thermal runaway.

Conclusion

Thermal runaway is one of the main threats to the safety and reliability of lead-acid batteries. Understanding its causes and adopting preventive strategies allows for optimized battery performance and minimized risk. With proper usage, continuous monitoring, and correct maintenance, energy storage systems can achieve greater safety and longevity.

ON LITE S.r.l. promotes innovative solutions for safe and efficient energy management, supporting businesses and individuals in the optimal selection and use of storage technologies. With our expertise in the sector, we provide advanced tools for battery monitoring and maintenance, helping to enhance the reliability and safety of energy storage systems.