What Is Battery Thermal Runaway?
Thermal runaway is a self-sustaining chain reaction in which a battery generates more heat than it can dissipate. Once triggered, internal temperatures climb uncontrollably. In lithium-ion cells, those temperatures can exceed 800°C, resulting in fire, explosion, or the release of toxic gases.
The process is self-reinforcing: rising heat accelerates the chemical reactions inside the cell, which in turn produce more heat. Once this cycle takes hold, it is nearly impossible to stop until the battery has fully decomposed.
Critically, lithium-ion batteries generate their own oxygen during thermal runaway and cannot be smothered like conventional fires. According to QBE Insurance risk manager Adrian Simmonds, thermal runaway fires can require up to ten times more water to extinguish than a typical combustion engine fire. The scale of the problem is growing: by 2025, UK fire brigades were tackling a lithium-ion battery fire once every five hours, a 147% increase over just three years.
The Three Triggers of Thermal Runaway
Thermal runaway is triggered by three categories of abuse, and understanding each one is essential for any business that stores, sells, or handles batteries.
Mechanical abuse covers physical damage such as crushing, puncturing, or dropping. Electrical abuse includes overcharging and internal short circuits. Thermal abuse occurs when batteries are exposed to high external temperatures, whether from direct sunlight, heat sources, or poorly ventilated storage areas.
Lithium-ion cells function safely between 0°C and 60°C. When a cell reaches 150°C to 180°C, an exothermic reaction begins that can trigger full thermal runaway. From that point, temperatures can surge from 100°C to 1,000°C in a single second.
One of the most dangerous and poorly understood aspects is delayed ignition. A battery damaged by impact during transit or handling may show no visible symptoms for hours or even days before thermal runaway begins. This is a serious concern for businesses handling returned, transit-damaged, or used stock, and it means standard visual checks at the point of receipt are not always sufficient.
The risk to commercial operations is real: 23% of all UK lithium-ion battery fires occur in commercial properties.
It's Not Just Lithium: The Hidden Risk in VRLA, AGM, and GEL Batteries
Most thermal runaway guidance focuses on lithium-ion cells, but the risk extends to other chemistries that many businesses rely on daily. VRLA (valve-regulated lead-acid) batteries, including AGM and GEL types, are widely used in UPS systems, alarm panels, and emergency lighting across offices, server rooms, hospitals, and schools. These batteries are also susceptible to thermal runaway.
Sealed VRLA batteries trap heat internally, making them more prone to thermal runaway than flooded lead-acid cells, which can vent and remain more stable. The primary triggers for VRLA thermal runaway are ambient temperatures above 25°C and overcharging on float charge. Both conditions are common in server rooms, plant rooms, and poorly ventilated electrical cupboards.
The temperature sensitivity is significant: every 8°C to 10°C rise above 25°C can halve VRLA battery life and increase thermal runaway risk. For organisations running critical UPS infrastructure, this demands active monitoring.
NHS England recognised this risk directly in technical bulletin NETB/2023/2, which recommends monitoring failure conditions that could lead to thermal runaway and installing automatic shut-down systems for battery energy storage on NHS estates. That guidance applies equally to any organisation running critical backup power systems.
While lead-acid chemistries are non-flammable and far less fire-prone than lithium-ion, thermal runaway in VRLA batteries can still cause case swelling, venting of toxic gases, and significant equipment damage. Dismissing this risk because the batteries are not lithium-ion is a costly mistake.
Warning Signs to Watch For, by Battery Type
Early detection can make the difference between a controlled response and a serious incident. The warning signs differ by chemistry.
Lithium-ion warning signs:
- Swelling or bulging of the cell or battery pack
- Unusual heat during charging or while at rest
- Hissing or crackling sounds
- Discolouration of the casing
- An acrid or sweet chemical odour
- Unexpected voltage drops
VRLA, AGM, and GEL warning signs:
- Battery case swelling or distortion
- Excessive heat from the battery or surrounding cabinet
- A sulphurous electrolyte smell
- UPS unit reporting a battery fault or elevated internal temperature
- Float charge current running higher than expected
Warning signs may be subtle or entirely absent in the hours before delayed thermal runaway, particularly in transit-damaged or returned stock. Any physically damaged battery should be treated as a thermal runaway risk, regardless of whether it shows immediate symptoms. Regular visual inspections and temperature monitoring are baseline controls for any business storing batteries at scale.
Safe Storage Practices for UK Businesses
Proper storage practices significantly reduce thermal runaway risk. These recommendations apply whether you hold a handful of replacement batteries or manage large-scale inventory.
- State of charge: Store lithium-ion batteries at 60% to 70% charge for long-term storage. Avoid storing them fully charged or fully depleted.
- Temperature control: Maintain storage areas between 10°C and 25°C. Avoid locations exposed to direct sunlight, heat sources, or poor ventilation.
- Quarantine damaged stock: Physically segregate damaged, returned, or suspect batteries in a dedicated quarantine area, away from main stock and staff. Never store them on the same racking as undamaged units. Account for the delayed ignition risk by holding quarantined batteries for an appropriate observation period.
- VRLA battery environments: For UPS or alarm applications, ensure the ambient temperature in the battery room does not exceed 25°C and that float charge voltage is correctly set per the manufacturer's specification.
- Fire-resistant storage: Use fire-resistant battery storage cabinets or dedicated battery storage rooms with appropriate ventilation for larger quantities of lithium-ion batteries.
- Avoid mechanical pressure: Do not stack lithium-ion batteries in ways that could cause crushing or cell damage.
- Inspection protocols: Implement a clear inspection and quarantine protocol for all returned, transit-damaged, or used batteries before they re-enter storage.
A real-world example underlines why these practices matter. In September 2023, a warehouse fire at a commercial mobility equipment retailer in Great Notley, Essex, originated in an area storing used lithium-ion batteries. Over 50 firefighters were deployed and the business was forced to temporarily close.
Sourcing also plays a role in risk management. A significant proportion of UK battery fires are linked to uncertified or counterfeit cells purchased through unverified online marketplaces. At hardwarexpress, we supply certified batteries from major manufacturers carrying the correct safety certifications, reducing that risk for our customers.
UK Regulatory Obligations Businesses Must Know
There is no single UK law dedicated exclusively to lithium-ion battery storage. Instead, businesses must navigate multiple overlapping frameworks:
- HSE workplace safety guidance covering general storage and handling obligations
- COSHH regulations addressing the risk of toxic gas exposure from battery failures
- The Regulatory Reform (Fire Safety) Order 2005 requiring fire risk assessments that account for battery hazards
- Environment Agency requirements for battery waste management
- The Product Regulation and Metrology Act 2025, which became law in July 2025, designating lithium-ion batteries as a priority product and requiring e-bike batteries to include mechanisms preventing thermal runaway
The WEEE Regulations 2025 Amendment has also strengthened reporting duties. Businesses must now separate batteries from host devices and report weights separately. Mandatory Digital Waste Tracking becomes fully mandatory for waste-receiving sites in October 2026.
In June 2025, UK Parliament debated Battery Energy Storage Sites Safety Regulations following two documented BESS fires in the UK (2020 and Kilwinning, Scotland, 2025). MPs called for enforceable national regulations for BESS design and construction.
There is also a growing insurance dimension. Insurers are increasingly scrutinising how businesses store, charge, and handle batteries. Organisations that cannot demonstrate adequate risk controls may face policy exclusions or significantly higher premiums. If your business stores batteries at any scale, your insurer will want to see documented procedures.
Choosing Safer Battery Chemistries for Business Storage
Not all battery chemistries carry equal thermal runaway risk. A practical comparison:
- NMC/NCA lithium-ion: Highest risk. Most energy-dense, but the least thermally stable cathode chemistry.
- Standard lithium-ion: High risk. Still susceptible to thermal runaway under abuse conditions.
- LiFePO4 (lithium iron phosphate): Significantly more stable. Thermal runaway is extremely unlikely under normal conditions. Increasingly used in UPS, solar storage, and mobility applications precisely because of this stability.
- VRLA AGM/GEL: Low fire risk, but susceptible to heat-induced degradation and toxic gas venting.
For energy storage applications where procurement teams have flexibility, LiFePO4 is the safer long-term choice. Chemistry selection should always be matched to the application.
Regardless of chemistry, verify battery certifications before purchasing. Look for UN38.3, IEC 62133, and CE/UKCA marking. Avoid uncertified cells from unverified online marketplaces; these have been a documented factor in a significant proportion of UK battery fires.
Key Takeaways for UK Businesses
Thermal runaway is not exclusive to lithium-ion batteries. VRLA, AGM, and GEL batteries in UPS and alarm systems also carry risk, particularly in warm or poorly ventilated environments. Every business that stores or handles batteries needs to take this seriously.
Three core actions will reduce your risk substantially:
- Implement safe storage practices: Control temperature (10°C to 25°C), manage state of charge, and use fire-resistant storage for lithium-ion batteries.
- Establish a quarantine protocol: Segregate damaged, returned, or suspect batteries immediately. The delayed ignition risk is real: a battery showing no symptoms after impact may still pose a serious hazard hours or days later.
- Verify regulatory compliance: Ensure your operations align with HSE guidance, COSHH regulations, the Fire Safety Order, and the WEEE 2025 Amendment.
Source your batteries from established, certified suppliers with a proven track record. This is one of the most effective ways to reduce the risk of counterfeit or uncertified cells entering your operations.
At hardwarexpress, we have supplied certified batteries from major manufacturers to UK businesses, NHS trusts, schools, and government organisations since 2004. If you need advice on battery selection, safe sourcing, or finding the right chemistry for your application, our team is here to help.