Rechargeable Battery Fire Risk in the Workplace
Walk around almost any workplace and you will find rechargeable batteries in more places than you might expect. There are lead-acid batteries in the forklift trucks and the UPS cabinet in the server room, and lithium-ion batteries in laptops, cordless tools, staff e-bikes and the electric vehicles in the car park. A growing number of sites also have a battery energy storage system sitting alongside their solar panels.
Many fire risk assessments still treat all of these as a single line item called "batteries". The risks they present are quite different, though, and the right precautions depend on what type of battery you have, where it is charged and how it is stored. In 2024 RISCAuthority and the Fire Protection Association published RC61, a guide to the fire and associated hazards of secondary (rechargeable) batteries. As members of the Fire Protection Association, we have drawn on that guide to set out what responsible persons need to know.
Why the type of battery matters
The main reason one battery is chosen over another is energy density, which means how much energy it can store for its weight and size. Lithium-ion batteries store far more energy than lead-acid batteries of the same size, which is why they now power everything from phones to cars to grid-scale storage. The same quality that makes them so useful also means there is more energy available to be released if something goes wrong.
Each battery type also fails in its own way. Lead-acid batteries mainly present an explosion risk from hydrogen gas during charging. Lithium-ion batteries present a risk of thermal runaway, which can lead to fire, toxic gas and explosion. Understanding that difference is the starting point for controlling the risk.
Lead-acid batteries and hydrogen
Lead-acid batteries are an old and well-understood technology, still widely used in forklift trucks, road vehicles and traditional uninterruptible power supplies. They contain lead plates in a solution of sulphuric acid and water, which is highly corrosive and can cause injury if spilled.
The main fire hazard comes from charging. Vented lead-acid batteries release hydrogen and oxygen while they charge, and the amount increases during boost charging or if the battery is overcharged. Hydrogen is highly flammable, so charging areas inside buildings need adequate natural or mechanical ventilation to stop it building up. So-called sealed, or valve-regulated, batteries produce little or no hydrogen in normal use, but RC61 advises that they should still be included in hazardous area assessments.
In the UK, the Dangerous Substances and Explosive Atmospheres Regulations 2002, usually known as DSEAR, require employers to identify places where an explosive atmosphere could form and to keep ignition sources away from them. For a forklift charging bay, that means good ventilation, no smoking, careful control of any hot works nearby, and making sure batteries are connected and disconnected correctly, since incorrect disconnection can cause arcing and short circuits can cause overheating.
Lithium-ion batteries and thermal runaway
Thermal runaway happens when a battery cell produces heat faster than it can lose it. The rising temperature speeds up the chemical reactions inside the cell, which produce still more heat, and the process becomes self-sustaining. It is rare, but when it happens it is serious.
RC61 lists the usual triggers as internal short circuits caused by manufacturing defects, the build-up of metallic lithium inside a cell, physical damage, heat from an outside source, and overcharging or over-discharging. In normal use, the risk is highest while a battery is charging, particularly if a cell is already faulty.
Once thermal runaway starts, the cell vents or ruptures and releases flammable gases, including hydrogen and carbon monoxide, along with toxic gases such as hydrogen fluoride. It also produces oxygen, which is one reason these fires are so hard to put out. The gases may ignite straight away, or they may spread unignited through a room and then explode when they meet an ignition source. That second scenario is a particular concern in small, unventilated rooms where batteries are charged.
Most battery packs are made up of many individual cells, so heat from one failing cell can pass to the cells next to it and spread through the pack. A pack that appears to have been extinguished can also reignite later, because damaged cells remain unstable. A fully charged battery carries a higher risk of thermal runaway and releases more gas if it fails.
Lithium-ion chemistries differ
There are several lithium-ion chemistries in common use, and they balance energy capacity, cost, lifespan and safety in different ways. RC61 focuses on three of the most widely used, and the temperature at which each goes into thermal runaway is a useful guide to how forgiving it is.
Approximate thermal runaway temperatures: lithium iron phosphate (LFP) at around 270°C, used in battery storage systems, forklifts and buses; nickel manganese cobalt (NMC) at around 210°C, common in portable devices, e-bikes and electric cars; and nickel cobalt aluminium (NCA) at around 150°C, used in some electric vehicles. A higher threshold means a cell can tolerate more heat before it fails.
Lithium polymer batteries, often found in drones and mobile devices, use soft foil pouches in place of a rigid metal case. That makes them easy to shape but more vulnerable to physical damage. If you are buying lithium-ion forklifts, storage systems or other large battery equipment, it is worth asking the supplier which chemistry they use and how its battery management system protects against overcharging and overheating.
RC61 also clears up a common confusion. Lithium-metal batteries are single-use cells found in watches, remote controls and some medical devices, and they contain small amounts of metallic lithium, which reacts with water. Rechargeable lithium-ion batteries contain no metallic lithium and behave quite differently in a fire.
Fighting a lithium-ion fire
Lithium-ion battery fires do not fit neatly into a single fire class, since they can involve solid materials, flammable liquids and flammable gases at once. The usual approach combines containing the fire, smothering it to reduce oxygen, and cooling the surrounding cells to stop it spreading while the burning cells burn out. Some specialist agents and fire blankets are available, but effective cooling normally needs large amounts of water.
RC61 is clear that only well-trained people should attempt to tackle these fires. For most staff, the right response is to raise the alarm, get people out and leave the fire to the fire and rescue service. We explain how to choose suitable equipment in our article on fire extinguishers for EV chargers.
Practical precautions for your premises
Based on RC61, the basic precautions for lithium-ion batteries in a workplace are:
- buy batteries and battery-powered equipment from reputable manufacturers and suppliers, and charge them only with the charger supplied
- take damaged batteries out of use straight away, never recharge them, and arrange safe disposal
- avoid charging in unoccupied areas, and unplug batteries once charging is complete
- keep batteries away from very high or very low temperatures, and never cover chargers while they are in use
- keep a clear distance between charging areas and any combustible or flammable materials
- store battery products in a cool, dry place, spaced apart so a fire in one cannot easily spread to the next
Damaged lithium-ion batteries also retain some charge, known as stranded energy, which can cause electric shock and reignition. Anyone handling batteries after an incident needs to treat them with care.
Newer battery technologies
Other rechargeable technologies are starting to appear, and responsible persons are likely to meet them more often. Sodium-ion batteries work in a similar way to lithium-ion, and some versions use water-based electrolytes. They can be fully discharged, which removes the stranded energy problem, though they can still go into thermal runaway depending on the electrolyte used.
Redox flow batteries, most commonly vanadium-based, store energy in liquid electrolytes held in tanks and are suited to large stationary storage. Because their electrolytes are usually water-based, they have a high level of built-in fire safety. Solid-state batteries replace the flammable liquid electrolyte with a solid material such as a ceramic, which improves fire safety, but rechargeable versions are still in development, with some car manufacturers aiming to offer them by around 2030.
What this means for your fire risk assessment
A good starting point is a simple inventory of the batteries on site, covering what type they are, where they are used, where they are charged and who maintains them. Lead-acid charging areas should be considered under DSEAR, with ventilation and ignition control. Lithium-ion charging and storage areas should be assessed for their location, separation from combustibles, fire detection and the risk of gas building up in enclosed rooms.
Your fire risk assessment should reflect all of this, and it should be reviewed when new battery equipment arrives, such as a switch to lithium-ion forklifts or the installation of a storage system. RISCAuthority publishes more detailed guidance for specific uses, including RE1 for commercial battery storage systems, RE2 for lithium-ion battery use and storage, and RC59 for EV charging, while the National Fire Chiefs Council covers e-bikes, e-scooters and mobility scooters. For sites that make or handle batteries in larger numbers, our guide to managing lithium-ion battery risk in production facilities goes into more depth, and our page on warehouse and industrial fire safety covers the wider picture.
Not sure what batteries are on your site?
We can identify the battery types in use across your premises, review charging and storage arrangements, and make sure your fire risk assessment and staff training reflect the risks. If you would like support, please get in touch.
This article provides general guidance only and does not constitute legal advice. It draws on RC61: Secondary batteries, fire and associated hazards (RISCAuthority and the Fire Protection Association, 2024). Responsible persons should seek professional advice tailored to their specific premises and circumstances. Fletcher Risk Management Ltd is registered in England and based in Chester.