Lithium-ion Battery Safety for Trades — Tool Batteries, Charging and Fire Risk (UK Guide)

Why Lithium-ion Batteries Catch Fire

The core failure mode is called thermal runaway. Inside every Li-ion cell, the positive and negative layers are separated by a thin plastic membrane. If that separator is breached — by physical damage, an internal short, overheating or overcharging — the cell starts to heat itself. Above a certain temperature the chemistry becomes self-sustaining: the cell vents flammable gas, gets hotter, and that heat spreads to the next cell, which does the same. The result is a fast, intense fire that can reach over 600°C, throw out jets of burning gas and reignite minutes or hours later even after being knocked down.

Two things make these fires especially dangerous compared to a normal fire. First, they are hard to extinguish — the cell is generating its own heat and oxygen-bearing reaction internally, so simply smothering the flames does not stop it. Second, the fumes are toxic: hydrogen fluoride and other gases that you must never inhale. A pack failing inside a van or a closed workshop overnight can produce a serious fire before anyone is aware of it.

The Main Causes of Battery Fires

Most lithium tool battery fires trace back to a handful of avoidable causes. Knowing them tells you exactly what to watch for:

• Physical damage: Dropped, crushed, run over or impact-damaged packs can suffer internal cell damage that is not visible from the outside. A pack that fell off a scaffold last week can fail on charge today.
• Non-genuine or incompatible chargers: Cheap third-party chargers and copy batteries are a leading cause. They may lack the protection circuitry that stops overcharging or detects a faulty cell.
• Overcharging: Leaving a pack on a faulty or unmatched charger that keeps pushing current after the cell is full drives the cell past its safe voltage.
• Charging unattended or overnight: A fault that would be caught and dealt with in seconds during the day becomes a building fire when nobody is there.
• Heat: Charging or storing a battery in a hot van, in direct sun or near a heat source raises the baseline temperature, leaving less margin before runaway.
• Water ingress: Water inside a pack can cause an internal short. Wet batteries are a real risk on UK sites.
• Internal short or manufacturing defect: A faulty cell can fail with no external warning — one reason you never charge unattended.
• End-of-life cells: Old, heavily cycled packs that no longer hold charge, run hot or have swollen are far more likely to fail.

Safe Charging Practice

Charging is where most things go wrong, and it is the part you have the most control over. Get this right and you have removed the majority of the risk.

• Use only the manufacturer's matched charger. The DeWalt, Makita, Milwaukee or Bosch charger designed for that pack contains the protection electronics it expects. Do not mix brands or use unbranded copies.
• Never charge unattended, and never while you sleep. Charge during working hours when someone can react. Do not put packs on charge overnight in the home, workshop or van.
• Charge on a hard, non-combustible surface. A bare concrete or metal bench, away from exits, away from anything that will burn — not on a wooden bench covered in dust, rags, packaging or sawdust.
• Keep the charging spot clear of combustibles and away from the route you would use to escape, so a fire does not block your exit.
• Never charge a battery that is damaged, swollen, hot or wet. If a pack is warm from heavy use, let it cool to room temperature before charging. If it is swollen, leaking or has been wet, take it out of service.
• Do not leave packs sitting on the charger for days. Take them off once charged.

Safe Storage Practice

How you store batteries between jobs matters as much as how you charge them — a poorly stored pack can fail while it sits in a drawer.

• Store in a cool, dry place out of direct sunlight and away from heat sources. Heat and damp are the enemies of stored cells.
• Store at partial charge for long periods rather than fully charged or fully flat. Around half charge is kindest to the cells.
• Never store packs loose with metal objects. Screws, nails, offcuts or keys in the same box or tote can bridge the terminals, cause a short and start a fire. Keep terminals covered or store packs separately.
• Keep stored batteries away from combustible materials and ideally in a metal cabinet or a designated non-combustible area.
• Inspect regularly for swelling, cracks, leaks, corrosion or damage to the casing or contacts.
• Take any damaged or suspect pack out of use immediately — do not keep using a battery that is swollen, runs hot or has visible damage.

Batteries in the Van

The van is the highest-risk environment for most trades. It gets very hot in summer, batteries roll around loose, and the temptation to charge overnight off a leisure battery or inverter is real.

• Secure batteries so they cannot move. Loose packs sliding around the load area get knocked, crushed and damaged. Use racking, cases or secured totes.
• Avoid extreme heat. A van in direct sun can exceed 50°C inside. Do not leave packs baking on the dashboard or in the sun, and avoid storing large quantities in an unventilated van in a heatwave.
• Beware overnight charging in vans. Charging unattended overnight in a parked van — especially off an inverter or with non-genuine kit — is a serious fire risk in an enclosed space with no smoke alarm and no one nearby to react.
• Carry packs in their cases rather than loose with tools and metal fixings that could short the terminals.

What to Do If a Battery Catches Fire

If a lithium battery goes into thermal runaway, your safety comes first. Do not try to be a hero with a small pack of batteries that is venting and burning.

• Get out and call 999. Evacuate the area or building, raise the alarm and call the fire service. Tell them it is a lithium battery fire.
• Do not inhale the fumes. The gases given off are toxic. Move upwind and keep others away.
• Lithium fires need a lot of water or cooling. Unlike a normal small fire, a single extinguisher rarely stops thermal runaway. The fire brigade uses large volumes of water to cool the cells. If you cannot apply that safely, do not try — evacuate instead.
• A fire blanket or a bucket of water/sand may help with a very small, early-stage incident if you can act safely, but do not rely on it and never put yourself at risk.
• Expect reignition. A pack that looks out can flare up again. Do not assume it is safe — let the fire service make that call, and keep a damaged pack isolated outdoors afterwards.

Disposing of Old Batteries Legally

Disposal is where a lot of trades unknowingly break the rules and create a hazard. Never put lithium batteries in your general waste or recycling bin. Crushed in a bin lorry or compacted at a waste transfer station, a damaged pack can ignite — battery fires in waste trucks and recycling centres are a growing problem across the UK.

Take old, dead or damaged packs to a proper battery recycling point. Tool retailers, trade counters and household waste recycling centres accept them, and under the Waste Electrical and Electronic Equipment (WEEE) Regulations and the Waste Batteries and Accumulators Regulations, larger battery sellers are obliged to take batteries back for recycling. Recycling recovers the lithium, cobalt and other materials and keeps them out of landfill and out of the waste stream where they cause fires.

• Tape over or cover the terminals before transporting or storing a battery for disposal, to prevent shorting.
• Keep damaged or swollen packs separate, ideally outdoors in a non-combustible container, until you can recycle them.
• Use a designated battery recycling collection point or your trade supplier's take-back scheme.

The Legal Position

There is no single law titled "tool battery safety," but several existing duties apply to you as a trade business:

• Health and Safety at Work etc. Act 1974: The general duty to ensure, so far as reasonably practicable, the health and safety of employees and others affected by your work covers how you charge, store and use batteries.
• Fire safety law: The Regulatory Reform (Fire Safety) Order 2005 requires a fire risk assessment for workplaces and premises. If you charge or store batteries in a workshop or unit, that risk must be assessed and controlled.
• DSEAR: The Dangerous Substances and Explosive Atmospheres Regulations 2002 can apply where larger quantities of batteries are stored or charged, given the flammable gases released during a failure.
• WEEE and Waste Batteries Regulations: Govern how batteries must be disposed of and recycled rather than binned.

For most sole traders and small firms, the practical takeaway is simple: charge and store batteries sensibly, assess the fire risk where you keep them, and recycle old packs properly. Document your approach if you employ others.

Quick Reference: Lithium Battery Do and Don't