Thermal runaway containment

How domestic battery design, location and emergency planning reduce the likelihood and consequences of thermal runaway.

Thermal runaway is a self-heating battery-cell failure in which rising temperature drives further damaging reactions. In a lithium-ion battery it can release heat, flammable and toxic gases, pressure and fire, and can propagate to neighbouring cells.

Containment is not one fireproof box or extinguisher. It is a set of measures that prevent an initiating fault, detect abnormal behaviour, stop energy flow, limit propagation and protect occupants and escape routes if those earlier measures fail.

Prevention comes first

A domestic battery relies on several safety layers:

cells and modules suitable for the intended application
a BMS monitoring voltage, current and temperature
fuses, contactors and electrical protection
compatible battery, inverter and firmware
mechanical and environmental protection
a location and enclosure suited to heat, moisture and impact exposure
correct cables, connections, torque and isolation
fault handling that fails safely when sensors or communications are lost

No single layer proves that runaway cannot occur. The installation design needs to consider foreseeable failure of controls as well as normal operation.

What can initiate a failure

Possible initiating events include internal cell defects, overcharge, deep discharge followed by unsafe recovery, external short circuit, high-resistance connections, impact, crushing, water ingress, unsuitable temperature, incompatible controls and fire originating elsewhere.

Ageing alone does not mean a battery is about to enter thermal runaway. Damage, repeated alarms or unexplained heating require assessment because they can reduce the safety margin.

The owner should never bypass a temperature sensor, contactor or BMS alarm to keep the system running.

The domestic fire-safety framework

PAS 63100:2024 is BSI’s specification for fire protection of small-scale battery energy storage systems in domestic dwellings within its stated scope. It covers battery units, BMS, power conversion, fault management, fail-safe controls, installation location and protection against fire.

As at 22 July 2026, PAS 63100:2024 remains the current edition. BS 7671:2018+A4:2026 introduced a battery chapter that calls for dwelling batteries to be installed in accordance with PAS 63100, with the BS 7671 transition described in the dedicated standards article.

The PAS has defined scope exclusions, including second-life systems, high-risk residential buildings and dwellings above its stated floor-area scope. Exclusion does not mean no fire assessment is needed. It means the designer needs the standards and specialist input appropriate to that installation.

Location limits consequences

Good siting reduces exposure to ignition, limits the route from a battery event into the dwelling and protects escape. The assessment considers proximity to openings, doors, windows, combustible material, sleeping areas, circulation and other equipment.

Outdoor installation can reduce some internal consequences but introduces weather, temperature, flooding, impact and security considerations. Indoor or attached locations need the specific separation, enclosure, detection and construction required by the applicable design.

An enclosure rating addresses defined access, dust or water exposure. It does not, by itself, establish fire resistance, gas management or safe battery placement.

Propagation and pressure

When one cell fails, barriers, spacing, cooling paths and module construction can slow or prevent propagation. These are product design features and should be supported by the relevant assessment and instructions.

Containment must also consider gas and pressure. A sealed improvised cupboard can allow gases to accumulate or direct a release into the home. Equally, adding unapproved openings can compromise a tested enclosure and expose live parts.

Do not modify the battery casing, vents or cabinet. The installer should follow the product and installation fire strategy as a complete system.

Detection and shutdown

The BMS may detect abnormal temperature, voltage or current and isolate the battery. Independent fire detection at the battery location provides a warning if an electrical control cannot contain the event.

Shutdown stops further commanded charging or discharging; it does not remove energy already stored in cells or guarantee that a self-heating failure will stop. Emergency isolation should be accessible and clearly labelled, but only used as instructed and without approaching smoke, heat or visible damage.

Monitoring alarms need a defined response. Silencing repeated notifications or repeatedly restarting a tripped battery removes valuable warning evidence.

Warning signs

Treat the following as urgent signs requiring distance and specialist advice:

smoke, vapour or an unusual sharp or solvent-like odour
hissing, popping or venting sounds
swelling, deformation or casing damage
unusual heat or a rapid temperature rise
liquid leakage or water exposure
repeated protection trips following impact or overheating

An app showing no data does not prove the battery is safe or de-energised.

What to do during a suspected event

If there is smoke, fire, venting or immediate danger:

Move people away and call the fire and rescue service.
Do not open the enclosure or attempt to remove modules.
Do not approach solely to operate isolation if doing so is unsafe.
Tell responders that a fixed battery system is present and where it is located.
Follow the site’s emergency information and responder instructions.

A small portable extinguisher should not be presented as a plan for controlling energy stored inside a fixed battery pack. Fire-service tactics depend on the battery, building and event.

After heat, fire, impact or flooding

Do not re-energise a battery exposed to fire, abnormal heat, impact, water or contaminated extinguishing runoff until it has been assessed through the manufacturer or competent specialist route.

Damaged lithium batteries can remain hazardous and need controlled storage, packaging and transport. The waste or service provider must be told their true condition. A visually intact outer case does not prove that internal cells are undamaged.

Incident logs, photographs taken from a safe position, alarm history and the isolation state should be retained for the service provider and insurer.

Maintenance and change control

Routine ownership checks are visual and non-invasive. Keep the specified clearances free, maintain ventilation paths, watch for water or impact damage and act on alarms. Do not stack combustible items around the battery or use its enclosure as a shelf.

Expansion, relocation or substitution can change separation, stored energy, BMS behaviour and protection. It needs a fresh design check. Mixing modules because connectors fit can invalidate the product safety case.

Handover information

The owner and emergency responders benefit from:

battery chemistry, model, number of modules and location
AC and DC isolation diagram
emergency contact and shutdown instructions
warning labels and circuit schedule
detection and alarm arrangements
product safety and installation records
the applicable PAS 63100 and BS 7671 design basis
damaged-battery collection route

Containment is credible only when these measures match the installed system and the people in the home know how to respond.