Battery Pack Thermal Runaway Spacing Screening Tool

Screens your pack layout against NFPA 855, UL 9540A and IEC 62619 and names every gap. It will not hand you a spacing number, because NFPA 855 has no formula — only a 3 ft default that real fire testing can reduce.

PRELIMINARY SCREENING ONLY

This is not a compliance sign-off and it cannot become one. NFPA 855 sets a 3 ft default separation between energy storage units and allows it to be reduced only where UL 9540A large-scale fire test data shows fire will not propagate, with the full report accepted by your authority having jurisdiction. No calculation substitutes for that test. Use this to find gaps before a review, never to justify a design.

The cells

Chemistry is not a detail here. Some cathodes release oxygen as they fail, which means they feed their own fire.

Cell chemistry i

Runaway onset temperatureFrom the chemistry — override with your cell’s datasheet figure

°C

Cells in the pack

nos

Advanced / Custom Input — the cells

Cell capacity

Cell nominal voltage

Energy densityHigher density means more energy to release in one place.

Wh/kg

State of charge at restOnset temperature falls as state of charge rises.

How it is laid out

Spacing between units iEnclosure to enclosure. The NFPA 855 default is 0.914 m (3 ft).

Cell-to-cell gapInside the module

Barrier between cells or modules

Advanced / Custom Input — the layout

Module-to-module gap

Show unit spacing in

Thermal management

What test evidence do you have?

This section decides more than any other. Without accepted test data, the code default applies whatever the design looks like.

UL 9540A test data i

What is it?

Where is it installed?

Advanced / Custom Input — the evidence

IEC 62619 certificateCheck which edition — the 2022 second edition added laser initiation.

Hazard Mitigation Analysis done?

What else is in place?

Fire suppression

Gas detection

Explosion controlNFPA 68 venting or NFPA 69 prevention

Advanced / Custom Input — mitigation

Distance to the nearest exposureA door, window, property line or occupied space.

Separation your insurer wantsOften 8 ft (2.44 m) or more, above whatever the code allows.

Screening result

Describe the pack and its layout, then press “Screen this design”.

Preliminary engineering screening only. This is NOT a substitute for UL 9540A large-scale fire testing, a Hazard Mitigation Analysis, or a certified ESS safety engineering review, and nothing here constitutes compliance sign-off. Verify with a qualified fire-safety engineer and your authority having jurisdiction before installation.