Understanding Thermal Runaway: Triggers and Mechanism
Thermal runaway describes a self-reinforcing heat reaction in a lithium cell: when the cell temperature exceeds a critical threshold, exothermic reactions begin — electrolyte decomposition, separator collapse, reaction of the released lithium with moisture or oxygen. The heat released accelerates the reaction further until the cell fails thermally. Adjacent cells are heated in the process and can also enter runaway — so-called cell-to-cell propagation.
Triggers fall into three categories: Mechanical — penetration by a conductive object (nail, splinter), severe deformation from impact or crushing; Thermal — external heat sources (fire in the vicinity, inadequate cooling), continuous operation outside the temperature range; Electrical — overcharging above the upper cell voltage, deep discharge below the lower limit, external short circuit. An intact BMS reliably prevents the electrical triggers. Mechanical and thermal influences require design measures on the pack and in the installation environment.
Safe Storage: SoC, Temperature, Clearances, Fire Compartments
Storage of lithium battery packs in industrial operations follows clear rules. State of Charge (SoC): For longer storage (weeks to months) a SoC of 30–50 % is recommended. Fully charged packs (SoC >80 %) age faster and contribute more energy in a fire event. Deeply discharged packs (<10 %) can fall below the lower limit through self-discharge, irreversibly damaging the anode.
Temperature: Lithium batteries should be stored between +5 °C and +25 °C. Cold reduces self-discharge, heat accelerates ageing and increases internal pressure. Direct sunlight and storage near heat sources must be avoided.
Spatial separation and fire compartments: Storage rooms for lithium batteries should be designed as separate fire compartments — fire resistance class F30 or better. Minimum clearances to combustible materials and escape routes must be maintained. For larger quantities, coordination with the fire protection officer and, if necessary, the competent authority is required.
Labelling: Stored packs must be clearly labelled (voltage, capacity, chemistry). Defective or damaged packs must be immediately isolated and stored separately — not on the same shelf as intact units.
Fire Protection Measures and Standards
Several standards and guidelines are relevant in a commercial setting. IEC 62619 defines safety requirements for stationary and motive lithium cells and batteries. UN 38.3 governs transport testing (vibration, shock, thermal load, overcharge, short circuit, crush, impact) — a corresponding test certificate must be available for every battery pack that is transported or delivered.
For fire protection in operations: smoke detectors in storage areas are mandatory. For larger battery stocks, fire protection engineers often recommend automatic extinguishing systems — water-based extinguishing systems (sprinklers) are fundamentally suitable for lithium-ion batteries. Fire suppression blankets can enclose individual packs during the onset of thermal runaway and temporarily limit flame and smoke development, but do not replace water.
In AGV operations additionally: vehicles with abnormal temperature behaviour (hot housing, unusual smell, swollen housing) must be taken out of service immediately and parked in a safer area — preferably on a non-combustible surface and with sufficient clearance from other vehicles and racking.
Correctly Extinguishing Lithium Fires
The widespread myth that water is prohibited for lithium fires originates from the chemistry of elemental lithium metal (e.g. in primary cells or lithium metallurgy) — not from lithium-ion batteries. When a lithium-ion battery pack is burning, water does not react with the lithium in the electrodes (which is present as intercalated lithium) but cools the pack. Large quantities of water are the most effective means of cooling the thermal runaway and preventing it from spreading to adjacent cells.
Fire services recommend in practice: placing the burning pack if possible in a water-filled container (metal tray, IBC container) or continuously flooding it with water. The goal is cooling below the reaction threshold — the chemistry can remain active for hours without reigniting as long as sufficient cooling is maintained. CO₂ extinguishers suppress the visible flame area but have no cooling effect on the cell reaction itself and are insufficient for larger packs. Dry powder extinguishers contaminate the equipment and impair visibility but also provide no cooling.
After a fire event the affected pack must be treated as hazardous goods and collected by a specialist disposal company. Even if the pack appears externally intact it must be taken out of service — thermally damaged cells can re-enter thermal runaway hours to days after the event.
How BMS and Cell Chemistry Reduce the Risk
A correctly designed BMS reliably prevents the main electrical triggers of thermal runaway: it blocks charging on overvoltage, discharging on undervoltage and both on overtemperature. It detects cell voltage spreads that may indicate a mechanically damaged or aged cell and reports anomalies via CAN bus or Profinet to the machine controller. For industrial use the BMS should additionally evaluate temperature sensors at multiple points on the pack — not just a single overall temperature.
Cell chemistry: LiFePO4 (lithium iron phosphate) is thermally more stable than Li-NMC (lithium nickel manganese cobalt oxide). The reason lies in the binding energy of the cathode material: LiFePO4 releases significantly less oxygen when heated than NMC cathodes, which substantially slows the exothermic reaction in thermal runaway. The self-ignition temperature of LiFePO4 cells is considerably higher than that of comparable NMC cells. For industrial applications where safety and longevity are prioritised over energy density, LiFePO4 is therefore the preferred choice — including at Hellpower Energy for AGV and FTS applications.