Thermal Runaway — The 5 Stages Your BMS Is Racing Against

Stage 1: SEI Decomposition (80–120°C)
Stage 2: Electrolyte Decomposition and Separator Melting (120–160°C)
Stage 3: Cathode Decomposition — The Point of No Return (160–220°C for NMC)
Stage 4: Full Thermal Runaway (>200°C)
Stage 5: Pack-Level Propagation
BMS Detection Strategy: What to Monitor and When
Key Takeaways

By Stage 3, the cathode is producing its own oxygen and the exothermic reactions are self-sustaining — the BMS intervention window has closed. Everything that matters happens in Stages 1 and 2.

TL;DRKey Points

Thermal runaway is a five-stage cascade: SEI decomposition, electrolyte decomposition and separator melting, cathode decomposition (point of no return), full thermal runaway, and pack-level propagation.
The BMS intervention window closes at Stage 3 for NMC (~160–180°C) — effective detection must happen at Stages 1 or 2.
LFP has a 90°C higher Stage 3 onset temperature than NMC 811 and does not produce oxygen during cathode decomposition, making propagation far less severe.
Temperature threshold monitoring alone only reliably detects Stage 4; rate-of-rise monitoring, voltage deviation, and gas sensing extend detection into Stages 1–2.
AIS-156 Phase 2 requires co-design of BMS detection capability and physical thermal barriers — both must be validated together to meet the 5-minute propagation requirement.

Thermal runaway is frequently described as a single catastrophic event — a cell suddenly catches fire. The reality is a five-stage cascade that unfolds over seconds to tens of minutes, with each stage producing specific chemical signatures that a well-designed BMS can detect, and each stage providing a narrowing but real intervention window. Understanding the sequence is the prerequisite for designing effective detection and mitigation strategies.

Stage 2 onset temperature (NMC 811, 100% SOC) | ~90–130 | °C

Stage 3 onset temperature (NMC 811) | ~150–180 | °C (point of no return)

Stage 3 onset temperature (LFP, 100% SOC) | ~240–270 | °C

Typical BMS temperature sensor response time | 1–5 | seconds

Maximum rate of temperature rise at Stage 4 | 100–1,000 | °C per minute

AIS-156 Phase 2 minimum warning time requirement | 5 | minutes to passenger hazard

Stage 1: SEI Decomposition (80–120°C)

The Solid Electrolyte Interphase (SEI) is a protective layer that forms on the anode surface during the first few charge cycles. It is necessary for the cell to function — it prevents the electrolyte from continuously reacting with the anode — but it is metastable. Above approximately 80–100°C, the SEI begins to decompose.

Reactions: The organic components of the SEI (primarily lithium alkyl carbonates) decompose exothermically, releasing CO₂ and heat. This is a self-limiting reaction in most cases — the decomposition product is a more thermally stable inorganic SEI (Li₂CO₃, LiF). But the heat released can raise the cell temperature if cooling is insufficient.

BMS detection opportunity: Temperature rise rate faster than expected for the current load. At this stage, the temperature rise is typically 0.5–2°C/minute above ambient — detectable but not alarming. A BMS with per-cell temperature monitoring and rate-of-rise detection (dT/dt threshold) can identify anomalous cells in Stage 1.

Intervention: Remove load immediately. Increase cooling. If temperature continues rising without load, cell is compromised — isolate the module.

QWhat is the SEI layer and why does its decomposition matter for thermal runaway?

The Solid Electrolyte Interphase (SEI) is a thin, stable layer that forms on the anode surface during the first few charge cycles. It is essential for normal cell operation — it prevents the electrolyte from continuously reacting with the anode. However, the organic components of the SEI are only metastable above ~80–100°C. Their exothermic decomposition releases CO₂ and heat, raising cell temperature. In isolation this is self-limiting, but if cooling is insufficient, the SEI decomposition heat can push the cell into the more severe Stage 2 reactions. Early detection of Stage 1 through abnormal dT/dt is the earliest possible BMS intervention point.

Stage 2: Electrolyte Decomposition and Separator Melting (120–160°C)

At temperatures above approximately 120°C, the liquid electrolyte itself begins to decompose. The organic solvents (typically EC/DEC or EC/DMC combinations) break down through oxidation and reduction reactions, releasing combustible gases including CO, CO₂, and short-chain hydrocarbons.

Simultaneously, if the cell temperature reaches the melting point of the separator (typically PE: ~135°C, PP: ~165°C), the separator begins to soften and collapse. Once the separator can no longer maintain physical separation between anode and cathode, an internal short circuit occurs — creating a low-resistance path that discharges the cell's stored energy as heat instantaneously.

Rates: Stage 2 temperature rise rate: 2–10°C/minute. The gas generation is detectable with in-pack CO/CO₂ sensors.

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Key Insight

Separator melting is the trigger for the irreversible phase of thermal runaway. Once the separator shorts, the stored electrochemical energy (which can be 250–300 Wh/kg at 100% SOC for NMC) is converted to heat on a timescale of seconds. The BMS cannot stop this conversion. The only meaningful intervention remaining is to prevent cell-to-cell propagation through the pack.

Stage 3: Cathode Decomposition — The Point of No Return (160–220°C for NMC)

Above approximately 160–180°C for NMC 811 (240–270°C for LFP), the delithiated cathode material decomposes. For NMC, this involves:

LiₓNiO₂ → Li₁₋ₓNiO₂ + x/2 O₂ (simplified)

The cathode releases oxygen. This oxygen reacts with the flammable electrolyte decomposition products present in the cell, creating a self-sustained oxidation reaction. The cell no longer needs an external oxygen source — it produces its own. This is why EV fires are so difficult to extinguish: you cannot cut off the oxygen supply by smothering.

Temperature rise rate at Stage 3: 10–100°C/minute. Beyond any cooling system's capacity to control.

LFP's advantage: The olivine (FeO₄) structure of LFP does not release oxygen during cathode decomposition. This is why LFP thermal runaway is fundamentally less severe:

Peak temperature: 300–500°C for LFP vs 700–900°C for NMC 811
Self-propagation tendency: much lower for LFP
AIS-156 barrier material requirement: lighter for LFP

Stage	Temperature Range (NMC 811)	Temperature Range (LFP)	Reactions	Detectable By
Stage 1: SEI decomposition	80–120°C	90–130°C	Li alkyl carbonates decompose	Temperature rate (dT/dt)
Stage 2: Electrolyte decomp. + separator	120–160°C	130–170°C	EC/DEC oxidation, separator melting	Gas sensor (CO, CO₂), temp
Stage 3: Cathode decomposition	160–180°C	240–270°C	O₂ release from cathode	Uncontrollable — smoke/gas
Stage 4: Full thermal runaway	200–500°C	300–500°C	Full oxidation, flames	Smoke, heat detector, IR
Stage 5: Pack propagation	Per cell → adjacent	Per cell → adjacent	Cell-to-cell heat transfer	Pack temperature sensors

QWhy does separator melting mark the irreversible threshold in thermal runaway progression?

Once the polyolefin separator melts (PE at ~135°C, PP at ~165°C), physical separation between anode and cathode is lost. The direct contact creates an internal short circuit — a low-resistance path that converts the cell's stored electrochemical energy to heat almost instantaneously. A 100% SOC NMC cell holds ~250–300 Wh/kg; releasing this over seconds rather than hours produces a catastrophic heat spike far beyond any cooling system's capacity. Unlike the earlier stages where load removal can arrest the progression, separator failure is mechanically irreversible and the subsequent heat release cannot be stopped by BMS action.

Stage 4: Full Thermal Runaway (>200°C)

At Stage 4, the cell is a self-sustaining chemical reactor generating heat at rates of 100–1,000 kW/kg of active material. The temperature rises to 700–900°C for NMC cells. All organic materials within the cell combust. The cell case ruptures mechanically from internal pressure, ejecting burning electrolyte and solid fragments.

Gas composition at Stage 4: CO (toxic, flammable), CO₂, H₂ (explosive), hydrocarbons, HF (toxic), and combustion products. The HF generation from LiPF₆ electrolyte salt decomposition is particularly significant — HF is acutely toxic and requires specialised respiratory protection for firefighters.

The rate of temperature rise at Stage 4 exceeds any thermal management system's dissipation capacity by orders of magnitude. The only BMS response remaining is to maximise the time before the next cell enters Stage 2.

Stage 5: Pack-Level Propagation

If cell-to-cell thermal runaway propagation is not prevented, Stage 5 describes the sequential thermal runaway of adjacent cells. The rate of propagation depends on:

Cell-to-cell thermal resistance: Higher thermal resistance barriers (intumescent materials, aerogel sheets) slow propagation
Inter-cell gap: Air gaps of even 0.5–1 mm significantly reduce conductive heat transfer between cells
Pack cooling system response: Active cooling can remove heat from non-failed cells if the cooling system remains operational
Pack architecture: Cell-to-pack (CTP) designs without cell-level modules have higher propagation risk than modular designs with thermal barriers at module boundaries

PYTHON

# dT/dt detection: rolling window derivative filter
from collections import deque
import time

class ThermalDerivativeDetector:
    """
    Computes smoothed dT/dt using a sliding window.
    Critical for Stage 2 to Stage 3 transition in thermal runaway FSM.
    """
    def __init__(self, window_s: float = 5.0, sample_rate_hz: float = 10.0):
        self.n = int(window_s * sample_rate_hz)
        self.temps = deque(maxlen=self.n)
        self.times = deque(maxlen=self.n)

    def update(self, temp_degC: float):
        """Returns dT/dt in deg C/s or None if window not full."""
        self.temps.append(temp_degC)
        self.times.append(time.monotonic())
        if len(self.temps) < self.n:
            return None
        dt_s = self.times[-1] - self.times[0]
        dT   = self.temps[-1] - self.temps[0]
        return dT / dt_s   # deg C/s

# Simulate: 5-minute observation with runaway starting at t=3 min
detector = ThermalDerivativeDetector(window_s=5.0)
for t_s in range(300):
    if t_s < 180:
        T = 55 + 0.05 * t_s           # slow rise, normal abuse
    else:
        T = 55 + 0.05*180 + 2.0 * (t_s - 180) ** 1.5 / 30  # accelerating
    rate = detector.update(T)
    if rate is not None and rate > 1.0:
        print(f"t={t_s}s  T={T:.1f} deg C  dT/dt={rate:.2f} deg C/s  STAGE 3 TRIGGER")
        break

Warning

AIS-156 Phase 2 requires that thermal runaway in a single cell does not propagate to cause a passenger compartment hazard within 5 minutes of onset. This requires both BMS detection (at Stage 2 or earlier) and pack physical design (thermal barriers, venting). Neither alone is sufficient. A pack that passes the BMS detection requirement but has inadequate thermal barriers can fail the 5-minute requirement at the propagation stage. Both systems must be co-designed and co-validated.

QHow does cell SOC at the time of a thermal event affect the severity of thermal runaway?

A fully charged NMC 811 cell (100% SOC) has a delithiated cathode dominated by Ni⁴⁺ ions that are thermally unstable and readily release oxygen above ~180°C. The same cell at 50% SOC has a partially lithiated cathode with more thermally stable Ni²⁺/³⁺, raising the onset temperature to approximately 210–220°C. For NMC, limiting pack operation to 80% maximum SOC provides a meaningful safety margin increase of ~30°C on the thermal runaway onset threshold. For LFP, this SOC-dependence on onset temperature is much less pronounced because the olivine structure is stable at all lithiation states.

BMS Detection Strategy: What to Monitor and When

An effective thermal runaway detection strategy monitors multiple signals, because no single signal is sufficient:

Temperature rate-of-rise monitoring

Compute dT/dt for each cell or module. Threshold: >2°C/min unexplained by current load → Level 1 warning. >10°C/min → immediate isolation. This catches Stage 1.

Voltage deviation monitoring

Compare cell voltage to expected OCV at current SOC and temperature. Large negative deviation (cell voltage lower than expected) indicates internal short — trigger even before temperature rises. This can catch Stage 2 before thermal signal.

Gas sensing

Integrate CO/CO₂ sensor in pack headspace. CO detection threshold: 50–100 ppm → Level 2 alert. This catches Stage 2 electrolyte decomposition.

Pressure monitoring

Cell pressure rise detected through pack enclosure pressure sensor. Useful for pouch and prismatic cells. Detects Stage 2 gas generation.

Multi-signal fusion

A single anomalous reading generates an alert. Confirmation from two independent signals (e.g., temperature rate AND voltage deviation) triggers load disconnect. Three signals → immediate maximum cooling + emergency alert to driver.

/* Thermal runaway FSM -- runs in BMS firmware every 100 ms */
#include <stdint.h>
#include <stdbool.h>

/* Detection thresholds */
#define STAGE1_TEMP_ONSET_DC   550   /* 55.0 deg C x 10 */
#define STAGE2_DTEMP_DT_DC_S   10    /* 1.0 deg C/s  x 10 */
#define STAGE3_DTEMP_DT_DC_S   100   /* 10 deg C/s   x 10 */
#define STAGE4_VDROP_MV        200   /* 200 mV sudden drop */
#define GAS_SENSOR_THRESHOLD   350   /* ppm VOC sensor ADC count */

typedef enum {
    TR_STAGE_NORMAL      = 0,
    TR_STAGE_ABUSE       = 1,   /* temp above warning threshold */
    TR_STAGE_ONSET       = 2,   /* exothermic onset, dT/dt rising */
    TR_STAGE_PROPAGATION = 3,   /* uncontrollable self-heating */
    TR_STAGE_VENTING     = 4,   /* gas sensor triggered */
    TR_STAGE_FIRE        = 5,   /* full thermal runaway */
} ThermalRunawayStage_t;

static ThermalRunawayStage_t tr_stage = TR_STAGE_NORMAL;

ThermalRunawayStage_t bms_update_thermal_stage(
    int16_t  temp_dc,          /* temperature in 0.1 deg C units */
    int16_t  dtemp_dt_dc_s,    /* dT/dt in 0.1 deg C/s units    */
    uint16_t cell_mv,          /* cell voltage in mV             */
    uint16_t gas_ppm_adc       /* VOC gas sensor ADC value       */
) {
    switch (tr_stage) {
    case TR_STAGE_NORMAL:
        if (temp_dc >= STAGE1_TEMP_ONSET_DC)
            tr_stage = TR_STAGE_ABUSE;
        break;
    case TR_STAGE_ABUSE:
        if (dtemp_dt_dc_s >= STAGE2_DTEMP_DT_DC_S)
            tr_stage = TR_STAGE_ONSET;
        else if (temp_dc < STAGE1_TEMP_ONSET_DC - 50)
            tr_stage = TR_STAGE_NORMAL;   /* cooled down */
        break;
    case TR_STAGE_ONSET:
        if (dtemp_dt_dc_s >= STAGE3_DTEMP_DT_DC_S)
            tr_stage = TR_STAGE_PROPAGATION;
        break;
    case TR_STAGE_PROPAGATION:
        if (gas_ppm_adc >= GAS_SENSOR_THRESHOLD)
            tr_stage = TR_STAGE_VENTING;
        break;
    case TR_STAGE_VENTING:
        tr_stage = TR_STAGE_FIRE;        /* no recovery from venting */
        break;
    default:
        break;
    }
    return tr_stage;
}

Note

Most production BMS systems in Indian commercial EVs rely solely on temperature threshold monitoring for thermal runaway detection. This catches Stage 4 reliably but typically misses Stages 1 and 2. Multi-signal fusion (temperature rate-of-rise + voltage deviation + optional gas sensing) can detect events in Stage 1–2, which provides a 30–120 second earlier intervention window — the difference between prevention and response.

/* BMS response actions per thermal runaway stage */
void bms_thermal_runaway_response(ThermalRunawayStage_t stage) {
    switch (stage) {
    case TR_STAGE_ABUSE:
        /* Reduce charge/discharge current limit by 50% */
        set_current_limit_pct(50);
        send_dtc(DTC_TEMP_WARNING);
        break;
    case TR_STAGE_ONSET:
        /* Stop charging, derate discharge to 25% */
        stop_charging();
        set_current_limit_pct(25);
        activate_cooling_max();
        send_dtc(DTC_THERMAL_ONSET);
        break;
    case TR_STAGE_PROPAGATION:
        /* Open main contactors -- isolate pack from vehicle */
        open_main_contactor_pos();
        open_main_contactor_neg();
        activate_fire_suppression();     /* if equipped */
        send_dtc(DTC_THERMAL_RUNAWAY);
        trigger_hazard_alert_to_vcu();
        break;
    case TR_STAGE_VENTING:
    case TR_STAGE_FIRE:
        /* Already disconnected -- keep logging for forensics */
        log_to_persistent_memory(stage);
        break;
    default:
        break;
    }
}

Key Takeaways

✦Key Takeaways

Thermal runaway is a five-stage cascade: SEI decomposition → electrolyte decomp. + separator melting → cathode decomposition (point of no return) → full thermal runaway → pack propagation.
The BMS intervention window closes at Stage 3 for NMC (~160–180°C). By Stage 3, the cathode is producing its own oxygen and the reactions are self-sustaining. Effective intervention requires detection at Stages 1–2.
LFP has a 90°C higher Stage 3 onset temperature than NMC 811 and does not release oxygen during cathode decomposition. These two differences fundamentally change the severity and propagation behaviour of thermal events.
Temperature threshold monitoring alone (the most common BMS approach) only reliably detects Stage 4. Rate-of-rise monitoring, voltage deviation detection, and gas sensing extend detection into Stages 1–2 and provide 30–120 seconds additional intervention time.
AIS-156 Phase 2's 5-minute propagation requirement demands co-design of the BMS detection capability and the pack physical thermal barriers. Both must be validated together, not independently.

Part of the thermal Series

← PreviousWhat Is Thermal Runaway? (And Why Your EV Won't Randomly Explode)Next →Thermal Runaway — What Actually Happens Inside a Cell Before It Catches Fire

Written by

Sai Chaitanya Dasari

Battery Systems Engineer · Volvo Eicher Commercial Vehicles

3+ years in commercial EV pack development. Writing about real battery engineering from the bench.

Frequently Asked Questions

At what stage does thermal runaway become unrecoverable?

Stage 3 — cathode decomposition — is the practical point of no return. At this stage the cell is generating its own oxidiser (oxygen from the cathode) and the exothermic reactions are fully self-sustaining. Stages 1 and 2 are technically recoverable if aggressive cooling and load removal happen quickly enough.

What gases are produced during each stage of thermal runaway?

Stage 1 (electrolyte decomposition, >80°C for some chemistries): CO₂ and water vapour from electrolyte oxidation. Stage 2 (SEI breakdown, ~90–130°C): CO₂, hydrocarbons, and trace HF from decomposing SEI and electrolyte salts. Stage 3 (cathode decomposition, >150–200°C): CO, CO₂, O₂ from cathode, and more HF. Stage 4 (full thermal runaway, >200°C): All previous gases plus intense combustion products. Stage 5 (venting/explosion): Explosive mixture of combustible gases and oxygen. Each stage's gas signature can be detected by specific gas sensors, which is why multi-gas sensor arrays are more effective for early thermal runaway warning than temperature sensors alone.

Can the BMS detect thermal runaway before it becomes unrecoverable?

In Stages 1 and 2, detectable precursors exist: cell temperature rising faster than expected for the current load, voltage deviating from the expected OCV-SOC relationship (indicating internal resistance changes), and gas sensor signals if the pack includes gas monitoring. By Stage 3, the rate of temperature rise has typically exceeded any practical cooling response capability. The BMS can detect and alert, but cannot prevent Stage 3+ progression. AIS-156 mandates at minimum a 5-minute warning from onset to passenger hazard, which typically requires BMS detection at Stage 2 or early Stage 3.

How does cell SOC at the time of abuse affect thermal runaway severity?

Higher SOC means more energy stored and more reactive electrode states. A fully charged (100% SOC) NMC 811 cell has delithiated cathode (Ni⁴⁺ dominant) that is thermally unstable and readily releases oxygen above 180°C. The same cell at 50% SOC has a thermally more stable cathode with partially lithiated Ni²⁺/³⁺, and the onset temperature rises to approximately 210–220°C. For LFP, the SOC effect is less pronounced because the olivine structure is thermally stable regardless of lithiation state. For NMC, operating the pack between 20% and 80% SOC rather than 0–100% meaningfully increases the thermal stability margin.

What is the difference between cell venting and cell thermal runaway?

Cell venting is a designed safety feature — a pressure relief valve that opens when internal gas pressure exceeds the design limit, releasing gas before the cell ruptures mechanically. Venting does not necessarily lead to thermal runaway. Many cells vent during aggressive overcharge and then recover if the overcharge is stopped. Thermal runaway requires that the cell's internal heat generation exceeds its heat dissipation — which may or may not accompany venting. A cell that vents hot gases and then the gas ignites from an external spark has undergone venting followed by fire, not necessarily thermal runaway. True thermal runaway is self-sustaining without any external energy input.

References

Stage 1: SEI Decomposition (80–120°C)
Stage 2: Electrolyte Decomposition and Separator Melting (120–160°C)
Stage 3: Cathode Decomposition — The Point of No Return (160–220°C for NMC)
Stage 4: Full Thermal Runaway (>200°C)
Stage 5: Pack-Level Propagation
BMS Detection Strategy: What to Monitor and When
Key Takeaways