Thermal Runaway Propagation in Lithium-Ion Battery Packs: Mitigation Strategies

The Problem

Thermal runaway in a single lithium-ion cell is a well-characterized failure mode. The more dangerous scenario — and the one that matters at pack level — is propagation: when one cell's thermal event triggers adjacent cells, creating a cascading failure.

In densely packed battery modules, cell-to-cell propagation can occur within seconds. The engineering challenge is not merely detecting the initial event, but ensuring that detection-to-response latency is short enough to prevent cascade.

Propagation Mechanisms

Thermal energy transfers between cells through three pathways:

1. Conduction through shared structural elements, bus bars, and thermal interface materials
2. Radiation from the failed cell's surface to neighbors
3. Convective heating from vented electrolyte gases

In our test configurations, conduction through copper bus bars was the dominant propagation pathway, accounting for roughly 60% of heat transfer to adjacent cells in the first 30 seconds after initial failure.

Detection Approaches

Voltage-Based Detection

Cell voltage collapse is the earliest electrical indicator of thermal runaway. A sudden drop greater than 0.5V within 100ms is highly specific to internal short circuits. The BMS must sample at sufficient frequency — we found that 10Hz sampling misses roughly 15% of onset events that are caught at 100Hz.

Temperature Monitoring

Surface-mounted thermistors detect thermal runaway reliably but with inherent latency. In our measurements, cell surface temperature lags internal temperature by 8-15 seconds depending on cell format and thermistor placement.

Detection latency comparison:
┌────────────────────┬───────────────┐
│ Method             │ Latency (s)   │
├────────────────────┼───────────────┤
│ Voltage collapse   │ 0.1 - 0.5    │
│ Surface temp       │ 8 - 15       │
│ Gas detection      │ 2 - 5        │
│ Impedance shift    │ 1 - 3        │
└────────────────────┴───────────────┘

Gas Sensing

Electrolyte decomposition produces detectable gases (CO, CO₂, H₂) before thermal runaway becomes self-sustaining. Gas sensors in the module headspace provide earlier warning than temperature measurement, though sensor placement and airflow design are critical.

Mitigation Strategies

Physical Barriers

Intumescent barriers between cells expand when heated, creating an insulating layer that slows conduction. In our testing, 2mm intumescent sheets between cylindrical cells increased propagation delay from 8 seconds to over 120 seconds — sufficient time for active cooling intervention.

Active Cooling Response

When thermal runaway is detected, the pack management system can:

• Electrically isolate the affected module via contactors
• Activate emergency cooling (forced air or liquid coolant pump override)
• Initiate controlled discharge of adjacent modules to reduce stored energy

Cell Spacing Optimization

Simply increasing inter-cell spacing is effective but reduces volumetric energy density. We modeled the trade-off and found that 3mm spacing (up from 1mm) reduces propagation probability by 85% while only reducing pack energy density by 12%.

Practical Recommendations

For engineers designing battery packs:

1. Sample voltage at ≥50Hz — slower rates miss fast-onset events
2. Combine voltage and gas detection — provides both speed and confirmation
3. Design for containment, not prevention — assume a single-cell event will happen and engineer the pack to survive it
4. Test with worst-case cell state — fully charged cells at elevated temperature are the correct test condition, not nominal conditions

Thermal runaway will remain a design constraint for lithium-ion systems. The engineering goal is not eliminating the risk but bounding its consequences.