What Is Thermal Runaway?

When a battery is connected to a charger, elevated temperature lowers internal resistance, allowing more current to flow. More current increases heat, which further raises temperature, repeating in a loop until the battery overheats and can fail catastrophically. In lead-acid systems, this may present as abnormal warmth, case bulging, and rapid loss of control over charge current.

In short: higher temperature → higher current → even higher temperature (a self-reinforcing loop).

Why Temperature Raises Float Current (Simple View)

In float charging, the charger maintains a small current to offset self-discharge. As temperature rises, the electrochemical kinetics accelerate and the effective float current increases, which adds thermal load. If cooling and charge control are insufficient, the loop can spiral.

Common Factors That Trigger Thermal Runaway

1. Excessive charge voltage — Over-voltage drives higher current and heat, raising risk.
2. Excessive charge current — Lack of current limiting overheats cells; typical practice is to cap charge current near 0.1C10 for lead-acid.
3. High ambient temperature — Rooms above recommended range (about 20–25 °C) push float current up and reduce heat margin; cabinets also need proper ventilation.
4. Shorted cell(s) — A failed cell can force other cells to higher per-cell voltage, increasing float current and heat.
5. Leakage to ground — Ground faults allow large unintended currents through the battery, driving heating and runaway risk.
6. Charger malfunction — Faulty regulation that outputs excessive voltage elevates risk.
7. Improper installation or maintenance — Insufficient know-how, poor setup, or skipped checks increases exposure to the above issues.

Practical Prevention Checklist

Adopt the following measures to reduce the likelihood and impact of thermal runaway:

• Use flame-retardant battery enclosures certified to standards such as UL94 and UL1778.
• Select battery models designed to resist thermal runaway and require a passed Thermal Runaway test for the specific model.
• Implement preventive maintenance following IEEE 1188 so issues are caught early.
• Control environment: keep battery rooms near 25 °C and ensure good cabinet airflow.
• Provide spacing: leave 1–2 cm between units for heat dissipation.
• Use a charger with temperature compensation so float/equalize set-points track temperature.
• Follow the manufacturer’s manual strictly for settings and handling.
• Install a Battery Monitoring System (BMS/monitoring) to continuously track health and detect anomalies early.

Conclusion

Thermal runaway is preventable. By keeping voltage and current within limits, managing ambient temperature, maintaining equipment, and monitoring batteries continuously, operators can avoid the self-heating loop that leads to failure—and extend system life and safety.