What is thermal runaway and how do I prevent it?

Thermal runaway is a self-reinforcing loop: elevated temperature increases charge current acceptance, the increased current generates more heat, the higher temperature increases current further. Left unchecked it dries the cell, deforms the case and can vent or rupture it.

This is a recognised failure mode in utility battery fleets managing critical infrastructure—not theoretical. It occurs when cells are over-charged by float voltage intended for the string as a whole, particularly where a weak cell sits in the circuit. This is why monitoring charge voltage and cell-level temperature on critical VRLA banks is standard practice.

The three contributing conditions are almost always present together:

1. High ambient temperature. Risk rises steeply above 40 °C.
2. A charger without temperature compensation, holding a float voltage appropriate for 20 °C at a cell that is at 45 °C.
3. An aged or damaged battery with reduced internal resistance and impaired recombination.

Prevention is unglamorous and effective:

• Temperature-compensated charging. This is now the modern standard in utility specifications. Many existing charger fleets lack the functionality, but current standards require it.
• Keep the battery below 25 °C where you can, and if you cannot, derate the expected life accordingly rather than being surprised.
• Monitor cell or monobloc temperature, and alarm on it. Critical infrastructure battery specifications typically require temperature logging at 15-minute intervals with high-temperature warning and over-temperature shutdown.
• Replace on condition, not on failure. An aged bank is the substrate the fault needs.

 

Note also that AS 60896.22 requires the manufacturer to provide thermal runaway sensitivity information. It is a disclosure requirement, not an immunity claim, but it means you are entitled to ask for the data, and comparing it across suppliers is a legitimate procurement question.