Thermal Runaway

What is it and How to Prevent it

Thermal Runaway

What is it and How to Prevent it

What is Thermal Runaway?


Thermal runaway begins when the heat generated within a battery exceeds the amount of heat that is dissipated to its surroundings. 


If the cause of excessive heat creation is not remedied, the condition will worsen. Internal battery temperature will continue to rise - causing battery current to rise - creating a domino effect. The rise in temperature in a single battery will begin to affect other batteries in close proximity, and the pattern will continue, thus the term “runaway.”

What Causes the Battery Overheating that Leads to Thermal Runaway?


There can be many contributing factors to thermal runaway in your UPS battery:


Ambient Temperature

Battery life is compromised when continually subjected to ambient temperatures above 77° F (25° C). A high ambient temperature reduces a battery’s ability to shed heat. This leads to an increase of the internal chemical reaction and increased float charging current, further increasing the battery temperature.

Age of the Battery

Batteries that are near end of life (commonly defined as a reduction to 80% of rated capacity) have been degrading over time, possibly requiring longer time to charge and/or being subjected to higher current and internal heat generation.

Float Charging Voltage

Excessive float charging voltage can cause an increase in float charging current and increased internal battery temperature.


Continuous overcharging will damage battery internals, leading to a shortened battery life.

What are the Dangers of UPS Batteries Overheating?


Left uncorrected, internal temperature will continue to increase, causing battery overheating and the outer casing to bulge, melt and/or rupture. When the casing of a VRLA battery becomes compromised, hydrogen sulfide gas (smelling like rotten egg) will escape. If this is observed, charging power to the battery needs to be removed immediately. If undetected, thermal runaway can lead to catastrophic results, including fire, explosion, sudden system failure, costly damage to equipment, and possibly personal injury.


Thermal runaway can occur in lithium ion batteries and VLA, but this discussion focuses mainly on VRLA batteries, since they are most common in the industry. Additionally, our lithium ion battery cabinets are already equipped with Battery Monitoring/Management Systems (BMS). The logging and network connection/ communication capabilities allow for data-driven decision and better management of the overall system.

What can be done to prevent thermal runaway?


Be on the alert for other battery anomalies, such as ground faults and shorted cells, which can negatively impact charging voltages and ultimately lead to thermal runaway. If you notice a battery overheating, disconnect it from charge and inspect it for other damage. Separate it from other systems to avoid runaway.


The best way to avoid the cataclysmic event that thermal runaway can become is through the application of the following:




Regularly scheduled maintenance of the batteries



Continuous monitoring of battery cabinet temperature and/or individual battery terminal temperature, and systematic review of the historic battery data

Photos on this page provided by Critical Power Solutions Inc. and C & C Power, Inc.