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 are the main Causes of 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 their nominal temperatures as stated in their battery specifications.
For Valve Regulated Lead Acid (VRLA) batteries, this is typically above 77° F (25° C) and for most lithium-ion batteries, this is above 82° F (28° C).
A high ambient temperature reduces a battery system’s ability to shed heat. This leads to an increase of the internal chemical reaction, a reduced impedance, and an increase in float charging current, further increasing the battery temperature.
Age of the Battery
The age of a battery is determined by the state of charge and the state of the overall battery system’s health.
For VRLA batteries, the end of life at the monobloc level is when the battery reaches 80% of its rated capacity when new. Lithium-ion batteries analyze end of life differently by factoring in the state of health of the battery at the cell level. The typical end of life of a lithium-ion system is state of health.
Battery systems that are cycled excessively beyond their intended use case or used improperly can also experience expedited aging over time.
Float Charging Voltage
In VRLA batteries, excessive float charging voltage can accelerate aging in the cells and increase internal battery temperature overtime.
In Lithium-ion batteries, the float voltage and charging parameters are strictly managed but any excessive float charging could cause serious internal battery temperature increases that may cascade into a thermal runaway event.
Overcharging
Both VRLA and Lithium-ion batteries require charge current control. Continuous overcharging and excessive float voltage can damage the internal battery design, leading to shortened battery life.
This concern is reduced or eliminated for VLRA and lithium-ion battery systems with battery management systems that passively or actively control the charge current and float voltage to the system.
What are the Dangers of UPS Batteries Overheating?
Excessive overheating can compromise the VRLA monobloc or lithium-ion module casing. When this happens in VRLA batteries, hydrogen sulfide gas (smelling like rotten eggs) will escape. In lithium-ion, the system will experience sudden fire and smoke.
In both battery types, charging power to the battery system needs to be removed immediately and the battery breakers should be opened automatically.
If undetected, excessive heat can lead to thermal runaway and can cause catastrophic results including fire, explosion, sudden system failure, costly damage to equipment, and possibly personal injury.
It should be noted that during a lithium-ion thermal runaway event, even after current to the system is removed, due to the chemical properties of the electrolyte and cell, it is possible that the system will continue to cascade further into thermal runaway.
For this reason, Mitsubishi Electric only provides lithium-ion battery products that meet acceptabletest resultsduring the UL 9540A test and is fully listed to UL 1973. Safety is extremely important to Mitsubishi Electric to meet the strict reliability expectations of our clients.
What can be done to prevent thermal runaway?
Be on 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 thermal runaway.
Both VRLA and lithium-ion batteries are required by domestic and international fire codes to have some kind of thermal runaway protection. Below we discuss the specific requirements to prevent VRLA and lithium-ion thermal runaway events.
VRLA Thermal Runaway Prevention & Control
To protect VRLA batteries from thermal runaway, typically, a battery monitoring system (BMS) is connected and integrated into the overall battery system to monitor the batteries.
The BMS measures and records the voltages, currents, impedance, and temperatures of the batteries monoblocs (but not the individual cells). These measurements are then used to monitor or alert to signs of rapid temperature increases and other potential thermal runaway characteristics.
When VRLA thermal runaway is detected, the listed breaker that is in line with the input and output of the battery cabinet is tripped to immediately remove current flow from the electrical circuit. The removal of current and the opening of the circuit reduces the potential spread of thermal runaway and can greatly reduce the damage within the facility.
For VRLA batteries, IEEE recommends two preventative maintenance visits per year to thoroughly inspect the system and measure critical battery characteristics.
Lithium-ion Thermal Runaway Prevention & Control
Most lithium-ion batteries seeking a UL1973 listing need to complete UL 9540A testing and are required to have a thermal runaway prevention listed control device or scheme.
A lithium-ion battery must have its own proprietary protection scheme or battery management system that is designed specifically for that chemistry and that battery system.
The protection scheme and battery management system for a lithium-ion battery includes recording and analyzing data down to the battery cell level. The cell level voltages, temperatures, and current are recorded and analyzed with the use of protection settings to specifically prevent thermal runaway. This is where a lithium-ion battery management system differs from a battery monitoring system.
Like VRLA batteries, when a lithium-ion thermal runaway event is detected, the breaker in line with the input and output of the battery rack is opened immediately and current is then removed from the circuit. The removal of current and the opening of the circuit reduces the potential spread of thermal runaway and can greatly reduce the damage within the facility.
Lithium-ion battery thermal runaway protection systems may include some kind of fire suppressant system,* such as NOVEC dry-type or FST aerosol fire suppression.
IEEE recommends one preventative maintenance visit per year. This maintenance visit is performed to download and inspect the lithium-ion battery management system data logging files. By analyzing the data, Mitsubishi Electric will identify any minor or major issues with the battery system. This will also maintain the warranty and performance guarantee guidelines and requirements.
*Currently, most UPS data center lithium-ion applications do not typically have an active fire suppression system included in the battery rack or cabinet, but that may change as technology advances.
The best way to avoid the cataclysmic event that thermal runaway can become is through the application of the following:
Preventative Maintenance
Promote the longevity of your equipment and ensure your system is ready to respond with regularly scheduled maintenance. Be proactive, not reactive.
Battery Monitoring
Continuous monitoring of battery cabinet temperature and/or individual battery terminal temperature, and systematic review of the historic battery data
Safety Codes
Obtaining a UL 9540 UPS & Battery System Listing can help prevent thermal runaway and meet the latest NFPA/IFC guidelines for fire safety. Mitsubishi Electric can support you in the listing process.
ESS Fire Safety Codes
The 2024 NFPA 1 Fire Code for Energy Storage Systems (ESS) now states to refer to NFPA 855 as the guide for thermal runaway protection. Electrochemical Energy Storage Systems Table 9.6.5 sorts the different electrochemical ESS technology-specific requirements by their general types.
This table also indicates if a certain electrochemical ESS technology and general type requires thermal runaway protection. Below is a UPS/ESS specific table that references table 9.6.5 from NFPA 855.
| Thermal Runaway Protection Required | |
| Lead Acid (VRLA) | Yes |
| Non-Vented Ni-Cd, Ni-MH, Ni-Zn1 | Yes |
| Vented Flooded Lead Acid & Vented Ni-Cd | No |
| Lithium-ion | Yes |
| Flow | No |
|
Sodium Nickel Chloride |
Yes |
|
Electrochemical Double-Layer Capacitor (EDLC) Energy Storage |
Yes |
|
*Other Electrochemical ESS & Battery Technologies |
Yes |
1Per the 2021 International Fire Code (IFC), all Ni-Cd batteries are required to have thermal runaway protection. This is the only major difference from NFPA 855.
*The protection in this column is not required if documentation acceptable to the authority having jurisdiction (AHJ) - including a hazard mitigation analysis complying with Section 4.4 of NFPA 855 - provides justification that the protection is not necessary based on the technology used.
The types of batteries with a “Yes” in the column for thermal runaway protection are required to have a UL or IEC listed device - or an approved method evaluated as part of the Energy Storage / UPS System.
The listed device or approved method is required to manage charging and discharging during normal operation, maintain safe operating parameters, and preclude thermal runaway.
Thermal runaway protection is provided by the battery or capacitor management system that has been evaluated as part of the UL 1973 or UL 9540 listing.
References
- 2024 NFPA 1 Fire Code, 2023 NFPA-855 Chapter 9 Electrochemical Energy Storage Systems Table 9.6.5 & Section 9.6.5.5
- 2021 International Fire Code (IFC) Chapter 12 Energy Systems Table & Section 1207