In product development, certain tests are routine. This isn't one of them. When you are creating components like advanced batteries or new building materials, you have a responsibility to know precisely how they will behave in a fire.
We believe a sharp focus on the Heat Release Rate (HRR) is one of the most productive things a product team can do. We've been in this field for years, and we can tell you that getting this one number right is what separates a truly safe product from a serious safety incident. It is about having a clear picture of what you're dealing with before it's too late.
So let’s break down what HRR is, why it's so important for battery applications, and how to properly read the results from a test.
Key Takeaways
- Speed Over Quantity: Heat Release Rate (HRR) measures how fast a material releases heat rather than the total energy it holds. This makes it the best indicator of how quickly a fire will spread.
- Critical for Battery Safety: For electric vehicles and energy storage, HRR predicts whether a single cell failure will stay contained or trigger a catastrophic explosion across the whole pack.
- The Peak Matters Most: The Peak Heat Release Rate (pHRR) is the most important data point for safety engineers. It reveals the maximum intensity of the fire and the worst-case scenario for your product.
- Automated Calculation: Modern testing relies on the Oxygen Consumption principle. We recommend using automated systems to handle the complex formulas and ensure precise data without human error.
- Testing for Compliance: Meeting standards like UL 9540A requires high-fidelity equipment. Solutions like the Battery Module Calorimeter provide the exact data needed to validate your safety designs.
So, What is Heat Release Rate, Anyway?
The official heat release rate definition is the speed at which a material unloads heat energy when it burns, typically measured in kilowatts (kW). A more useful way to see it is as the accelerator on a fire.
It's easy to confuse this with the total amount of energy a material holds, but that would be a mistake. To see the difference, think about a heavy wooden log versus a bucket of gasoline. They might actually hold a similar amount of total energy potential. However, if you light them up, the log burns slowly over hours (low HRR), while the gasoline releases everything in a massive, instant fireball (high HRR).
In scientific terms, the HRR reflects the intensity and severity of combustion processes (Ping et al., 2015; Voigt et al., 2021; Kim et al., 2022). Accurate measurement here is crucial for assessing fire hazards because it helps predict flame behavior and explosion risk (Ping et al., 2015; Kim et al., 2022; Fu et al., 2015).
A number that climbs quickly points to a fire that is growing exponentially, drastically cutting down the time for evacuation or suppression. For an engineer, this is the kind of variable that demands attention.
Why This is So Critical for Battery Testing
For anyone working in electric vehicle or energy storage industries, the HRR isn't just data. It is a key predictor of system failure.
When a lithium-ion battery begins to overheat, it can set off a chain reaction where one failing cell triggers the next. The heat release rate calculation is what indicates the likelihood of this happening. Research indicates that HRR depends heavily on factors like battery chemistry, physical configuration, and the State of Charge (SOC), with higher SOC generally leading to higher peak HRR and more intense fires (Ping et al., 2015; Kim et al., 2022; Fu et al., 2015; An et al., 2022).
For instance, consider the difference between battery chemistries. An LFP (Lithium Iron Phosphate) cell usually produces a lower, flatter heat curve compared to an NMC (Nickel Manganese Cobalt) cell. The NMC might show a sharp, aggressive spike that is much harder to contain. Understanding these heat generation rates is vital for the design of effective thermal management systems (Ravdel & Puglia, 2022; Hu et al., 2021).
This is why standards like UL 9540A have become so central to the industry. We feel, however, that simply meeting a standard is the starting line. The real objective should be to push your product to its limits using a specialized Battery Module Calorimeter so it never reaches those limits in the field. That is how you engineer a product with a true margin of safety.
The Formula and How We Determine the Rate
The science behind this measurement is a clever principle called Oxygen Consumption. For most materials, there is a direct relationship between the heat being produced and the amount of oxygen the fire consumes.
While various experimental methods exist, including sensible enthalpy rise approaches, each comes with specific sensitivities and limitations (Voigt et al., 2021; Vanderwege & Petersen, 2025). However, oxygen consumption calorimetry remains the foundation for major standards like ASTM E1354 and ISO 5660.
The textbook heat release rate formula looks like this:
q̇ = E × (ṁO2∞ − ṁO2)
How it works in the lab:
In practice, no one is standing by with a calculator during a live fire test. Attempting this by hand is simply impractical and prone to error. Instead, the process relies on sophisticated equipment to do the work.
- The system ignites the sample in a highly controlled environment.
- It analyzes the exhaust to measure how much oxygen the fire used.
- It simultaneously measures the flow rate of that exhaust.
We believe that relying on manual calculations or outdated systems is an unnecessary risk in modern safety testing. A contemporary system, like our Battery Module Calorimeter, automates the entire process. It handles the complex math instantly, allowing your team to concentrate on the most important part: interpreting what the results mean for your product.
Making Sense of the Data on an HRR Chart
After a test, the data is presented on a heat release rate chart. This graph provides a full story of the fire event over time.
- Ignition: The line jumps from zero as the material catches fire.
- Growth: A steep upward slope is a clear warning sign of a fast-developing fire.
- The Peak (pHRR): The most telling feature is the peak of the curve. To visualize this, imagine testing untreated foam versus a fire-resistant panel. The foam would likely create a terrifyingly sharp "needle" on the graph, representing a massive spike in seconds. The resistant panel, on the other hand, would look more like a low, rolling hill. That shape tells you everything about flashover risk.
- Decay: As the material is consumed, the line will fall back down.
Our professional opinion is that the Peak HRR (pHRR) is the single most important value for a safety engineer. A low total energy output means little if the peak release is intense enough to cause immediate damage or propagation.
Partner with Qualitest for Better Safety Solutions
Whether you are running standard material tests or putting batteries through demanding abuse scenarios, you need equipment that delivers reliable data.
We understand that budgets are a key consideration. We also see proper testing equipment as a critical investment in product integrity. We offer a full range of accurate and cost-effective battery testing equipment. They are built to manage the complex parts of safety testing so you can be confident in your data and your compliance.
Ready to improve your lab's testing capabilities? Explore our Battery Testing Equipment here and reach out to us. We would be happy to help you find the right solution for your needs.
References
- An, W., Lu, Y., Liu, F., Wang, T., Xu, W., & Wang, Z. (2022). Prediction of heat release rate of single/double 32,650 lithium ion batteries. Journal of Thermal Analysis and Calorimetry, 148, 2057-2067.
- Fu, Y., Lu, S., Li, K., Liu, C., Cheng, X., & Zhang, H. (2015). An experimental study on burning behaviors of 18650 lithium ion batteries using a cone calorimeter. Journal of Power Sources, 273, 216-222.
- Hu, Y., Choe, S., & Garrick, T. (2021). Measurement of heat generation rate and heat sources of pouch type Li-ion cells. Applied Thermal Engineering, 189, 116709.
- Kim, S., Park, S., & Lee, E. (2022). Assessment of the explosion risk during lithium-ion battery fires. Journal of Loss Prevention in the Process Industries.
- Ping, P., Wang, Q., Huang, P., Li, K., Sun, J., Kong, D., & Chen, C. (2015). Study of the fire behavior of high-energy lithium-ion batteries with full-scale burning test. Journal of Power Sources, 285, 80-89.
- Ravdel, B., & Puglia, F. (2022). Thermochemical Approach to Determining Battery’s Heat Release: RI² Formula. Journal of The Electrochemical Society.
- Vanderwege, B., & Petersen, B. (2025). Impact of Thermal Gradients on Calorimetry Testing of Battery Cells. SAE Technical Paper Series.
- Voigt, S., Sträubig, F., Palis, S., Kwade, A., & Knaust, C. (2021). Experimental comparison of Oxygen Consumption Calorimetry and Sensible Enthalpy Rise Approach for determining the heat release rate of large-scale lithium-ion battery fires. Fire Safety Journal.
