Key points
Laptops and smart-devices are now staples for us all – whether that’s at home, at play, in the office or at a place of study etc. We don’t give a second thought to the fact that like so many devices and appliances, our laptops generate large amounts of heat when in use and deploy methods to dissipate this heat i.e. an integral fan.
The battery that powers the laptop will generate heat as it provides power to the microchips which themselves generate heat, as does the power unit that charges the battery, and also the screen.
As such, a critical element of new device development and manufacture is to understand the heat generation of the various component parts to ensure that the end product can operate safely and effectively in typical environments, and to still safely function when boundaries are pushed.
The consequences of not fully understanding the heat generation or the associated risks are exemplified by episodes such as with the Samsung Galaxy S8 Note smartphone where battery problems cost the company several billion dollars in lost revenues and huge reputational damage. [REF1]
Calorimetry techniques are used to understand the thermal profile of reactions and the thermal behaviour of electronic components such as batteries, processor chips, or power supply units – providing critical data which allows potential hazards to be identified early in design and development.
These hazards can then be designed out or mitigated during the transfer into manufacturing. When appropriately applied, calorimetry can also help in optimising component performance through the deployment of effective thermal management strategies or by defining appropriate operating parameters.
This article will focus on the use of calorimetry during battery design and development, however many of these approaches are equally applicable when considering other components that contribute to a device or appliance’s thermal behaviour.
Reaction Calorimetry
Reaction calorimetry is typically used to understand the heat generation (or adsorption) profile of a reaction under certain conditions. The data helps identify hazards associated with specific materials used or any intermediaries that are generated during the reaction. In battery development, researchers may use reaction calorimetry to test synthetic routes for component manufacture.
Adiabatic Calorimetry
Once the chemistry has been assessed, typically abuse testing of the assembled battery or cell is conducted to define the safe limits of operation (Figure 1). Adiabatic calorimetry defines the limits of the dangerous and safe zones of operation. Isothermal testing can support defining the optimum zone of operation.
During development and transition into manufacturing, it is desirable to expand the limits of the safe and optimum zones, producing batteries that work effectively in a range of applications and environments.
Legislation defines how batteries should be tested for safety – for example, most Li-ion batteries require testing against the UN 38.3 standards. Although not all test criteria can be satisfied with calorimetry approaches, one would expect this destructive safety testing to include some form of adiabatic test.
This involves subjecting the battery to a range of extreme conditions such as thermal abuse, mechanical stress, or external short circuiting. Data generated during these tests inform a battery developer on the mitigation required to prevent a catastrophic failure when the battery is in regular use
Whilst subjected to these stresses, the adiabatic calorimeter will mimic a worst-case scenario where the unwanted thermal energy generated is not able to dissipate, and the cell is not able to cool down.
This typically drives a thermal runaway which will cause structural failure. This can lead to deformation, release of gas, the battery catching fire or, in some cases, an explosion.
In many cases, the ultimate cause of battery heating and thermal runaway is a short circuit, or external heating (Figure 2). Normal battery use and aging can also lead to an unwanted failure, so lifetime performance testing must be conducted.
Performance testing – Isothermal and Isoperibolic Calorimetry
There are two types of calorimetry which can be used to study performance characteristics of the battery, with isothermal and isoperibolic testing both being non-destructive performance testing approaches. Typically, a battery will be connected to a charge/discharge unit (a cycling unit) during these tests so performance can be assessed under a range of operating conditions.
In isoperibolic testing, the battery’s test environment is maintained at a constant temperature. In isothermal testing, where the battery or component under test is maintained at a constant temperature.
While both approaches can provide useful data on battery performance, isothermal testing is generally regarded to be the superior testing procedure. To understand why this is the case, let's briefly look at how these two different types of testing work.
Isoperibolic Calorimetry
Isoperibolic testing methodology involves placing the sample in a calorimeter device and keeping the ambient temperature surrounding it constant. For example, the battery can be placed between heating/cooling plates or directly into a (non-electrically-conducting) liquid which is heated or cooled.
While in the test device, the battery will be connected to the cycling unit. Once the battery has attained thermal equilibrium with the cooling system, the charge/discharge protocol can be commenced. As the battery heats up, there will be an increasing flow of heat to the cooling system.
If the battery cools down at any point during the process, heat will be absorbed by the battery. Once the charge/ discharge protocol is complete, the battery will again attain thermal equilibrium with the cooling system. The bigger the difference between the battery temperature and the test set point, the more rapidly the battery will lose or gain heat.
This means that while this approach describes the heating and cooling process at the test temperature, it does not inform the user about how the battery responds to these tests at different temperatures. Due to changes in the heat flow rate during a test, isoperibolic testing requires every operating environment to be tested. In contrast, isothermal testing allows inference of behaviour at operating temperatures not directly tested.
Isothermal Calorimetry
The isothermal methodology works by closely monitoring and maintaining the sample temperature. The battery, once connected to the cycling unit, is placed into the calorimeter with an effective thermal contact between the battery and the cooling plates and a temperature probe is directly attached to the sample. To be highly responsive to sudden changes in the sample temperature, both heating and cooling are applied to the sample simultaneously.
To heat the battery, electrical heaters are attached. These respond quickly to changes in the battery temperature, maintaining the battery at the set point throughout the experiment [Figure 3]. The power used by the heaters can be measured to identify the heat generated or consumed by the battery under different test conditions.
The battery sample is only in thermal contact with the cooling solution via a suitable adaptor attached to cooling plates, which makes it suitable for testing electronic components as there is no liquid contact.
Isothermal testing has the following advantages over isoperibolic testing:
- power released from the battery is easier to calculate – largely due to consistent rates of heat-flow
- the battery is maintained much closer to the experimental set point – due to highly responsive battery heating and cooling
- behaviours at temperatures not directly examined can be accurately inferred – as heat losses do not require calculated compensation, behaviours at any temperature can be interpolated from experimental data
- the isoperibolic testing profile can be inferred from isothermal testing data, while the reverse is not as straightforward
Experimental data obtained from reaction calorimetry can be used to define the expected lifetime of the battery in terms of charge/discharge cycles, the ideal operating temperature and the optimal charging and discharging rate. By charging and discharging the battery at higher rates than is expected in normal use, accelerated aging tests are possible.
It is important to determine the optimum operating temperature of a battery due to the wide range of applications across all climates and environments. If a cell is seen to heat up under certain use conditions, then it’s important to know whether the cell would exceed the safe zone of use in hot climates. Such information is useful to battery developers to define the charging rates for batteries, or the maximum operating temperature.
In figure 4a, we can see the thermal behaviour of a gel battery when charged and discharged at its intended rate at a range of different temperatures. During charging at 60oC, the battery is endothermic. As we reduce the temperature, we see this endothermic phase switch over to exothermic between 40oC and 20oC. At 0oC, there is a strong exothermic effect during charging.
If this battery was to be used in a device with direct skin contact (e.g. smart watch, headphones or a phone) and the device were to be charged in an environment between 10oC and 20oC, or lower, the exothermic effect could lead to the device overheating and causing discomfort, or possibly even burns. This is on top of the risk of a broader thermal runaway occurring within the device. In this case, the data showed that the gel battery was unsuitable for the intended application.
In figure 4b, we compare the profile of the heat-flow characteristics of the same gel battery at different temperatures. During charging and discharging of the cell, not only does the transition from endothermic to exothermic occur, but we also see changes in different parts of the charge and discharge cycle indicating spikes of heat generation. These changes in the heat flow profile indicate further studies are required to understand the cause of the issue.
Conclusion
Calorimetry is an important tool for many phases of battery development as it answers fundamental questions around cell safety and the necessary steps required to mitigate any risk. Calorimetry can also be used to identify how to optimise battery usage in real world situations providing not only an additional layer of safety, but also how to get the best possible performance and longest lifetime from a cell in a given application or environment.
Often combined with other technologies and techniques such as over-charging and discharging, or physical or thermal damage, the data generated from calorimeter tests provides critical insight in a world where batteries are ever-more important as a source of sustainable energy.
References:
1 – “How much money has the Galaxy Note & debacle cost Samsung?”, Newsweek 2016
2. Heat Wait Search Video Using H.E.L Battery Testing Calorimeters (BTC) – YouTube, October 2022
