A battery pack, comprised of a collection of modules enclosed together, forms a crucial component within electric vehicles (EVs). It is often a large assembly integrated into the vehicle's structure.
In this edition of our Battery Safety series, we will focus on the safety considerations concerning a single battery pack assembly.
The first safety consideration is structural integrity, as the whole pack constitutes a significant mass. The strength and stiffness are unquestionably important as the mass influences the way the battery pack structurally behaves, both separate from the vehicle and when installed in the vehicle. The pack and its structure must be capable of withstanding many static and dynamic loadings.
Statically, there are bending and torsional loadings that must not overstress the materials. Dynamically, there are forces and accelerations in all directions that need to be considered, and optimising the structure and mounting regime requires the use of conventional analysis techniques for stiffness, strength, and fatigue.
Assembling the battery pack brings together high voltage (HV) harnesses to electrically connect each module, and the high voltage safety is now a consideration as connecting modules brings the hazard of dangerously high voltage.
To mitigate these risks, careful thought as to the connection methodology for the high-voltage harness is a must. The low voltage system includes the Battery Management System (BMS) connections from the modules to the master, and any other connections including sensors, communications etc.
The BMS is now a full system with the master able to communicate with the ‘slave boards’ inside the modules and to report back to the Vehicle Controller.
Depending on the system specification, the BMS may include drivers for switching coolant circulation pumps, fans, and other components, otherwise, these will be controlled by the Vehicle Controller to maintain the cell temperatures in their comfort and safety zones.
The coolant system connections are typically within the battery pack assembly, and these can be proprietary quick-release connections or others, however, there may be design rules that prohibit this.
A large European OEM has corporate design rules that do not allow any liquid coolant connections within the battery pack enclosure. This is to mitigate any potential short circuits due to coolant leaks within the pack itself.
Overall, short-circuit protective devices would ultimately prevent this, however, this corporate rule adds an additional level of safety protection.
In our previous articles, we discussed the importance of maintaining the temperature of the cells for the whole battery system. Another key element that EV battery manufacturers need to consider is the pack’s mechanical integrity.
The typical thermal management of an HV battery system is based on a ‘conventional’ automotive 50/50 water/glycol solution to absorb excess heat and transport it from the battery pack. It may or may not be force-cooled using the onboard air conditioning or refrigeration system.
While the process is a very cost-effective way of maintaining the cell temperature, there are alternatives that manufacturers can use. A sophisticated thermal management system may be heat-pump based which, albeit more effective, adds a level of complexity.
The simplest form of thermal management is to rely on the thermal mass of the pack itself. This is passive cooling - there is no active warming or cooling at all. When the cells start to work, they will take some time to start warming up. This is particularly the case for large format cells.
Some electric buses and trains utilise this method, as it is suited to large-capacity, low-performance packs. This method was used on the first generation of the Nissan Leaf passenger car.
Air cooling is a very simple method that flows air through a battery pack and around the cells and modules. The trade-off for this simplicity is that it is not very effective. This can be seen particularly in high-performance battery packs where the cells are being worked hard.
As the pack performance increases, so does the cooling requirement. Liquid cooling systems have been previously described. However, an alternative to this is using a dielectric fluid, which is a non-electrically conductive fluid – an oil of some sort.
Although the specific heat capacity compared with water/glycol is lower, the benefit is that it can directly flow over the cells and electronics, providing an immediate point of contact with the components that require cooling.
Due to the hazard of short circuits, a water/glycol solution needs to be somewhat remote from the ‘live’ cooled surfaces, relying on the inefficient thermal conductivity through whatever medium material it flows through (cooling plate, tubes) before the heat is transferred to the liquid.
Alternatives to thermal management include using phase- or state-change materials, PTC (Positive Temperature Coefficient) elements, and heat tubes. However, these examples are beyond the scope of this article.
An interesting modern battery pack variation is the Cell-To-Pack (CTP). This is where the module stage is bypassed, and the cells are mechanically assembled directly into the whole pack and integrate body or chassis structural elements as part of the pack.
Depending on the OEM, there are further alternatives such as Cell-To-Chassis or Cell-To-Body, where the cells are directly integrated into the vehicle structure assembly.
The design for the safety of the whole pack is an integration of all the subsystems into one single unit. Mechanical design, high- and low-voltage electrical design, sensors, pumps, valves, and all the elements of thermal management together, form the basis of the whole battery pack design and development.
At a concept level, whichever methodology is implemented requires the management of many trade-offs or compromises, which is a typical demand in the domain of whole vehicle design.
To recap our Battery Safety series, we discussed how to keep EV batteries safe at the cell level through chemistry. Manufacturers use cell chemistry to manage trade-offs in the batteries’ characteristics such as specific energy, specific power, safety, performance, lifespan, and cost.
We also looked at how battery safety is prioritised once the different cells are combined into a single module. This is where we found out how the construction of the cells in cylindrical, prismatic, or pouch formats can play a key role in the stability of modules.
In this article, we just finished exploring how modules are assembled into battery packs, with a particular focus on the structural integrity of each one. Having the right coolant system can determine whether the pack stays safe and functional once placed in an electric vehicle. All of these elements are important to battery safety and are carefully considered, monitored, tested, and integrated by Tembo’s expert team to ensure the highest standards of safety to operate an Electric Utility Vehicle (EUV) in a large variety of terrains, climates, and use cases.
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