How battery pack heat management enhance performances of EV?

In this presentation, we will succinctly summarize the main factors that come into play in limiting heat fluctuations in battery performance: why batteries are a critical technology for all types of electric and hybrid vehicles, the importance of battery temperature (both for safety and performance), a summary of heat-management considerations in both high and low temperature conditions, how to limit uneven temperatures in battery packs and a Summary of the choices available to deal with all these heat management challenges.

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Heat Management of Battery Pack

Batteries: a critical technology for all types of electric and hybrid vehicles

In our first article on Thermal Management Systems (TMS) and the choice of Thermal Interface Materials (TIMs), we looked at general issues such as thermodynamics, heat and power transfer, insulation, conductivity, convection, condensation, radiation, etc.

This time, we’ll be focusing on limiting battery heat fluctuations. So, to begin with, let’s reiterate why the Battery is a key critical technology for all EDVs, HEVs and PHEVs:

• They enable efficient input of electrical power in all types of electric vehicles
• They also provide energy to all motors during acceleration phases
• They enhance the autonomy and the distance range of all electric vehicles
• They help to downsize the size of all engines, including fossil-fuel motors in HEVs
• They enhance regenerating braking (friction heat converted into energy)
• They must still be improved to address key issues that have limited the uptake by consumers of electric vehicles:
  • Safety concerns
  • Added cost, weight and volume
  • Fears about reliability and durability of batteries
  • Decreased performance over time
    

Why is battery temperature so important?

Before we look at the technical factors that contribute to controlling battery temperature fluctuations, let’s be very clear on the absolute priority in all power charging and battery-back systems: SAFETY! The reason for this is very clear. First, designers and carmakers have a responsibility to protect drivers and passengers against all risks that could cause injuries or fatalities. Second, heat-control is the key factor that could potentially generate problems and so it is the primary factor that current R&D is focusing on.

Here are the considerations in ensuring both safety and performance:

• The electrochemical system of the battery must be perfectly managed
• The battery must be safe and efficient when it is recharged, in particular in fast-charging mode when heat is generated
• The battery temperature must be limited to safe parameters in demanding conditions, such as high-speed driving or stop-and-start urban usage
• Power and energy availability must include reliable and clear warning signals integrated into the driver interface and information system.

How to deal with factors affecting battery temperature

In our first blog, we dealt with the importance of controlling the heat generated not only by battery operation but also by external factors, such as ambient temperature, from arctic to tropical conditions. The main points you need to remember are :

• To protect your battery system from the external environment, you need to work on thermal insulation from the design step of your whole battery pack. For the thermal management materials aspect, insulation foams are the only way to achieve proper results, since it’s all about insulating your battery pack from what can affect it from the outside (extreme heat or cold).
• On the other hand, to protect and manage the heat generated from the inside of your battery pack, here you have two different options! Either you can insulate each component from each other, once again, elastomeric foams (such as silicones) are the best option because of their intrinsic heat resistance properties, as well as their lightness (see our blog post about the importance of lightweight thermal management materials). The second option is to evacuate the heat, which is slightly more complex because it involves a cooling system (cooling plate + cooling fluids). Here, the goal is to transfer the heat from the heating battery cells to the cooling plate. To get to this, having a proper Thermal Interface Material (TIM) in between the two elements is key, but you need to know it will impact the lightness of the whole design because thermally conductive materials can’t be foamy textured: they are very dense. The main element in insulating or dissipating heat is largely dependent on the Thermal Interface Materials (TIMs) chosen that can be made of various materials in the form of pads, greases, liquid gap fillers, adhesives, etc. Increasingly, silicone is being used as the material of choice, either on its own in specially designed elastomeric silicone formulations or in composites that include specific fillers.

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So, what are the main parameters to be considered in Battery-Pack Heat Management?

The main objective of battery pack heat management is to reduce uneven temperature distribution, i.e. the homogeneity of the temperature within the battery pack in a range between 3°C to 4°C, in ambient conditions that range from -35°C to 50°C. As a reminder, an electric vehicle battery reaches its optimal performance if operating between 15°C and 35°C internal temperature. To master these very tight specifications, one way to succeed is to set up a system that evacuates the heat generated by the battery cells outside of the pack. To do so, the most common solution is to use thermally conductive materials between the battery modules and a cooling plate (with cooling fluids circulating inside). The thermally conductive materials (ie: silicones, polyurethanes, or epoxy technologies – see our eBook to understand the strengths and weaknesses of each solution) play the role of a heat conductor from the very inside of the battery pack to a cooling system that regulates the average temperature.

The second objective of battery pack heat management is to control potential hazards related to extreme temperature rises in any given part of the pack referred to as Thermal Runaway. This means avoiding that any part of the battery array overheats, since it may catch fire and/or propagate heat to other components, thus inducing a chain reaction. This is why it is absolutely essential to insulate all battery cells and other elements from each other with the proper flame retardant materials. For thermal insulation, foam materials are commonly used. Silicone foams, in that regard, should be considered as a material of choice because of their intrinsic insulation characteristics, but also because they are efficient fire retardants or even self-extinguishing materials. 

Having a performant potting of each battery cell retard heat propagation from one cell to another, thus giving the driver and passengers enough time to leave the vehicle in all safety before the fire to start.

Summary

Battery pack heat management is a primary element of overall Thermal Management Systems (TMS), which ensures proper cooling within the entire system, including tubing and pipes that allow cooling products to circulate and heat insulations between different parts. Of course, this overall protection has a slightly higher initial price, compared to other materials such as polyurethanes. However, this is widely offset by safety and long-term performance considerations so that, in the long run, the Total Cost of Ownership (TCO) is lower and quality and lifespan are greatly improved.

All the factors mentioned in this overview can be integrated upstream into the design of your battery pack and downstream in the manufacturing process. At any given point, you might need to identify a specific TMS solution.