Heating elements

Let’s start by addressing the heating elements in the pack. All batteries rely on chemical processes to generate a current. These processes slow down when external temperatures drop. As a result, they deliver less power, and charge slower. The same applies for lithium-ion batteries used in electric vehicles. When batteries are exposed to extremely cold temperatures, it can lead to additional issues such as plating, which we’ve addressed here. 

Well-established solutions for heating of battery packs include self-regulating positive temperature coefficient (PTC) heaters, usually consisting of polyester or polyimide films. The heating elements are screen-printed onto them with a PTC ink. They effectively self-regulate their resistance and thus temperature, formulated for each application.

Heater film bonding PSA tapes are often a preferred option for mounting heater films to a module or pack enclosure for extended battery life in cold temperatures. Not only is this a simpler alternative to mechanical fasteners, but their visco-elastic nature also provides the additional benefit of accommodating expansion rate differences. 

Insulation is another way to go. Thermally insulating layers, such as reflective coatings and heat shields, protect battery packs from external heat, ensuring optimal performance and longevity. These materials reduce heat transfer and maintain efficient temperature management for improved energy efficiency and cell durability.

It’s important to note that how the heat is spread in the cells is critical; if hot spots build up in individual cells this can already lead to local decomposition leading to complete cell failure. If a cell is getting too hot – even before thermal runaway happens – good practice is to prevent the heat from spreading to other cells. Each cell is unique. Even though the process to make them is highly automated, they are made up of many different layers and components – all with their own variation, however tiny. When these layers come together in the cell, variations accumulate and can lead to wide differences in reaction to heat between the finished cells. 

Cooling elements

Cooling elements in battery packs rely on heat transport mechanisms such as radiation, conduction, and convection. An effective design maximizes these methods to prevent hot spots and ensure heat is efficiently dissipated from the cells.

Basic heat management can be achieved using the outer sheathing of the cells, Thermal Interface Materials (TIMs) to conduct heat away, and heat spreaders, particularly in pouch cells, which are prone to hot spots due to the poor heat conductivity of the PET film. Lightweight alternatives like graphite sheets are increasingly used, but these require careful attachment, often with PSA tapes to meet performance and productivity needs.

There are two main approaches to cooling in battery packs: passive and active cooling.

Passive cooling

Passive cooling relies on natural heat transfer without active systems, utilizing conduction, radiation, and convection to regulate temperature. One example is passive air cooling, where air flowing through the vehicle during operation cools the battery pack. This is found in earlier EVs, such as the Nissan Leaf, which had limited range and charging speed. Passive cooling is typically used in lower-performance vehicles or commercial vehicles with moderate power requirements, where slow charging is sufficient ­– such as buses. These vehicles sometimes place the battery on the roof for safety and may use heating foils to warm the cells when necessary.

Active cooling, on the other hand, involves more complex systems like liquid cooling, where pumps and temperature management components (such as heaters and radiators) work to precisely control cell temperatures.

Active cooling

This method allows for higher energy densities and faster charging speeds, as seen in vehicles like the Tesla Model Y, which can handle much higher energy densities compared to earlier models. Liquid cooling systems typically involve coolant passing through plates or channels near the cells to draw away heat, and in some cases, immersion cooling, where the fluid surrounds the cells, is used for more efficient heat dissipation. Fluids such as ethylene glycol are commonly used to prevent freezing and corrosion, while inert fluids like oils are required for immersion cooling.

A more advanced form of active cooling is phase change cooling, where the coolant changes phase (e.g., from liquid to gas) as it absorbs heat, enabling more efficient heat capture. The design of these cooling systems is highly optimized using Computational Fluid Dynamics (CFD), which helps to determine the best fluid flow and heat transfer configurations for the battery's cooling needs.

Phase change cooling enhances liquid cooling by using a fluid that changes phase (e.g., liquid to vapor) as it absorbs heat from the cells. This allows the fluid to capture more heat energy, as vapor can often be heated to higher temperatures than the liquid phase, improving thermal management efficiency.

Pros of passive cooling

• Less expensive

• Lower weight and volume

• No possible coolant leaks

• Simpler to manufacture

• Easier service and repair options

Cons of passive cooling

• Hot and cold ambient limits

• Charge and discharge rate limited by thermal mass and heat loss to surroundings

• Not able to use coolant circulation to minimise cell to cell temperature differences.

Active cooling overcomes these cons but comes with added costs and complexities.

What role do PSA tapes have in addressing these challenges?

• PSA films to electrically insulate cold plates and ribbons

• PSA on seals and gaskets to prevent fluid loss in active systems

• Bonding of heat spreaders – e.g. graphite foils – to cells.

• Bonding of thermally insulating cell pads in cell stacks (e.g. foams, aerogels)

• PSA bonding of TIM (Thermal Interface materials) to pack enclosure

• Fixing of heating films inside packs

• Thin bonding solutions with high surface wetting to reduce heat flow resistance through laminated layers

• Bonding of heat insulating layers to pack enclosure (e.g. insulating heat shields to pack bottom or lid) to reduce heat loss to the environment