The Challenges:

Batteries need to operate between 25-50°C to function effectively both for charging and discharging. Straight away this creates particular challenges in colder climates. To mitigate this OEMs like Tesla use pre-heating while the vehicle is charging in your garage or driveway, this ensures optimal temperature of the entire pack before the vehicle is driven - without impacting range. However, maintaining these conditions is a costly use of energy when the vehicle is in use - drastically impacting range in very cold conditions.  On the flip side, vehicles parked outside in blazing sunshine during the summer will need to activate cooling systems to keep the cells below 50°C (listen out next summer, you can often hear this in action).

Uniform temperature control between cells is another major challenge—any significant deviation in temperature (more than 3°C) can lead to faster battery degradation. Thermal runaway, where a cell enters an uncontrollable, self heating state can cause venting, high temperatures, smoke and fires. As one cells enters thermal runaway it can cause adjacent cells to enter the same state, posing a major safety risk to both passengers and the vehicle. I found this article on UL's website great for better explaining thermal runaway.

Lastly, complexity, and most importantly the cost that inevitably comes with it, is another critical issue as I need not cover the well documented cost hurdle of BEVs adoption. Maybe this is why we are yet to see widespread adoption of immersion cooling in the market. As engineers push for better thermal systems, they often have to compromise between thermal performance and cost. Water-based systems, while effective at cooling, introduce the risk of leaks, particularly dangerous in high-voltage EV systems.

Typical solutions today.

Each of the current cooling methods—air, water, and immersion—has their own set of trade-offs:

• Air cooling: It’s cost-effective but limits performance and struggles to maintain a consistent temperature across all cells.
• Water cooling: More efficient, and the most popular but introduces potential leakage risks and is very complex if you wish to cool a higher percentage of the cells surface.
• Immersion cooling: While it offers the best performance and good temperature uniformity, it comes with high costs, added weight, and mechanical integration challenges.

Thinking differently by combining approaches.

At Calyos we have taken a different approach. Recognizing that air and water are the most mainstream, widely adopted solutions we looked to see how we could boost the performance without introducing revolutionary design changes.

Our Micro-Channel Heat Pipes (MCHP) were invented directly for this purpose. These lightweight, low cost, aluminum heat pipes can be easily integrated due to their flat form factor. It works by targeting hot spots—such as the sides of prismatic cells—where heat tends to accumulate and captures this heat and transports it directly and passively to the water cold plate or the air.

The effect it can have on your battery pack is:

• Improved thermal performance during charging/discharging
• Increases cell temperature uniformity
• Reduces cell-cell temperature delta

Source: Calyos

The key here is to apply the MCHPs exactly where they are needed, to provide just enough cooling to meet your user requirements. This minimizes the impact on weight and volume, while boosting the performance to the required level. A bonus is also the reduced flow rate of water or air required during typical conditions, saving energy and boosting range.

3 Mistakes to avoid when implementing MCHPs:

1. Over Engineering‍

Part of the benefit of adding MCHPs to your battery pack is that you can choose the number of MCHPs to add to your pack. This allows you to provide just enough cooling performance based on your use case.
High Demanding = Many MCHPs
Moderately Demanding = Some MCHPs
Lightly Demanding = Few MCHPs
Not Demanding = No MCHPs

2. Poor Thermal Contact

‍It is important for the MCHPs to have a good mechanical contact with the cells to reduce the thermal resistance. A TIM (paste/grease) can be used to further reduce the thermal resistance. The MCHPs can be embedded inside module casing to provide a double function.

3. Ignoring Orientation

‍MCHPs use gravity to assist with the liquid return and therefore can only transport heat upwards. While they operate great across varying orientations (think vehicle’s on slopes), they will not work if placed upside down. Therefore the design must be optimized to remove heat from the top of the pack.

Source: Calyos

A proven example

Calyos tested an OEM battery module with the solution pictures above. Our inverted L-shaped MCHPs provide cooling to the sides of the prismatic cells and transported that heat to the existing water cold plate. It had a minimal impact on size, +6mm in width, +3mm in height.

To add 125Ah of charge, our system achieved 27% faster charging time (45 to 33 min). Plus the following thermal data is for a 2C charging rate:

• 10°C lower max temperature of the module (from 62°C to 52°C)
• 52% lower cell internal temperature difference (from 3.8°C to 1.8°C).
• 72% lower cell-cell temperature difference (from 6.7°C to 1.9°C).