Looking at the benchmarks there are a number of pouch cell cooling options that have been used and threads where we can see improvements to designs.
Firstly thought it is worth just reminding ourselves of the heat generation mechanism in a battery cell.
Heat Generation in a Cell can be defined quite simple for the case where the cell is operating within it’s normal limits. The first expression gives the heat flow [W].
The first part of this equation is the irreversible Joule heating term, the I2R term.
The second part is the reversible entropy term or Reaction heat terms. The charge and discharge reaction can be exothermic or endothermic under certain conditions.
Passive Cooling
The mobile phone has a passively cooled battery. In this application there are a number of advantages in that the charge and discharge rates are low, the phone is normally kept close to you and hence in a 5°C to 45°C environment and you are used to replacing your phone every 1 to 3 years.
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The original Nissan Leaf and all of the subsequent iterations have been passively cooled. However, this does mean that the charge and discharge performance is limited, plus a difficult lifetime equation in very hot climates.
Nissan Leaf
The original Nissan Leaf had 4 cells in a 2s2p configuration in each of the “sardine tin” modules.
There is no cooling other than radiation and convection from the outer surface of the pack to the environment. Air flow cooling increases over the outside of the battery pack when the car is moving.
The pack also absorbs heat from the environment, radiated and reflected from road surfaces.
The pack is simple, but the downside is the performance is limited and lifetime is limited in very hot climates.
Surface Cooling
Surface / face cooling of pouch cells is a great way of increasing the contact area with the cell, there are though a number of challenges with this approach: downside is number of cooling joints, cell expansion and contraction, cell expansion over time, volume requirements for the cooling plates and electrical isolation. Thus making this complex to engineer, build and difficult to make durable.
Chevy Volt
A complex arrangement of thin aluminium cooling plates with a serpentine channel for the water glycol cooling fluid. The “D” shaped connection points connected the plates to a cooling inlet manifold and a cooling outlet manifold. This required a lot of O-rings and produced a lot of potential coolant leak paths.
The heating and cooling capability of this pack was very good.
The management system monitors feedback from 16 thermal sensors arranged throughout the battery pack to maintain a spread of no more than 2°C from the optimal temperature across the pack.
GM Newsroom [1]
This approach can also use forced air as the cooling medium rather than liquids. This helps to reduce some of the complexity, however, air is very limited in terms of the rate of heat removal.
Heat Transfer Plates
This idea sits between surface and edge cooling. Here aluminium plates between the cells are used to transfer heat to one edge.
A123 Module
This has an aluminium plate with a 90° flange on 3 sides. The cell sits in this plate and is electrically connected to the next cell on the top edge, the cell/module can then be cooled on the other 3 edges. You can see that compression bands are used to apply the overall pressure to the cells.
The aluminium plate acts as a thermal spreader, thermal conductor and mechanical support for the cell.
However, these plates add weight, volume and complexity to a pouch cell that has electrodes that can themselves conduct heat to the edges of the cell.
Therefore, this design approach has become redundant.
Edge Cooling
Fundamentally pouch cells are made from multiple layers of active material all applied to copper and aluminium electrodes that are sandwiched and compressed together. The copper and aluminium electrodes have very good in-plane thermal conductivity and hence cooling along the longest edge of the cell makes perfect sense.
This is perhaps the most common approach to cooling pouch cells in automotive applications. The cells are held upright within a module (Audi e-tron module has 12 pouch cells) with one edge in contact with the bottom inner surface of the module.
These modules are then arrange on a larger cooling plate that will accommodate multiple modules.
This design has gradually been optimised to reduce cost, weight and improve the thermal connection between the cells and the cooling plate.
Note: a separate benchmarking post will look at the optimisation of this design and compare the benchmark designs.
Porsche Taycan
This is an 800V pack designed for fast charge and fast discharge.
The modules fit into a robust waterproof frame and the cooling elements are glued on underneath the bulkhead plate using heat conductive adhesive.
This approach is simple and robust in terms of it’s approach to possible coolant leaks, but it does reduce the overall thermal performance of the pack.
Tab Cooling
The tabs of a pouch cell are thermally connected to the electrodes of the anode at the negative end and cathode at the positive tab. This electrical connection also provides a very good thermal connection.
Tab cooling has been shown by Hunt et al [2] to be a superior method of keeping thermal control of pouch cells. Their conclusions are very compelling:
- at higher rates, discharging the cell in just 10 minutes, surface cooling led to a loss of useable capacity of 9.2% compared to 1.2% for cell tab cooling
- after 1,000 cycles, surface cooling resulted in a rate of loss of useable capacity under load three times higher than cell tab cooling
- thermal gradients being perpendicular to the layers for surface cooling leading to higher local currents and faster degradation
- For automotive applications where 80% capacity is considered end-of-life, using tab cooling rather than surface cooling would therefore be equivalent to extending the lifetime of a pack by 3 times, or reducing the lifetime cost by 66%.
However, a tab cooling system needs to be thermal connected and electrically isolated from the cell tabs. Thus this is a challenging and complex system to design.
Immersion Cooling
For pouch cells this is a difficult concept and brings a lot of complexity in terms of designing a pressure system that also allows the dielectric fluid to directly contact the cell surface.
Battery Cooling Options
There are many battery cooling options, which is better or best depends on the cell selection and application. Passive, Forced Air, Cooling Plates, Dielectric and Refrigerant.
Summary
It is difficult to summarise and rank all of the cooling options as the application of the battery will determine the weighting of the factors. However, below is a pugh analysis of the pouch cell cooling options that have been rated based on an automotive application that needs to work in all markets and have a 10 year lifetime.
The best overall option comes out as Edge Cooling and this is the most common pouch cell cooling system that you will see in battery electric vehicle applications.
As we pursue faster charging and we solve the electrical isolation and thermal conductivity in more cost effective ways the tab cooling approach is likely to become more accepted.
References
- Cooling Fins Help Keep Chevrolet Volt Battery at Ideal Temperature, GM Newsroom
- Ian A. Hunt, Yan Zhao, Yatish Patel and G. J. Offer, Surface Cooling Causes Accelerated Degradation Compared to Tab Cooling for Lithium-Ion Pouch Cells, Journal of The Electrochemical Society, Volume 163, Number 9