Electric vehicles use large batteries to store energy. The energy flowing into the battery pack as it is charged either from regenerative braking or from the grid and discharged from the pack to power the vehicle and its accessories is measured by electrical current and voltage. The flow of current causes heating in the battery cells and their interconnection systems proportional to the square of the current flowing multiplied by the internal resistance of the cells and the interconnect systems. The higher the current flow the more the heating effect will be.
The performance of Lithium-Ion battery cells is greatly impacted by their temperature, they suffer from the Goldilocks effect, they do not perform well when too cold or too hot, which can lead to permanent and extreme damage of the cells or accelerated degradation. So in addition to cooling, heating of the cells may also be required at lower ambient temperatures to prevent damage during fast charging when the cells are too cold.; this is because the internal resistance of the cells rises when they are cold. Most lithium battery cells cannot be fast charged when they are less than 5oC and cannot be charged at all when they are below 0 oC. Lithium cells also begin to degrade quickly when their temperature is above 45oC.
In the past, the largest battery packs did not necessarily need any special cooling as the physical size of the packs was sufficient and the relative flow of current was not large compared to the overall capacity of the pack. As ever faster battery charging rates are demanded with recharge power of over 200kW to deliver times of 30 minutes or less, higher performance electric vehicles with a requirement for consistent performance and adequate durability in global markets has meant that special thermal management methods for the battery pack are now required.
There are 3 common battery thermal management methods used today:
- Convection to air either passively or forced.
- Cooling by flooding the battery with a dielectric oil which is then pumped out to a heat exchanger system.
- Cooling by circulation of water based coolant through cooling passages within the battery structure.
Recently published work by Hunt et al from Imperial University has found that not only the temperature but also the cooling method is critical to preserve the performance of the cells over their lifetime. In particular, the direction of cooling was found to be critical in order to maintain good internal temperature gradients across the layers internal to each battery cell. They made this great animated explainer which is now on youtube, link below:
Air cooling is not suitable for most new high-performance applications due to the power density required and the inability to cope with a wide range of ambient temperatures. It is simply not possible to remove sufficient heat from within the battery with an air cooling system alone, some air cooling will generally occur on a battery that is positioned on the underside of the vehicle due to air flow during driving but this is insufficient to meet the full cooling needs of the battery.
The conclusion of Imperial’s work was that tab cooling of the cells offers significant benefits over surface cooling as it prevents a temperature gradient developing between the layers of the cell. This is an interesting conclusion and would suggest that cooling needs to be carefully targeted in order to prevent damage and preserve life cycle performance.
Typically a flooded dielectric cooling system will be primarily cooling by extracting heat from the surfaces of the cells. Tab cooling is best achieved by a water-based coolant or an organic refrigerant circulated through a cold plate system built into the battery pack by a pump [such as the high-efficiency electric pumps manufactured by AVID Technology]. The coolant can be used to remove heat from the pack and to also provide heating of the pack for fast charging at low temperatures.
Tab cooling is not commonplace in the industry, for example, both the BMW i3 and the Chevy Bolt use a bottom cooling plate in its battery with the cooling medium being a refrigerant for the BMW and water glycol for the Chevy. Fitting the cooling plate to the bottom of the pack is not necessarily for directional cooling, however it is more likely to be linked to simple construction and failure mode analysis of leaks within the pack itself. Although the BMW does use top mounted terminal tabs; this will be directional cooling in the correct direction but at the opposite end to the hot spot in the cell. The Chevy has side-mounted tabs meaning the cooling base plate is potentially drawing heat from the wrong edge of the cell. It can be seen though that the Chevy Bolt has moved away from cold plates interleaving the battery cells and cooling through the surfaces as used on the Volt and Spark, as can be seen in this video:
Considering the difficulty in engineering such an interleaving cooling system and the results from Imperial that predict this will lead to accelerated degradation of the pack it is unlikely that such a system will be used again.
The Tesla models S battery cooling system consists of a patented serpentine cooling pipe that winds through the battery pack and carries a flow of water-glycol coolant, thermal contact with the cells is through their sides by thermal transfer material. Again this will remove heat from the side of the cells rather than from the tabs, and overheating a Tesla battery pack under hard driving is easy to do.
The Audi battery module design probably comes the closest to directional tab cooling due to the orientation of the battery cells. For example, on the A3 e-tron there are 4 heat exchangers which are interleaved within the battery pack, but the heat exchangers appear to be deliberately positioned at the base end of the cells away from the tabs
Tab cooling is difficult due to the need to electrically isolate the cooling system to prevent a short circuit of the pack and also to ensure that no failure of the cooling system at a joint results in the release of coolant into the battery pack itself. It is possible to design an effective contact cooling system utilising electrically insulating but thermally conductive material to remove heat from the tab area of the battery pack.
Effective cold plate design typically leads to a higher pressure drop across the battery pack due to the long length and narrow coolant channels required. This needs an electric coolant pump that can generate both high flow rates and also high static pressures. 2 phase heat pipes can be used in the system to increase specific heat flux from the relatively small cell tabs and into a lower pressure drop cooling plate or as in the BMW direct expansion of the organic refrigerant.
Once the coolant has passed through the battery pack it is then circulated through a heat exchanger where heat is transferred to ambient air flow which is being blown by a fan, sometimes a refrigerant chiller system will be used to achieve sub-cooling. This is important if the vehicle is intended to be used globally where ambient temperatures could come close to the max acceptable temperature for the battery. 2 phase cooling is needed to allow the battery to be kept at an optimum temperature that is below ambient. Because of the coefficient of performance of refrigerant “heat pump” systems, this reduces the overall power consumption of the system but adds more components and cost.
Due to the relatively small temperature gradients between the battery system and ambient relatively large air flows, heat exchangers can be required even though the amount of heat being rejected from an EV is around 90% lower than the comparable internal combustion vehicle driveline. This means that the power consumption of the battery cooling system can be one of the largest parasitic power draws of the EV driveline, and is closely linked to consistent vehicle performance.
Because of this high potential parasitic power consumption and ability to impact on overall vehicle performance, it is a false economy to select cheap but inefficient fans and pumps for the battery cooling system. The overall power consumption of the system can be reduced by more than 75% through good system design and the use of efficient electric pumps and fans.
About the author; Ryan Maughan is the Managing Director of the AVID Technology Group Ltd. AVID is based in the North East of England and is a leader in the design and manufacture of electrified powertrain systems for heavy duty and high-performance vehicles such as traction motors, coolant pumps, cooling fans, air compressor and steering drive systems. AVID supports its customers from innovation to production with vehicle systems integration, component design, testing, and manufacture.