HEV Battery Pack Thermal Management Design and Packaging Solutions.
Ford's current model C-Max and Fusion hybrid battery packs are built with 76 Lithium-ion cells that are wired in series and are nearly identical in their design. The battery pack provides 1.4kWh of energy and has a 35kW charge and discharge capability. It also houses a 165 Amp DC-DC convertor. All of the battery cells and the DC-DC convertor are cooled by a shared air cooled system.
HV battery packs can be utilized efficiently only when they are operated within a specific temperature range. At higher temperatures the battery life span is reduced, battery performance declines and efficiency decreases. The battery cooling system must therefore be designed to keep the lithium-ion cell pack operational temperature below a certain threshold while at the same time maintaining uniform air flow across the battery cells and providing sufficient cooling to the DC-DC convertor. While meeting the primary purposes of cooling battery cells and the DC-DC convertor, the cooling system had to be designed to meet luggage space and vehicle cargo requirements. Additionally, vehicle cabin and climate control performance could not be compromised and air handling/air flow Noise, Vibration and Harshness (NVH) had to be minimized.
This paper presents how the HV battery pack cooling system was designed and packaged to meet all thermal management functions and vehicle attribute challenges. This paper also describes the system validations that were performed utilizing Computational Fluid Dynamics (CFD) analysis and thermal measurements to obtain the optimum configurations of the cooling system on the C-Max HEV.
The high voltage battery assembly is placed behind the second row seat in the cargo area of the C-Max HEV as shown in Figure 1.
Cabin climate air is used for battery thermal management. A single fan assembly which can provide required air flow is packaged at the outlet and is used to cool both the battery pack cell arrays and DC-DC convertor. Figure 2 shows the general layout of the grille openings, inlet ducts, battery pack, DC-DC assembly and cooling fan in the C-Max vehicle. Cooling ducts inlet opening area, duct sizing, ducting interface, and grille pattern were all designed to minimize the NVH impact resulting from air inrush noise at the duct inlets.
Cooling ducts were correctly sized to minimize the pressure drop along the cooling path, which has the benefit of allowing a lower fan speed to drastically reduce the interior noise.
For the C-Max battery cooling system, cabin air is drawn in from both sides of the vehicle through equally sized ducts. This allows the cabin interior climate equilibrium to be maintained and reduces any thermal impact to passengers. The cooling ducts are designed and packaged to provide 70% of air flow to the battery pack cell arrays and 30% of the air flow to the DC-DC convertor (1). All the air flow from right side ducting is directed solely to the battery pack, whereas the left side ducting is designed to provide cooling air to both battery cell arrays and the DC-DC convertor.
Additionally, the thermal management system is designed to attenuate any NVH resulting from cooling fan operations and to reduce its impact on vehicle cabin climate control. The cooling fan is well covered and packaged under the cargo management load foor, behind the battery enclosure. The fan's chute or outlet duct length and opening is designed to minimize air outlet noise. A virtual ducting path was created within the cargo management box and rear body panel for the cooling fan exhaust flow path (2). The ducts route the exhaust air to the vehicle extractors which are located on both sides of the vehicle.
Exhausting all of the outlet air through the air extractors can lead to a significant temperature and humidity load on the vehicle climate control system. To reduce this load, the battery thermal system was designed to promote some mixing of the battery exhaust air with the cabin climate air. The virtual duct formed by the cargo management system and the vehicle body structure ends at the inner trim area, where the flow path is open to both of the air extractors and to the open area/cavity between the outer body panel and inner trim panel where part of the battery cooling exhaust air is recirculated back into the vehicle cabin environment. This design reduced the impact on the cabin climate system while drawing required air for the battery cooling from the given cabin climate air volume. Figure 3 shows the virtual ducting path for the cooling fan exhaust and mixing area between the two body panels.
The entire virtual ducting path is sealed to eliminate air leakage and associated NVH. The air inlet duct interface with inlet grille areas and ducting interface with battery pack assembly are sealed to prevent any micro-recirculation of exhaust air into the battery air inlet.
HV Battery Pack Assembly Design
The battery pack assembly is designed to provide uniform air flow through all cells in the pack as well as to provide the required air flow to the DC-DC convertor. The general layout of all the components in the battery pack assembly is shown in the figure 4. The entire 76 lithium-ion cells are grouped into two arrays connected in series, one cell array in the front and the other cell array in the back. The DC-DC convertor is housed next to the battery cell arrays. Both the cell arrays and the DC-DC convertor are secured to the battery tray and enclosed with an outer sheet metal enclosure. The battery enclosure also has the air inlet ducting interface at the front, and cooling fan interface at the rear. A jumper duct is packaged between the DC-DC convertor outlet port and cooling fan inlet port.
The Battery Electronic Control Module (BECM) controls the cooling fan operation based on inputs from the inlet temperature sensor inside the battery pack assembly and battery operating conditions.
The battery pack tray and the battery enclosure are designed to provide fresh cabin air to the front array, while the rear array receives a mixture of both fresh cabin air and partially heated air which has already passed through the front array. The battery pack enclosure design and tray design influence the amount of fresh air to the front and rear arrays as well as the direction of air flow. The battery pack tray and battery pack enclosure provide a combination of heated and non-heated air for effective cooling of the rear array (3). Figure 5 shows the air flow path to the front and rear arrays.
The multifunction nature of these two parts eliminated the need for unique ducts for the front and rear arrays. This reduced the overall battery cost, weight and packaging complexities. Additionally, the built-in air ways enabled a reduced battery pack height and width, which provided the customer with improved cargo space. The angled plane of the ducting interface and the opening areas for the air flow on the battery enclosure directed the cooling air to all the cells in the cell array. This kept the differential temperature between the cells in both arrays to the minimum desired level.
As previously mentioned, a single fan unit is used to cool both cell arrays and the DC-DC convertor. A jumper duct between the DC-DC convertor outlet and cooling fan inlet enabled tuning of the air flow between the battery pack cell arrays and the DC-DC convertor to the required level. This eliminated the cost, weight and package space required for separate cooling ducts for the two heat generating units inside the battery pack assembly. Figure 6 shows the cooling fan and jumper duct in the cooling system.
Thermal system verification and validation was carried out at various stages of development utilizing both Computational Fluid Dynamics (CFD) analysis and actual thermal measurements to optimize the configurations of the cooling system. CFD analysis was very instrumental in defining the battery tray configurations, the height of the air flow cross section opening below the front row cell array to the battery tray and the position of the blocker on the tray below the second row cell array. These two features determined the amount of fresh air sent to the rear cell array vs the amount of cross flow air sent through the front cell array and in turn the differential temperature between the cells across the entire array assembly. Figure 5 shows the air flow under the two battery cell arrays.
CFD analysis was also very helpful in optimizing the inlet ducts' cross sectional area to minimize pressure drop across the system. Additionally, the optimized duct sizing reduced the ducts' intrusion into the cargo area of the C-Max HEV.
The reduced pressure drop across the entire thermal system allowed the cooling fan to run at a lower speed which minimized system NVH and ultimately leads to improved customer satisfaction. Figure 7 shows the one of the CFD study results.
CFD analysis helped to fine tune the cross-section of the jumper duct between the DC-DC convertor and cooling fan. Figure 8 shows the various percentages of cross-sections of the jumper duct that were studied to restrict the flow through the DC-DC convertor air path.
For the CFD analysis, both Ted surface mesh and Tetra volume mesh were employed. Multiple layers were applied to capture higher resolution of temperature gradients. Steady state analysis was used in the modeling. Turbulent model with enhanced wall treatment was used wherever applicable in the simulation to capture an accurate heat transfer between the boundary walls. Multiple Reference Frame (MRF) method was used for the fan modeling.
Thermal measurement data from the battery packs that were fitted with jumper duct having no restriction for air flow showed more flow through the DC-DC convertor from the left side ducts, which resulted in the higher temperatures on the cells cooled by the left side ducts. Conversely, battery packs fitted with 40% flow restricted jumper ducts showed a well-balanced temperature distribution throughout the packs.
Flow measurements were conducted in the lab using non-functional battery packs. Figure 9 shows the test set up to measure total flow through the system and flow through the battery pack.
The flow measurement bench test data confirmed the CFD analysis conclusions. The 70:30 air flow split between the cell arrays and DC-DC convertor, coupled with the use of the 40% flow restricted jumper duct at the DC-DC convertor outlet yielded the desired optimum result. Figure 10 shows the actual flow measurement of this optimum design compared to the CFD results.
Thermal measurement data from a battery pack assembly instrumented with thermistors on every cell confirmed the reduction of cell temperatures and a reduction in the cell temperature variance.
This served as supporting evidence that the air flow thorough the battery pack cell arrays were improved by employing the 40% flow restricted jumper duct at the DD-DC convertor outlet.
Thermal data collected with the instrumented prototype packs during various drive cycles were analyzed using 6 - Sigma statistical methodology.
A Paired t-test analysis verified the effect on the battery pack cell array temperature rise resulting from the 40% flow restricted jumper duct versus the jumper duct having no flow restriction.
The Paired t-test procedure was selected for analysis as this study results in a smaller variance and greater power of detecting differences as the data before and after measurements are paired and analyzed. Figure 14 shows the Paired t-test analysis on the thermal data sets for one of the battery current profiles evaluated.
The 95% Confidence intervals (CI) for the mean difference between the two data "does not include zero" which suggests a difference between these two data sets. The small P-Value (p=000) further suggests that the two data do not perform equally.
Specifically, the mean value (36.988) from the battery pack that has 40% restricted jumper duct is lower than the mean value (39.016) from the battery pack that has 100% opening jumper duct suggests more flow through the pack with the use of 40% jumper duct in the DCDC convertor flow path.
The 6-Sigma statistical analysis results corroborate the outcome of the CFD analysis and the flow test results on a non-functional battery pack.
The Ford team was able to design, develop, package, verify, and validate a high voltage battery assembly thermal management system common to both the Ford C-Max and Ford Fusion HEVs' with minor variations to support the two different vehicle architectures. The Ford Fusion is a four door sedan whereas Ford C-Max is a five door hatchback.
This paper covered how the high voltage battery cooling system was designed and packaged to meet all the thermal management targets and the vehicle attribute challenges. System verifications and validations were performed using CFD analysis and thermal measurements to obtain the optimum configurations of the cooling system on the C-Max hybrid vehicle. The thermal management system used cabin climate air as a cooling medium and used a single fan unit to cool both the battery pack cell arrays and DC-DC convertor. This saved cost, weight and package space by eliminating the need for a separate cooling system for the battery pack and DC-DC convertor.
The battery pack enclosure and tray were designed with built-in air ways to provide cooling air to two cell arrays, and eliminated the use of separate cooling ducts for the front and rear arrays further reduced the complexity of the system. This also enabled a lower pack height and width which improved the cargo space.
CFD analysis was extensively used to optimize the duct cross sections and inlet openings to reduce the pressure drop along the cooling path. These optimized cross sections reduced the potential air inrush noise at the duct inlet and enabled the cooling fan to operate at lower speeds which drastically reduced the vehicle interior noise.
Virtual ducting for the battery pack outlet air was created by using the space between the body panels and cargo management box. This saved piece cost, tooling cost, inventory cost, and cost of labor, while at the same time reducing weight. This design recirculated a portion of the battery cooling exhaust air inside the vehicle, which in turn supported the Heating, Ventilation, and Air Conditioning (HVAC) unit's objective to balance and maintain cabin interior climate temperature.
The entire thermal system was verified and validated using instrumented packs on various drive cycles. Thermal data was analyzed using six sigma statistical methodology to prove the system met the thermal requirements.
The authors would like to acknowledge Brian Utley for his design and development of cell separators which influenced the air flow across the cell array pack, George Garfnkel and LeeAnn Wang for their CFD analysis, and Hsiao-An Hsieh for his NVH assessment. The authors would also like to acknowledge Battery pack CAD team and test team for their support in packaging and conducting multiple tests to verify and validate the design.
Sury Janarthanam, Sarav Paramasivam, Patrick Maguire, James Gebbie, and Douglas Hughes
Ford Motor Company