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The Race to Design Batteries for Urban Air Mobility

Generating Battery Pack Concepts for eVTOL Aircraft Propulsion using Voltt ‘Design’


the future of urban air mobility, a city with helicopters


Developing high-performance eVTOL battery packs is a big challenge in revolutionizing urban transportation. These batteries need to be really good in terms of power, energy, safety, durability, and charge time while also being lightweight and compact to fit in eVTOL aircraft. Making sure the batteries are powerful and safe, lasting a long time, and charging quickly are the main goals for battery makers.


According to a report by Bloomberg NEF, the UAM battery market is expected to grow a lot from 2023 to 2030, with a annual growth rate of 45%. This growth is driven by the use of eVTOL aircraft. The report says that the electric propulsion system is the biggest opportunity in the UAM battery market, with a projected value of $50.2 billion by 2030. The battery management system (BMS) market is also expected to be worth $21.3 billion by 2030.


The chemistry of the battery cells is really important for the overall performance and safety of the battery. Well-known battery cell makers like Amprius, Ionblox, Cuberg, and Molicel are actively working on batteries for eVTOL aircraft. Because of safety requirements, certification standards, and battery degradation, only part of the stored energy can be used for the flight. Usually, around 35% of the energy is used for a typical eVTOL flight [1]. Strict safety standards, certification procedures, and battery degradation all contribute to this energy loss. Battery makers and eVTOL developers are trying new things to make the batteries more efficient and reduce energy loss. Cooling systems and cell balancing for even power distribution are really important. Aerospace battery packs also have to meet environmental standards and pass safety tests. Battery thermal runaway is a big concern, so a good plan is needed to manage the risks.


To sum it up, designing eVTOL battery packs means finding the right balance between power, energy, safety, weight, and size. We need to keep making advancements in battery technology, cooling systems, and safety measures to get the most out of eVTOLs and revolutionize urban transportation.


Spider Charts Comparing eVTOL Battery Requirements to EV Batteries.
Figure 1. Spider Charts Comparing eVTOL Battery Requirements to EV Batteries [2]


Leveraging Voltt for eVTOL Battery System Design and Evaluation

The Voltt Design web app (Figure 2) makes battery pack design and architecture analysis super easy for eVTOL and aerospace applications. Cell manufacturers can use it to dig deep into their products, find areas to make them even better, and level up their offerings to meet the needs of these applications. System designers can check out how different cells and system architectures perform to make the battery system as awesome as possible while keeping costs down and ensuring safety. The web app also lets you compare how things are at the beginning and end to make sure the battery performs consistently throughout its life.


Screenshot for Voltt Design - A Pack Configuration and System Architecture Development Tool.
Figure 2. Screenshot for Voltt Design - A Pack Configuration and System Architecture Development Tool.

The tool is super useful in the project definition phase of the engineering V-model (Figure 3), which is a fancy way of saying a structured approach to designing, developing, and testing complex systems. This approach is perfect for battery system design because it focuses on checking and making sure everything is safe and performs well right from the start.


 Typical Battery Engineering Process Landscape: V-model.
Figure 3. Typical Battery Engineering Process Landscape: V-model.

Case-in-Point: Generating Aerospace Battery Concepts using Voltt Design

E-One Moli Energy Corp is a top battery cell maker for eVTOL aircraft, known for their high-performance Molicel brand lithium-ion cells. These batteries are designed for awesome power output, long cycle life, and wide-temperature operation, making them perfect for eVTOLs (source: Exploring Molicel's New Power Cell). Using the Molicel P45B as an example, let's check out the system requirements and come up with concepts using Voltt Design. We'll also compare performance at the beginning and end of battery life, and think about trade-offs between weight, available power, and thermal efficiency. It's worth noting that most eVTOL aircraft have multiple batteries to handle energy, power, space limitations, and system-level safety strategies.


Before we go any further, let's make a few assumptions.


Chosen cell format: Cylindrical 21700

  • Cylindrical cells are widely used because they have a standard shape and safety features like CIDs and PTC thermistors. Their smaller energy capacity makes designs safer and more modular.

  • The 21700 cell size offers a good balance between weight and safety, making it suitable for eVTOL battery systems. Cylindrical cells are cost competitive due to high-volume production. They can be easily integrated into devices, allowing for quick adoption of new battery technologies. Manufacturing considerations also contribute to their structural robustness.

Number of cells in series and parallel:

  • Module: 8 cells in parallel with 12 cells in series

  • Pack: 20 modules in series with 1 in parallel

  • This results in a pack with a nominal energy of 31.1 kWh, operating between 600 - 1000 V.

Just a note, the investigations are done at a pack level. There will be further considerations when looking at system-level performance.


Mass multipliers:

  • Cell-to-Module: 1.15

  • Module-to-Pack: 1.15

Miscellaneous resistances:

  • Module (accounting for cell connections, busbars, etc.): 0.002 Ω

  • Pack (accounting for module connections, fuses, etc.): 0.100 Ω

SOC window (0-100%):

  • Lower bound (accounting for reserve requirement): 20% (may be presented as 0.2 in results below)

  • Upper bound: 95% (may be presented as 0.95 in results below)

End-of-Life (EoL) multipliers:

  • Capacity EoL multiplier: 0.9 (this means that capacity available at EoL is 90% of that at BoL)

  • DC Resistance EoL multiplier: 1.3 (this means that resistance at EoL is 130% of that at BoL)

Cost & Utilisation:

  • Let's leave those out for now.

We've put together a battery pack with a nominal capacity of 31.1 kWh, but only 24.4 kWh is usable. As shown in Figure 4, the available power decreases from 289 kW to around 221 kW as the battery pack discharges from 95% SOC (State of Charge) to 20% SOC. Additionally, even though the battery pack has a nominal capacity of 186 Wh/kg, the usable capacity at BoL (Beginning of Life) is 145 Wh/kg due to limitations on the SOC window and thermal losses, among other factors.



Pack Level Available Power Estimated for 240s8p Molicel P45B Aerospace Battery Pack
Figure 4. Pack Level Available Power Estimated for 240s8p Molicel P45B Aerospace Battery Pack

The main challenge in designing battery packs for eVTOL applications is ensuring the safety of power delivery. This becomes particularly crucial during landing. Even without considering go-arounds, a minimum of 200 kW is needed for up to 90 seconds to ensure a safe landing (as shown in Figure 5 for a typical eVTOL profile). By comparing the available power in Figure 4 with the requirements in Figure 6, it becomes evident that it becomes increasingly difficult for the battery to supply the necessary power at lower SOC levels. Note, the go-around (GOA) procedure is a critical aspect of aviation safety, allowing pilots to abort a landing attempt and climb back to a safe altitude if necessary [5]. This procedure is particularly important for eVTOL (electric vertical take-off and landing) aircraft, which operate in complex and dynamic urban environments.


Typical Aerospace eVTOL Power Profile at Battery Pack Level for 240s8p Configuration
Figure 5. Typical Aerospace eVTOL Power Profile at Battery Pack Level for 240s8p Configuration


Understanding Design Complications: Using Voltt Design

This complexity is further compounded by factors such as aging (Figure 6), especially when power is needed at even lower SOCs, such as during go-around landing events. According to calculations by Voltt Design, the continuous discharge power available at 50% SOC at beginning of life (BoL) is 254 kW. However, as the battery ages from 100% state of health (SOH) to 90% SOH, this power drops to 240 kW, representing a 5% decrease (please note that these estimates are based on the end of life assumptions made earlier). Additionally, it is important to note that as the pack level resistance increases from 0.5 Ω to 0.61 Ω, this not only affects electrical performance, but also significantly increases heat generation within the pack. This highlights the need to consider battery thermal management system design in light of end of life conditions.



Pack Level Available Power Estimated for 240s8p Molicel P45B Aerospace Battery Pack at EoL
Figure 6. Pack Level Available Power Estimated for 240s8p Molicel P45B Aerospace Battery Pack at EoL

As depicted in Figure 7, when the power drawn from the battery during discharge increases, the thermal efficiency decreases due to greater heat generation within the battery pack. This effect becomes more pronounced as the battery's SOC decreases. Alongside the increase in resistance, the battery currents must also rise to meet the power demand. However, this increase in current can lead to dendrite formation, which compromises safety. To tackle these issues, battery manufacturers are working on developing new thermal management technologies. By enhancing thermal management, we can prolong battery lifespan, enhance safety, and broaden their range of applications.



Typical Thermal Efficiency Values for Molicel P45B Battery Pack for 240s8p Configuration
Figure 7. Typical Thermal Efficiency Values for Molicel P45B Battery Pack for 240s8p Configuration

The operating voltage of a battery pack is important to ensure that it does not exceed any thresholds imposed by the limitations of the inverter, given a certain power. As depicted in Figure 9, as the discharge power increases, the operating voltage decreases. This implies that if the inverter had a minimum operating voltage limit of 660 V, the total available power would not be accessible. For instance, instead of being able to draw 460 kW at 95% SOC, the battery would only be able to supply around 440 kW at the same SOC.



FTypical Voltage Values for Molicel P45B Battery Pack for 240s8p Configuration
Figure 8. Typical Voltage Values for Molicel P45B Battery Pack for 240s8p Configuration

Now comes the fun part.


Imagine your manager has asked you to determine the relationship between the available pulse discharge power at 50% SOC (both BoL and EoL) and the weight budget allocated. Currently, for a 240s8p battery pack weighing 168 kg, the available continuous discharge power at 50% SOC is 343 kW at BoL and 282 kW at EoL. Let's consider some additional assumptions: the number of cells in series is fixed, as dictated by the inverter limits, and the number of battery packs is also fixed.


Using Voltt Design, we can define the following table to easily understand the relationship between adding cells in parallel and the pulse discharge power available at BoL and EoL. As shown, the available power at BoL and EoL increases as the number of cells in parallel increases. This is because the battery pack has a higher capacity and can deliver more power. However, keep in mind that the weight of the battery pack also increases with more cells in parallel.


The table also demonstrates that the available power at BoL and EoL decreases as the battery ages. This is because the battery's capacity decreases and its resistance increases over time. This means that if a battery pack needs to deliver 282 kW pulse power for an 8p battery system at EoL, it should be designed for 343 kW available pulse power at BoL.

Cells in Parallel

Pack Weight [kg]

BoL Power [kW]

EoL Power [kW]

6

126

277

224

7

147

311

254

8

168

343

282

9

189

373

309

10

210

401

334


In a nutshell, this simple analysis highlights the importance of considering both weight and performance requirements when designing eVTOL battery packs. By optimizing the number of cells in parallel, battery designers can balance the available power with the desired weight budget, ensuring that eVTOL aircraft can meet the demanding power requirements of both take-off and landing maneuvers, even as the battery ages. Voltt Design provides a valuable tool for making these informed decisions and enabling the development of high-performance eVTOL battery systems.


Conclusion

Voltt Design is a super useful tool for checking out the main system requirements and coming up with cool ideas for eVTOL battery systems. It can be used to evaluate how different cell chemistries, architectures, and setups perform, helping you make smart choices that boost the battery system's performance, save money, and ensure safety.

In a nutshell, developing top-notch eVTOL battery packs is a pretty complex and tough job that needs a deep understanding of battery tech, system needs, and engineering principles. Voltt Design is a simple-to-use tool that can help cell manufacturers, eVTOL developers, and battery system designers make informed choices and bring awesome eVTOL battery solutions to market.


If you think this is a good start and you want more detailed info on specific cells or modelling tools to predict cell performance in your system-scale simulations, check out our Voltt platform.


References

  1. Yang, Xiao-Guang et al., “Challenges and key requirements of batteries for electric vertical takeoff and landing aircraft” Joule, Volume 5, Issue 7, 1644 - 1659, 2021.

  2. M. Al-Hussein, et al., "The Future of Battery Cell Chemistry for eVTOL Applications," Journal of Power Sources, 2023.

  3. Viswanathan, V., Epstein, A.H., Chiang, YM. et al. The challenges and opportunities of battery-powered flight. Nature 601, 519–525 (2022).

  4. Baker McKenzie, "Regulation and Certification of Electric Vertical Take-off and Landing (eVTOL) Aircraft | Insight" (2022)




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