Framework and Classification of Battery System Architectures: History
Please note this is an old version of this entry, which may differ significantly from the current revision.

In this paper, battery system architectures are methodologically derived in order to find the key type differences. In a first step, the system levels are identified and distinguished. In order to be able to completely cover the solution space of battery system architectures, a distinction is also made between mono- and multifunctional materials. Based on the system levels, a framework for possible architectures is derived. Four system architecture generations with a total of eight different types are identified and analyzed in the dimensions “Nomenclature”, “Approach”, “Omitted Components” and “Industry Examples”. In this way, upcoming system architectures, such as cell-to-pack and cell-to-chassis, can be clearly differentiated. Finally, fundamental product characteristics for the four system generations are derived and compared.

  • battery electric vehicle
  • battery system architecture
  • Cell-to-Pack
  • CTP
  • Cell-to-Chassis
  • CTC
  • Cell-to-X
  • structural battery

1. Introduction

In the years from 2010 to 2020, electromobility was for a long time only a marginal phenomenon in the international automotive industry. This is changing, and battery electric vehicles are finding their way into the portfolios of major car companies. In order to complete the mobility transformation, the acceptance of electric vehicles must be further improved. The two biggest obstacles for electric cars cited in surveys are high purchase costs [1] and a short range [2,3] (p. 44). Both arguments can be traced back to the battery, which is why development priorities are being set here in order to achieve full EV market penetration in the future.
Figure 1 shows the typical modular product structure of the battery system in the automotive sector. The voltage of individual cells is limited by the basic chemical elements. Therefore, single battery cells are interconnected in series and/or parallel to form a battery module. This encapsulates the intercontacted cells and a battery management unit (BMU)-Slave with the corresponding voltage measurement and temperature sensors. A connection or the cooling system itself, e.g., in the form of a cooling plate, can also be part of the battery module. For voltages up to 60 V (DC), no special high voltage safety measures apply during module assembly. Above 60 V (DC), the regulations of the DGUV have to be observed [6] (p. 9). For this reason, the voltage of 60 V (DC) was usually not exceeded in the past within the individual modules. To achieve the required battery pack voltage, several battery modules are connected in series to a battery string [7] (pp. 31–32). To increase capacity, serial strings of battery modules can be connected in parallel to form a larger battery pack [8] (p. 300). The battery pack contains the BMU-master interfaces for the external systems and has a high IP protection class. In the passenger car sector, the battery pack is usually equivalent to the battery system that is integrated into the vehicle chassis. In the commercial vehicle sector, several battery packs are typically connected in series and/or parallel to form the commercial vehicle battery system. In the passenger car sector, battery systems currently have voltages of up to 924 V [9] and usable energy contents of up to 210 kWh [10]. For example, the Tesla Model S has a 100 kWh storage unit weighing over 500 kg [11]. The size of the battery pack is significantly limited by the available installation space inside the vehicle.
Figure 1. Simplified BEV product structure based on a conventional battery system architecture.
The energy density of available battery packs is on average around 50% of the volumetric energy density at the cell level [4]. There is thus potential to increase energy density at the overall system level through efficient battery cell integration. Disruptive system architectures, e.g., Cell-to-Pack (CTP) or Cell-to-Chassis (CTC), are promising approaches to exploit the existing potential. The usual composition of the system layers—battery active materials, battery electrodes, battery electrode stack, battery cell, battery module, battery pack/system, (vehicle) chassis, full battery electric vehicle—is abandoned by skipping individual system levels, i.e., the cells are directly integrated into the pack housing/vehicle chassis or may in the future even replace chassis/vehicle parts. 

2. Generation 0—0 Layers Omitted

Generation 0” (G 0) are “Conventional System Architectures” that use all previous identified system levels and are therefore based on eight layers—active material, electrode, electrode stack/roll, cell, module, pack/system, chassis/body (in white) and the overall vehicle/BEV.

2.1. Generation 0—Conversion Design

  • Nomenclature: The first generation of LIB mass market BEVs was based on the conversion of existing internal combustion engine (ICE) vehicle platforms, leading to the nickname “Conversion Design” [38] (p. 54).
  • Approach: To save both investments in development and production cost, the already developed ICE chassis/vehicle platform as well as the existing production lines were used to build the first BEVs. This leads to significantly lower costs and risks for the manufacturer, but to disadvantages in the technical implementation, as innovations and package advantages can only be achieved within limits.
  • Omitted Components: Compared to vehicles with combustion engines, the ICE is replaced by an electric drive train.
  • Industry Examples: The arguably first highway legal serial production of an all-electric car to use lithium–ion battery cells was the Tesla Roadster 2008 [39], which was partly based on a Lotus Elise platform, even though they did not share the same production line [40]. Another well-known example is the e-Golf, which VW assembled on the same production line in Wolfsburg as the Golf combustion models [41]. In the early 2020s, there are still many models available with both combustion engines and pure battery electric vehicles (BEVs). These include, among others BMW 4, 7, X1 and X3 [42]; Citroën C4 [43]; DS 3 Crossback [44]; Fiat 500 [45]; Hyundai Kona [46]; Mini [47]; Opel Corsa, Mokka and Zafira [48]; Peugeot 208, 2008 and Traveller [49]; Renault Master [50]; Toyota Proace and Verso [51]; Volvo XC40 [52], VW e-up [53].

2.2. Generation 0—Purpose Design

  • Nomenclature: Instead of converting existing ICE vehicles, the next generation system architecture is a dedicated “Purpose Design” BEV platform, often executed as a skateboard platform [54].
  • Approach: The Purpose Design platforms aim to exploit the advantages of electric drive trains. Battery systems no longer have to be squeezed into existing installation spaces, but the platform design is optimized for a purely electric powertrain—a dedicated EV platform.
  • Omitted Components: New degrees of freedom result from the elimination of the vehicle tunnel, which enables a considerably simplified vehicle underbody and thus also simplified operating equipment in the area of chassis construction [38] (p. 55). By eliminating the combustion engine, the wheelbase can be extended to gain more space between the axles and accommodate a larger battery in the underbody of the vehicle. [55] In comparison with different BEV conversion designs, it can be argued that the multi-pack variants are omitted. Instead of electrically connecting two mechanically separated battery packs to form one battery system, as in the conversion design of the 2015 FORD Focus EV [56], for Purpose Design platforms there is predominantly only one installation space for the battery system. This reduces the number of pack housings from two to one and also lowers the number of necessary thermal and electrical interfaces (connectors).
  • Industry Examples: Generation 0 in the purpose design manifestation can be considered as state-of-the-art in 2023, as the BEVs in mass production are predominantly based on purpose design architectures with all system levels. Examples include Geely SEA platform [57], Hyundai E-GMP [58], Rivian Skateboard [59], Tesla [60], Volkswagen Group MEB platform [61] and Xpeng SEPA [62], among others.

3. Generation I—1 Layer Omitted

Generation I” is based on the partly omission of one system level, which leads to seven instead of eight main system layers. Since the battery materials, electrodes and electrode stacks display lower boundary conditions and the chassis (BiW)/BEV the upper ones for the application of electrochemical energy storages inside a battery electric vehicle, there are three possible Generation I manifestations.

3.1. Generation I.1—Module-to-Chassis

  • Nomenclature: The “Generation I.1” expression is based on the partly omission of the pack/system level. Based on the nomenclature defined in the introduction of this chapter, Generation I.1 can be described with the nickname “Module-to-Chassis”.
  • Approach: Cells are assembled into modules and these are then integrated into the vehicle chassis (parts). Previously separate pack and chassis components can be brought closer together/combined, such as the battery pack lid with the chassis base. It is possible to integrate former pack functionalities/components into the underlying assembly group—the battery module. The pack level is partially omitted, as probably not all pack components can be merged with components of other levels. Examples for components that cannot be easily transferred to other system levels can be found in the pack domains of the thermal system and high-voltage. The pack component groups of the temperature control system (cooling and heating elements) and the battery junction box (contactors, fuses, currents sensors, BMU master, etc.) can be partially moved from the pack level to the neighboring module or chassis level on paper, but only lead to an increase in energy density if they are intelligently functionally integrated.
  • Omitted Components: The battery pack housing and the vehicle chassis merge into one component/assembly.
  • Industry Examples: Until the beginning of 2023, there are no announcements for Module-to-Chassis system architectures.

3.2. Generation I.2—Cell-to-Pack

  • Nomenclature: “Generation I.2” partly skips the module level. Cells are directly integrated into the pack housing, which is then married to the chassis—a “Cell-to-Pack” system architecture. In the existing literature, this approach is also known under the description “module-less” [63] or “module-free” [64] battery pack technology.
  • Approach: “Cell-to-Pack” describes a new type of structure of battery systems, which is characterized by the direct integration of the battery cells into the battery pack [13]. This allows for the reduction in the passive/non-energy storing components of the battery module. Inside this Generation I.2, different types of execution can be observed. Some manufacturers only eliminate the module housings, but preserve cell sub-assemblies, while other manufacturers rely on one complete cell block (cf. industry examples).
  • Omitted Components: Incremental improvements can be observed by omitting the module housing [12]. The functions of the module housing, e.g., the mechanical clamping of pouch and prismatic cells, are no longer carried out by module pressure plates and module tie rods, but instead are achieved on the high pack level [65]. The previously required fastening of the individual modules as well as the space needed around these modules in order to be able to integrate them into the pack housing can be significantly reduced; thus, the cells can be packed in a denser manner and a higher overall system energy density is achieved. The saved parts also lead to a cost reduction.
  • Industry Examples: Multiple cell manufactures as well as leading vehicle original equipment manufacturers (OEMs) have filed patents for Cell-to-Pack system architectures, announced plans for a product market launch or already have a product in production. These companies are, among others, BYD (Patent 2019 [65]—Announcement 2020 [66]); CATL (2019 [67], 2022 “CTP 3.0 battery “Qilin” [68]); LG Chem (2020 [69]); Mercedes-Benz Group (2020 [70]); Nio (2020 [71]); Stellantis (2021 [72]); SVOLT (2021 [73] & 2022 [74]); Tesla (2020 [75]) and Volkswagen Group (2021 [76]).

3.3. Generation I.3—Electrode Stack-to-Module

  • Nomenclature: “Generation I.3” skips the classic battery cell and is called “Electrode Stack-to-Module” system architecture based on the neighboring system levels of the skipped battery cell level.
  • Approach: Instead of building individual cells, a battery module is built up directly out of electrode stacks. Lithium–ion battery cells based on LNMC/LNCA cathodes exhibit a typical nominal voltage of around 3.6 to 3.7 V or 3.2 Volts if the cathode is based on LFP. A key performance indicator of the Electrode Stack-to-Module approach therefore is a larger voltage difference between the two module terminals, in the region of current battery modules, i.e., 7.2–59.2 V (nominal). Starter batteries in ICE vehicles are predominantly lead–acid batteries and can be described as an approximation to the Electrode-to-Module approach, as the individual cells do not have their own fully enclosed housing, but the 12 V lead acid battery consists of multiple serially connected electrode sets, which are separated by insulation walls [77] (pp. 247–264). A similar approach is conceivable for an advanced battery chemistry in which the inner jelly rolls/electrode stacks of a battery are not (only) connected in parallel, but (also) in series. Liquid electrolytes of current LIBs decompose under voltage differences higher than 4.2 V [78], which is why until now no industry implementation of LIB-based ETM system architectures can be found. However, next gen solid-state electrolytes can resist higher voltage differences and therefore enable bipolar stacking/a serial connection of the individual monocells [79,80].
  • Omitted Components: The passive battery cell/module housing material is significantly reduced, which leads to a better ratio of cell housing mass to active material mass. If bipolar electrodes are used, the external wiring (tabs and wires) of the individual electrode stack can be omitted, as all electrodes are connected in series and only the two tabs at the end of the stack need to be connected [81].
  • Industry Examples: By early 2023, there are different announcements for electrode stack-to-module system architectures based mainly on solid-state electrolytes, but no systems in series production, e.g. the company ProLogium announced an EV battery pack based on solid-state technology and describe their approach as “Cell is Modul (CIM)” [82]. Additionally, the first research results provide an impression of what an implemented approach may look like [81,83,84].

4. Generation II—2 Layers Omitted

Generation II” is based on the partly omission of two system layers, which leads to six instead of eight main system layers.

4.1. Generation II.1—Cell-to-Chassis

  • Nomenclature: “Generation II.1” is known as “Cell-to-Chassis” (CTC). By (partly) avoiding the module and the pack level, the battery cells are directly integrated into the chassis. Therefore, this approach is also known under the nicknames “Cell-to-Body” (CTB) referring to the body in white, “Cell-to-Vehicle” (CTV), “Cell-to-Car”(CTC) [85] or “Cell-to-Frame” (CTF) [86]. Since the battery cell and chassis levels are considered necessary for conventional LIB cells, Generation II.1 represents the methodological limit of what can be achieved with conventional battery cells by omitting the system levels in the vehicle.
  • Approach: The functions previously taken over by the module and pack have to be redistributed to the remaining system levels. The degree of function integration increases parallel to the decrease in the number of system levels. The module and pack housing functionalities have either to be taken over by the chassis or by the cell. The cells are mechanically connected to the chassis and contribute significantly to their stiffness, which is why the system is also called a “Structural Battery Pack” [87]. The difference to the previously described Cell-to-Pack approach lies in the interface between the pack and the chassis, whereas with a CTP the battery system can still be detached from the chassis and the vehicle interior without leaving a hole in the bottom of the vehicle chassis. With CTC technology, the interior components (e.g., the seats) are directly connected to the battery system lid, which leads to challenges during the disassembly [88]. Current announcements by manufacturers are limited to the integration of cells into the vehicle floor. The integration of cells into the cavities of adjacent body components, such as the A-, B-, C-pillars or the doors, has not been announced. The reasons can be found, among other things, in the safety of LIBs in the event of a crash. As the key technology battery cell increasingly merges with the entire vehicle, this leads to changes in the development process. Due to the significantly higher system energy density of the CTX approaches, a development focus lies in the field of battery system safety, and more specifically in the topics of cell selection, gas flow and thermal propagation prevention. Due to the increased mechanical integration of the cells, repairs become more challenging [89], which must lead to higher quality requirements to prevent field failures due to a lack of repair capability.

4.2. Generation II.2—Electrode Stack-to-Module-to-Chassis

  • Nomenclature: “Generation II.2” is an “Electrode Stack-to-Module-to-Chassis” system architecture.
  • Approach: This G II.2 pursues the idea of skipping the cell level and then integrating these battery modules into the vehicle chassis. Therefore, this approach is a combination of the previously introduced Generations G I.1, Module-to-Chassis, and G I.3; Electrode Stack-to-Module.
  • Omitted Components: Battery cell housings and merging of pack housing and chassis.
  • Industry Examples: Until 2023, there are no announcements for Electrode Stack-to-Module-to-Chassis system architectures. This generation can only be expected after the previous generations G I.1—MTC and G I.3—ETM have been successfully implemented.

4.3. Generation II.3—Electrode Stack-to-Pack

  • Nomenclature: “Generation II.3” is an “Electrode Stack-to-Pack” system architecture.
  • Approach: The G I.3—Electrode-to-Module system architecture idea is taken and developed on step further by aiming to build up system level characteristics coming from an electrode stack level.
  • Omitted Components: The cell housings as well as the module housings are (partly) omitted.
  • Industry Examples: Until 2023, there are no announcements for electrode stack-to-pack system architectures. Nevertheless, there are patents that could be interpreted in this direction [94].

5. Generation III—3 Layers Omitted

Generation III” is based on the partial omission of three system layers, which leads to five instead of eight main system layers.

Generation III—Electrode Stack-to-Chassis

  • Nomenclature: “Generation III” is an “Electrode Stack-to-Chassis” system architecture.
  • Approach: G III is enabled by realizing the electrochemical energy storage function as a chassis component or the other way around. The extreme approach combines the previously separate levels of the battery cell and the chassis components. One way of approaching this highly function-integrated Generation would be to use SBC electrode stacks surrounded only by the matrix material of the carbon-fiber-reinforced polymer (CFRP). This drastically reduces the number of housings/intermediate system layers—the active material housing is concurrently the chassis component. Decentralized segments of body structure/panels can be electrically connected to form the entire battery system.
  • Omitted Components: Components of the classic battery system with predominantly mechanical functionality, such as cell, module and pack housings, will be (partly) omitted. The electrical interface between the battery system and the vehicle (BMU, contactors, etc.) will be probably retained in part.
  • Industry Examples: Until 2023, there are no announcements for electrode stack-to-chassis system architectures. Nevertheless, different research teams are working on structural integrated LIBs [95] as well as structural battery composites [32], which could lead to viable products in the future. As the ETC cost would be high at the beginning, an application in the aviation or even aerospace industry is initially more likely than in the car industry [96].

This entry is adapted from the peer-reviewed paper 10.3390/wevj14040088

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