BFFT Techblog August: Design of high-voltage batteries at cell level
Subject: Lithium-ion batteries for BEVs (battery electric vehicle). Approach to battery design in the concept phase. From testing the performance requirements to estimating the cooling capacity and installation space required.
The importance of electromobility will increase significantly over the next few years. As soon as the necessary charging infrastructure is in place and affordable electric cars are available in car dealerships, electromobility will also catch on with the majority of the population. The progress made in terms of affordable electric cars is now also being advanced by the OEMs – even if it is only to achieve the EU’s CO2 emissions targets or due to the new competitors in the automotive sector.
This techblog deals with the subject of the electrochemical energy source of purely electric vehicles. The following explanations illustrate the details of a battery based on lithium-ion cells and why it is not possible to reach a range of 1000 km in everyday life with the current technology, not even in an SUV without an additional trailer.
A high-voltage battery or battery pack of an electric vehicle (EV) is composed of a multitude of individual lithium-ion cells, which are organized in modules, also called stacks. The number of cells required ranges from a few hundred for prismatic or pouch cells up to nearly five figures for round cells. The above-mentioned pouch cells have been used for years in a multitude of consumer products such as smartphones and have replaced round cells in many places.
As already mentioned, the cells are generally mechanically combined into modules. For example, the Nissan LEAF (version 2012) has 48 modules wired in series. In each module, there are 4 cells wired as 2s2p, respectively 2 in series and 2 in parallel. Due to the wiring in series, the voltage is “set” (mostly in the nominal range of 400 V), whereas the additional parallel wiring of the cells increases the capacity of the battery and allows a higher current flow, thereby achieving the desired range and power. This is necessary because, depending on the system, the current carrying capacity of a cell is not sufficient to provide the required power for the vehicle. In addition to cellular chemistry, the current load capacity depends significantly on the capacity. The power and energy densities of the current cell chemistries simply limit the possible capacity and therefore the current that can be drawn from it. It is only through new technologies that it will be possible to pack 80 kWh into a lower weight than ~ 450 kg (cf. Tesla Model 3) or to achieve a higher capacity at the same weight and thus significantly increase the range of the vehicles. The industry’s hope is currently vested in so-called solid-state cells. In these cells, whose use in standard production is not expected before 2025, the previously liquid electrolyte of the cell is replaced by a solid substance. This new technology promises a range comparable to combustion engines, while taking up the same installation space as current batteries. The properties of the solid-state cell also facilitate improved fast charging capabilities and increase safety.
The use of batteries with a voltage level of 800 V (nominal) is foreseeable. The Mission E (Taycan) by Porsche is an example. The higher voltage can be used to work with lower currents at the same power. This makes it possible to save weight due to smaller wire cross-sections. The hurdle currently lies in the availability of components that have the necessary electric strength.
The maximum current that can flow per cell during charge and discharge is called the C-rate by the manufacturers. This is the specification of the maximum current in relation to the nominal capacity of the cell. Example: The indication “Discharge 10 C” of a 44 Ah cell corresponds to a discharge current of 440 A. Here it must be noted that due to the internal resistance, the operation of the cell with higher or maximum C-rates leads to increased heat development, a reduction in the number of cycles, resulting in a shorter service life.
In addition to the structural design of the cells, wiring and crash requirements, thermal conditioning of the cells is of vital importance. Depending on the cell type, there are various possibilities for the design of a thermal management system. The design of such a system has a significant influence on the installation space, the complexity and therefore also on the price of a vehicle battery. This is primarily a distinction between passive and active temperature-controlled systems. With active cooling, for example, different cooling media can be used, such as air, water or refrigerant.
In the Energy Systems and Electrification Department, batteries of this type are designed, developed and prototyped based on customer requests. Depending on the level of detail and the availability of requirements, a first concept can already be provided in great detail. The most important electrical requirements are the voltage level and the charge and discharge capacity of the battery.
Since the performance and aging of the lithium-ion cells are highly dependent on the temperature, the thermal management of the battery must already be taken into account during in the concept phase in order to ensure feasibility. Only a correctly dimensioned temperature control makes it possible for the specified power to be drawn from the battery for the entire life cycle. The optimum temperature range for lithium-ion batteries lies approximately between 15 °C and 45 °C (depending on cell chemistry and manufacturer). Below this range, there is a sharp increase in the internal cell resistance, which leads to loss of performance. In addition, charging at low temperatures leads to significant cell aging. This is why, for example, recuperation is very limited or not available at all in cold batteries in order to protect the cells. Aging also accelerates at temperatures above the optimum range. Cell components such as the electrolyte begin to degrade, or the material properties change. Cell manufacturers often specify an upper limit of 60 °C. If the temperature increases further, this can cause “thermal runaway”, which can lead to a fire and in the worst case scenario, to the complete destruction ofthevehicle.
Electric vehicles which only have passive thermal management (e.g. VW E-Golf, Nissan Leaf) often have the problem that the range drops sharply in winter and the battery ages faster at high ambient temperatures than in vehicles with active thermal management. In addition, the charging of such vehicles can take longer, as the battery cannot be actively placed and maintained in a temperature range which is optimal for the charging process.
The following provides a broad overview of the most important steps in the conceptional design of the batter:
- Checking requirements
The voltage level of the battery is probably the most important requirement. It determines the number of cells to be switched in series and as a result, the type of battery (low-voltage/high-voltage). In addition, depending on the focus of the prototype, the energy content and therefore the range or alternatively the charge and discharge capacity may be of paramount importance. This specifies the system in terms of the number of cells to be switched in parallel and the cell chemistry.
- Determination of C-rate
Determination of the necessary C-rate to achieve the required continuous and pulsed charge and discharge capacity depending on the cell wiring. The cell wiring is mainly dependent on the cell type and chemistry. The nominal voltage also differs in the cell chemistries and the number of cells that are switched in series varies as a result.
- Selection of realistic C-rate
Comparison of the C-rate with the data of the cell manufacturer. A C-rate must then be selected, which represents a realistic load for the cells. It should be noted that the maximum specified load in the data sheet is usually not a practical solution. Such a load is not advisable because it causes the battery to age too quickly, as well as due to the cooling capacity required.
- Comparison of possible power with requirements
Based on the selected wiring and C-rate, the power has to be compared to the requirements. Two things must be kept in mind in this context. On the one hand, in practice the cells are only operated in a usage window, called the depth of discharge (DoD). This protects the cell, ensuring a longer service life.
On the other hand, it must be possible to draw down the power when the charge in the cell is low, called the state of charge (SOC). The voltage of the cell depends on the state of charge and decreases in line with this. It is therefore expedient to calculate the maximum power based on the minimum load state and therefore the minimum cell voltage of the defined usage window.
- Evaluation of energy content
The actual usable energy and consumption of the vehicle are relevant for the range of the vehicle. The specified consumption must be seen as a rough indication, as it is dependent on the driving behavior of the user, the functions used, such as air conditioning and the existing power dissipation in the drive train. This is also the reason why the ranges specified by manufacturers are not suitable for everyday use and a lower range must be anticipated.
In contrast to range data based on the NEDC and WLTP standardized driving cycles, information based on a type of customer consumption cycle (CCC) is more realistic.
For a professional examination of the energy content, the usable energy must be considered in the use window and at the end of the life cycle, called the end of life (EoL), of the cells. In the case of electric cars, the battery is already repurposed for second life applications, such as use as stationary storage, when its capacity has decreased to 80% of the initial value.
- Estimating the thermal management required
To estimate the power dissipation, the electrical resistance of the battery and the expected RMS current (root mean square) of the system are used. The total electrical resistance of the battery is determined on the basis of the cell wiring and the internal cell resistances. Here, an EoL analysis is useful again, since the internal resistance of the cells increases with aging.
Depending on the estimated power dissipation, a passive thermal management system may be sufficient. For the optimal operation of batteries, however, an active system is advisable from a technological perspective, in order to keep the cells at the ideal operating point at all times wherever possible. The challenge lies in cooling the battery, since the operation of the battery at high temperatures is a more significant aging and safety-critical factor than operation at low temperatures.
In an ideal scenario, sufficient information is available to create simulation models and to determine the temperature behavior of the battery or individual cells as realistically as possible.
For this purpose, 3D models (Bild 5) can be used in the further development process to represent the spatial distribution of the temperature. This makes it possible to visualize hotspots and temperature gradients and take them into account in further development. Excessive temperature gradients (> 5 K) within or between cells should be avoided in order to avoid an uneven aging of the battery.
- Estimating the installation space, weight and costs
Based on existing experience, mechanical components and conditions can be taken into account in addition to the weight and volume of the cells and compared with the requirements.
The mechanical requirements for the battery include crash resistance, vibration stability and location in the vehicle. The planned quantity is also relevant and has an influence on the concept.
When designing the thermal management system, it is necessary to find a compromise between existing installation space, weight and costs compared to the optimal temperature control variant.
The design of the thermal contacting between the battery cells and the cooling system is a decisive factor for the cooling performance and has a different effect on the installation space. In this context, the design of the battery cells (cylindrical, prismatic, pouch) plays an important role. Typical variants of a cooling connection are shown in Bild 6 . However, the space requirements limit the choice of solutions.
All departments concerned must work closely together to create a well-founded battery concept. This is the only way to ensure that the concept can be implemented at a later stage.
In an ideal scenario, the individual stages of the design will only be stepped through once and then result in a first impression of the possible battery specifications. In reality, however, several reiterations are required, as intermediate steps can result in an unimplementable solution. For example, estimating the power dissipation may require a cooling capacity which is too high, making the system unattractive from an economical perspective or impossible to implement from a technical standpoint. Another example would be a necessary power reduction of the battery, since the required number of parallel-switched cells does not fit into the intended installation space.
Author: Daniel Walters, Sebastian Daniel (Energy Systems & Electrification)
- R. Korthauer. Handbuch Lithium-Ionen-Batterien. 1. Edition. Berlin: Springer-Verlag, 2013