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Lithium Ion Battery Pack: How is it made?

Li-ion battery

Introduction

Increased reliance on electric powered transportation, wireless devices, and electronics worldwide has caused an uprise in demand for the development of lithium-ion batteries. Electric vehicles, which use electrochemical cell batteries, have risen due to both increasing popularity and a cultural shift to reduce carbon emissions. In 2018, according to the Global Carbon Project, CO2 emission from transportation hit an all-time high of 37.15 billion tonnes (Global Carbon Budget, 2019).


With the price of fossil fuels and government subsidies increasing, it reduces the price gap between traditional combustion engines and electric-powered engines. 


Figure 1 shows the increase in the usage of lithium-ion batteries coinciding with increased consumer use, showing positive development in the electric vehicle sector (Yoshino, 2012) and thus an increased need for batteries.

Figure 1 Expansion in demand for Lithium-Ion batteries, GWh – Gigawatt hours (Deng, 2015)
A complete battery pack is made up of the number of cells with low voltage and power specifications fixed in a parallel format. Different chemistry of battery cells decides compatibility for a specific application, i.e. domestic or industrial, stationary or portable, high power or high voltage. Despite the electrochemical versatility, battery pack modelling affects many parameters such as lifetime, cycle-life, efficiency, specific power and energy, and safety and cost. Overall, battery pack modelling and testing adapt to the particular requirement of the application.

Manufacturing Of The Battery Pack

Battery cells come with specific current, voltage and power capacity. To run automotive, computer or aircraft applications, they require high voltage and current. For high voltage and amp power capacity, many cells need to be connected in series or parallel format to meet the correct requirements (Ko et al., 2019).

Selection Of Cell

The capacity and weight of the battery cell differ as per its construction and type. The initial step to designing the battery pack is to select the appropriate battery cell which should meet the criteria of the application as shown in Table 1 (Rajasekhar & Gorre, 2016) (Amp & Cells, 2014). The chemical properties affect the voltage and Amp hour (Ah) capacity of the battery cell. Due to the advantages of the lithium-ion battery, it is used for the formation of the battery pack.


Table 1 Pros and cons of cylindrical and prismatic cell

Type of cellProsCons
Cylindrical CellStandardized sizeRequire more space
 Easy construction, Low costTolerance issue
 

Can handle high internal

pressure

Become hard to increase

capacity

Prismatic CellThin and Light in weightMore expensive
 More stableComplicate construction
 High volumetric efficiencySwelling issue

Cell Configuration

The second step is to determine the number of cells in the series or parallel configuration of the battery cell. The setup of the battery pack is based on the voltage and current rating of every cell. To make the battery pack compact, the series and parallel configuration can be optimized (Maiser, 2014). A series configuration consists of batteries connected by the positive of one cell to the negative of the next cell. As shown in Fig. 2, a series configuration connection adds the voltage capacity of each cell to deliver the total terminal voltage.

Figure 2 Series configuration of the battery cell (University, 2019)
In parallel configurations, the positive and negative of every cell are connected as shown in Fig. 3. This adds more Ah capacity to each cell.
Figure 3 Parallel configuration of the battery cell (University, 2019)
Battery packs consist of a combination of series and parallel configurations. For example, a laptop usually requires four lithium-ion battery cells in series (4×3.6 V) to achieve 14.4 V nominal voltage and two cells in parallel (2×2400 Ah) to produce a 4800 Ah capacity battery pack.

Structural Design

The third step to consider for battery pack design is the architectural design that will best increase the life and safety of the battery pack. This includes detailed knowledge of electrical connections, mechanical support (battery cell pressure and support), the material used, and thermal management of the battery pack.

Electrical Interconnection

The battery pack carries high voltage, meaning the electrical interconnection of the battery pack should be capable of carrying the maximum current capacity. Improper design and consideration can lead to damaged connections to nearby components due to excessive heat loss (Rajasekhar & Gorre, 2016). The primary electrical interconnection in the battery pack is in between the cells. A major part of this process includes soldering nickel where the cells connect to carry high current.

Nickel Strip

The main advantage of using pure nickel strips as the connection material is that it has very high corrosion resistance and is very easy to spot weld. Over the past decades, many bike battery manufacturers have used an automated spot welder, which is excellent for the low-amp battery cell (Lewchalermwong et al., 2018). The numbers, size and configuration of the nickel strip directly depend on the battery’s maximum amount of current capacity.

The table below shows the acceptable current levels for pure nickel strips.

Size Optimal Acceptable Poor
0.1 mm × 5 mm
< 2.1 A
~3.0 A
> 4.2 A
0.1 mm x 7 mm
< 3.0 A
~4.5 A
> 6.0 A
0.15 mm x 7 mm
< 4.7 A
~ 7.0 A
> 9.4 A
0.2 mm x 7 mm
< 6.4 A
~ 9.6 A
>12.8 A
0.3 mm x 7 mm
< 10.0 A
~ 15.0 A
> 20.0 A

Table 2 Acceptable current levels for pure nickel strip (Mike, 2017)

The division of maximum battery current to the current carrying capacity of the nickel strip gives the number of pieces needed. For example, if two parallel groups with four cells in each configuration are being used to create a total of 20 A battery current with the help of 0.15 mm × 8 mm nickel strip (5A current varying capacity), then it requires 20 A/ 5 A = 4 strips to carry the present (Figure 4). Similarly, if the battery current is 40 A, then eight nickel strips are needed because two pieces are needed for a single connection. The best battery pack design is to stay within the criteria of the nickel's optimal current capacity. If the dimension of the nickel strip is chosen from the poor current position, it could start to overheat and damage the thermal ability of the battery pack.

Figure 4 Nickel strip connection for long series-parallel configuration
Figure 5 Nickel strip configuration as per the current carrying capacity, Fig. A for 20A current draw, Fig B for 40A current draw

Multiple nickel strips can be also used for long-series parallel configuration connections to design a compact battery pack (Figure 5).

Spot Welder

For mass production of battery packs, automated robot spot welders have become popular (Lewchalermwong et al., 2018) considering the computerized system has made spot welding easy and productive. Unfortunately, due to limited production and startup, the automated system is not yet an economical option.

It is important to note quality welding plays a vital role in optimized battery packs since uneven welding creates weak conduction and hotspots. Traditional welding methods are not sufficient, especially when working with lithium-ion cells. Nowadays, more people are looking for custom built or rebuilt battery packs. In order to fulfill demand, small spot welders are handy and provide outstanding quality when it comes to readily available custom welding.

Mechanical Structure

Mechanical construction affects the performance and longevity of battery packs. Four parameters need to be considered while designing the mechanical development of the battery pack.

I.   Thickness and expansion of the cell

During charging and discharging, the battery cell expands and retracts to its original position. The amount of expansion depends on the load on the cell. As the cell ages, the thickness of the cell increases between 3-5% of its initial diameter (Rajasekhar & Gorre, 2016).

II.    Insulation

The battery cell is an electrochemical component meaning it is necessary to insulate each cell from one another and any other conducting materials such as the outer environment and electrode terminals of the battery.

III.    Ventilation to the cell

During the charging cycle of the battery, the electrodes of the cell produce gaseous compounds and create pressure on the surface of the cell. To avoid gaseous compounds, the battery pack should have provisions to vent out the gases.

IV.    Physical protection of the cell

 

The battery pack contains the battery cell as the main component and a well designed battery cell will offer the best and safest battery pack. The cell of the battery should be protected from vibration, shock, dirt and water. Physical protection should also consider the thermal management of the pack.

Thermal Management

There are many thermal management approaches for the lithium-ion battery. One of the most critical parts of the battery management system is thermal management. This system aims to maintain the temperature of the battery within its specification and create even heat distribution throughout the pack (Rothgang et al., 2012). All batteries are classified as flammable components because they contain electrochemical construction and can start burning very quickly. 

Lithium-ion batteries do not have good thermal stability, meaning special attention needs to be paid during the thermal management manufacturing process. The control design (active or passive) and medium (air and/or water) are used for the cooling of heating and define the thermal management system.

For an electric bike battery system, the air is the best suitable medium for the thermal management system. Based on this, passive cooling, passive heating and cooling, and active heating and cooling methods are used. Active cooling techniques involve a fan like in a laptop.

Battery Management System (BMS)

Development in the BMS has increased as the demand for customized battery packs continues to grow. The BMS is an electronic component that helps control the critical parameters of the battery pack while working, charging and discharging (Weicker, 2014)(Jossen et al., 1999). BMS systems use sensors to monitor cell voltage, current, impedance and other mandatory conditions of the battery pack. Monitoring of these parameters helps to optimize the performance, increase the life cycle and capacity and decrease charging time.

The modern battery management system includes:

  • Cell monitoring (voltage, current, and temperature)
  • Supervising the battery pack behaviour (the state of charge, energy, performance)
  • Cell balancing
  • Charging and discharging limit of the battery pack

BMS is a vast area to discover. It comes in the form of an integrated circuit with necessary programming and sensors. The main advantage of the BMS is that it’s programmable and the limit of the functions is manageable as per the requirement and application.

There are many BMS system providers in North America such as:

  • Nuvation Engineering (U.S.)
  • Valence Technology, (U.S.)
  • Linear Technology Corporation (U.S.)
  • Eberspaecher Vecture (CANADA)
  • Nuvation Energy (USA)
  • ON Semiconductor (USA)
  • JTT Electronics (CANADA)

References:

Amp, M., & Cells, H. N. (2014). Battery Pack Design , Validation , and Assembly Guide using A123 Systems. 1–71.

Deng, D. (2015). Li-ion batteries: Basics, progress, and challenges. Energy Science and Engineering3(5), 385–418. https://doi.org/10.1002/ese3.95

Global Carbon Budget. (2019). No Title. https://www.globalcarbonproject.org/carbonbudget/

IEA. (2019). Global EV Outlook 2019. In IEA. https://www.iea.org/reports/global-ev-outlook-2019 Jossen, A., Späth, V., Döring, H., & Garche, J. (1999). Reliable battery operation - a challenge for the battery management system. Journal of Power Sources84(2), 283–286. https://doi.org/10.1016/S0378-7753(99)00329-8

Lewchalermwong, N., Masomtob, M., Lailuck, V., & Charoenphonphanich, C. (2018). Material selection and assembly method of battery pack for compact electric vehicle. IOP Conference Series: Materials Science and Engineering297(1). https://doi.org/10.1088/1757- 899X/297/1/012019

Maiser, E. (2014). Battery packaging - Technology review. AIP Conference Proceedings1597(February), 204–218. https://doi.org/10.1063/1.4878489

Mike. (2017). How Much & What Size Nickel Strips Should You Use.

Rajasekhar, M. V., & Gorre, P. (2016). High voltage battery pack design for hybrid electric vehicles. 2015 IEEE International Transportation Electrification Conference, ITEC-India 2015, 1–7. https://doi.org/10.1109/ITEC-India.2015.7386876

Rothgang, S., Nordmann, H., Schaper, C., & Sauer, D. U. (2012). Challenges in battery pack design.

Thomas Industry Update. (n.d.). Top US and International Battery Suppliers and Manufacturers. https://www.thomasnet.com/articles/top-suppliers/battery-manufacturers-suppliers/

University, B. (2019). Series and Parallel Battery Configurations.

Weicker, P. (2014). A Systems Approach to Lithium-ion Battery Management. Artech House. World Energy Council. (2019). Energy Storage Monitor: Latest trends in energy storagewww.worldenergy.org

Yoshino, A. (2012). The birth of the lithium-ion battery. Angewandte Chemie - International Edition51(24), 5798–5800. https://doi.org/10.1002/anie.201105006

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