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Lithium Ion Battery Pack: How is it made?

Introduction

The unappeasable demand for consumer electronics devices, fuel-efficient, and wireless electric equipment has drastically increased the demand for development in the Lithium Ion Battery Pack. Especially for the automation sector, electric vehicle equipped with green technology has created the way the high need for Li-Ion batteries. A reason for the success behind the electrification in automotive is the essentiality of CO2 emission reduction. According to the Global Carbon Project, in 2018 CO2 emission due to transportation is set to hit an all-time high of 37.15 bn tones, which share 23% of overall radiation (Global Carbon Budget, 2019), this amount will again rise due to increase in demand of the transportation.

On the other side, the rise in the price of fossil fuel and new standards for vehicles like the Clean Air Act is reducing the price gap between the conventional vehicle and electric one. Figure 1 shows the progress in the usage of Li-Ion battery for an electric vehicle compared to consumer use, which shows the development in the electric vehicle sector (Yoshino, 2012).

Figure 1 Expansion in demand for Lithium-Ion batteries, GWh – Gigawatt hours (Deng, 2015)
The major part of an electric vehicle is an electrochemical cell, battery. The number of cells with low voltage and power specification, fixed in the series or parallel format, make a complete battery pack. Different chemistry of battery cells decides the characteristic and built the base of the battery pack for its compatibility for a specific application, i.e., domestic to industrial, stationary to portable, high power to high voltage. Despite this electrochemical versatility, battery-pack modeling affects many parameters of the battery pack like a lifetime, cycle life, efficiency, specific power, specific energy, safety, and cost. Overall, battery-pack modeling and testing adopt the battery pack to the particular requirement of the application.

Manufacturing of Battery Pack

Battery cell comes with specific current, voltage, and power capacity. To run the automotive, computer, aircraft facility, it requires high voltage and current. For high voltage and amp power capacity, many cells need to be connected in series or parallel to meet the requirement (Ko et al., 2019).

Selection of cell

The capacity and weight of the battery cell differs as per the construction and type of the cell. The initial step to designing the battery pack is to select the appropriate battery cell, which can meet the criteria of the application, as shown in Table 1 (Rajasekhar & Gorre, 2016) (Amp & Cells, 2014). The chemical property of the cell affects the voltage and Ah capacity of the battery cell. Due to the advantages of Li-Ion battery, it is being 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 series and/or parallel configuration. The setup is based on the voltage and current rating of every cell, which will form a whole battery pack. To make the battery pack compact, the series and parallel configuration can be optimized (Maiser, 2014).

Connecting the battery cell in series means connecting the positive of one cell to the negative of an immediate cell. As shown in Fig. 2, Series connection adds the voltage capacity of each cell to deliver the total terminal voltage.
Figure 2 Series configuration of the battery cell (University, 2019)
On the other hand, Connecting the battery cell in parallel means connecting the positive of every cell and negative. As shown in Fig. 3, the parallel connection adds the Amp-power capacity of each cell.
Figure 3 Parallel configuration of the battery cell (University, 2019)
The battery pack consists of a combination of the series and parallel combination. Such as, for the laptop, it usually requires four Li-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 4800 Ah capacity battery pack.

Structural design

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

Electrical Interconnection

The battery pack carries high voltage, and for that, the electrical interconnection of the battery pack should be capable of carrying the maximum current capacity of the battery. Improper design and consideration lead the damage to self as well as a nearby component due to excessive heat loss (Rajasekhar & Gorre, 2016). The primary electrical interconnection in the battery pack is in between cells. The major part of the process is soldering, and the nickel strips are required to connect two cells and carry high current.

Nickel Strip

The main advantage of using the pure Nickel strip as a busbar material is that it has very high corrosion resistance, and it is very easy to spot-weld. Over the past decades, many bike battery manufacturer is using it with the 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 upon the battery's maximum amount of current capacity. The table 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 grow 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/ 5A = 4 strips to carry the present (Figure 4). Same way, if the battery current is 40 A, then eight nickel strips are needed means two pieces for a single connection The best design of the battery pack is to remain in the criteria of the nickel's optimal current capacity. If the dimension of the nickel strip chosen from the poor current position, it starts to heat the spots 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
Same way, multiple nickel strip can be used for long series-parallel connection to design the compact battery pack (Figure 5)

Spot Welder

Recently, due to the high demand of the battery packs, many battery cell manufacturer has applied hand in battery pack industry. For the mass production of the battery pack, automated robot spot-welder is popular (Lewchalermwong et al., 2018). The computerized system makes the spot welding easy and productive. On the other hand, for the limited production and startup, the automated system is not an economical option.

Even and equal welding plays a vital role in the excellent quality battery pack. Uneven welding creates the weak conduction and hotspot of the heat. Traditional welding methods are not sufficient, especially while working with Li-Ion cell. Nowadays, more and more people are looking for a custom-built or rebuilt battery packs. To accomplish the demand, many small spot welders are accessible, which serves an outstanding quality of the welding readily.

Mechanical Structure

The mechanical construction of the battery pack affects the performance and life of the battery pack. Mainly four parameters need to be considered while designing the mechanical development of the battery pack.

I.   Thickness and expansion of the cell

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

II.    Insulation

The battery cell is an electrochemical component so, it becomes necessary to insulate each cell from each other as well as from any other conducting material such as the outer environment, electrode terminals of the battery.

III.    Ventilation to the cell

While charging cycle of the battery, especially in overcharging event, the electrodes of the cell produce gaseous compound and create the pressure on the surface of the cell. To avoid such an event, 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. A well-designed battery cell can give the best and safest battery pack. The cell of the battery pack should have isolated from the vibration, shock, dirt, and water. Physical protection should also consider the thermal management of the pack.

Thermal Management

All batteries are classified into the flammable components as it contains the electro-chemical construction. The primary safety thing issue while working with the battery is that battery can start burning so quickly. Notably, it's proved that the thermal stability of the Li-Ion battery is not so good. Thus, the battery pack developed with the help of the Li-Ion cell needs more care for the thermal management of it.

There are many thermal management approaches for the Li-Ion battery. The thermal management system aims to maintain the temperature of the battery pack within the battery’s specification and create even heat distribution throughout the pack (Rothgang et al., 2012). The control design (active or passive) and medium (air and/or water) used for the cooling of heating defines the thermal management system.

For an electric bike battery system, mainly air is the best suitable medium of the thermal management system. Based on this, Passive cooling, passive heating and cooling, and active heating and cooling methods are being used, which contains a fan for the cooling.

Thermal management is also one of the critical work of the battery management system.

Battery Management System (BMS)

Development in the Battery management system has increased as the demand for customized battery pack grown. The battery management system is an electronic component which helps to control the critical parameters of the battery pack while working, charging, and discharging (Weicker, 2014)(Jossen et al., 1999). BMS system monitor cell voltage, current, impedance, and other mandatory condition of the battery pack with the help of sensors. 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 behavior (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 system is that its programable. The limit of the functions is manageable as per the requirement and application.

There are many BMS system provider 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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