To Study the Mechanical Design of Battery pack.

AIM : To Study the Mechanical Design of Battery pack.

Battery Pack :

A battery pack is a set of any number of (preferably) identical batteries or individual battery cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density. The term battery pack is often used in reference to cordless tools, radio-controlled hobby toys, and battery electric vehicles.

Components of battery packs include the individual batteries or cells, and the interconnects which provide electrical conductivity between them. Rechargeable battery packs often contain a temperature sensor, which the battery charger uses to detect the end of charging. Interconnects are also found in batteries as they are the part which connects each cell, though batteries are most often only arranged in series strings.

When a pack contains groups of cells in parallel there are differing wiring configurations which take into consideration the electrical balance of the circuit. Battery regulators are sometimes used to keep the voltage of each individual cell below its maximum value during charging so as to allow the weaker batteries to become fully charged, bringing the whole pack back into balance. Active balancing can also be performed by battery balancer devices which can shuttle energy from strong cells to weaker ones in real time for better balance. A well-balanced pack lasts longer and delivers better performance.

OBJECTIVES :

Battery pack capacity : 18 kWh

Cell : ANR26650M1 - B

Prepare a detailed battery pack drawing along with its enclosure. State your assumptions.

STUDY - 1 :

Preparing a detailed battery pack drawing along with its enclosure and Stating my assumptions :

As we know that the electric vehicle market is gradually increasing. With the growth of an electric vehicle, the growth and interest in Lithium-ion cell are also increasing. The energy source for an electric vehicle is stored in cylindrical cells.

Lithium-ion cells are capable of delivering very high power. Lithium-ion cells are used in aerospace, automotive electric vehicle, mobile devices, medical devices and other electronic devices which is based on these cells. The battery pack is installed in lower-floor of the electric vehicle so that weight may distribute equally and also the centre of gravity also maintain properly.

The high voltage battery pack is one of the most important components of any electric vehicle. The traction battery of an electric vehicle must achieve the following thing :

• Provide maximum traction motor torque
• Vehicle range in sense of kilometre
• To achieve the proper life cycle of battery

The battery pack of an electric vehicle is generally having a series or parallel combination. Large battery pack have multiple cell modules, in which many numbers of cells are kept together to build a large battery pack. Basically, a series connection is used to compound the voltage whereas the parallel connection is for compound current capacity. A battery cell consists of five major components that are electrodes – anode and cathode, separators, terminals, electrolyte and enclosure.

Mechanical Design of Battery Pack -

Safety and reliability are two major things for large battery packs. While mechanical designing of large battery pack we should follow some technical criteria so, that the battery may not fail.

• Mechanical vibration should decrease
• Controlling on high impact force
• Better thermal runway for cooling
• Serviceability and recyclability

For the designing of 18 kWh battery pack here, We use a base model of ANR26650M1B Lithium-Ion Cell. ANR26650M1B cell is consist of lithium iron phosphate chemistry (LiFePO4) and it is best for use in a wide range of temperatures and conditions. The ANR26650M1B Lithium Ion cell may achieve the following features :

• Stable chemistry
• Faster charging
• Consistent output
• Excellent cycle life
• Superior cost performance

The provided battery cell type is ANR26650M1-B and its datasheet is used to get all the required information about it. After that, we will figure out how many cells do we require to achieve the said 18 kWh capacity and also the configuration of their connection. In the end, a detailed battery drawing along with its enclosure is given to understand the working of it more deeply.

Technical Specification ANR26650M1B -

Common terms used in Battery -

Voltage : Total voltage, voltages of individual cells, minimum and maximum cell voltage, or voltage of periodic taps. To maximize the battery's capacity and to prevent localized undercharging or overcharging, the BMS may actively ensure that all the cells that compose the battery are kept at the same voltage or the State of charge, through balancing. The maximum voltage at the time of charging and nominal voltage during the operation of the battery is with to mention her.

Temperature : Average temperature, coolant intake temperature, coolant output temperature or temperature of individual cells.

State of Charge(SOC) : Depth of charge(DOD) to indicate the charge level of the battery.

State of Health(SOH) : A variously defined measurement of the remaining capacity of the battery as % of the original capacity.

State of Power(SOP) : The amount of power available for a defined time interval given the current power usage, temperature and other conditions.

Charging-Discharging Current : Current In or Out of the battery respectively.

Calculation as Per Assumptions : 18kWh Battery Pack

The mathematical calculation for designing an 18kWh battery pack, basically 18kWh is energy require to achieve the vehicle its proper range. It is depending upon the voltage (V) parameter and ampere-hour or capacity (Ah).

Here We used to design a high voltage battery pack for an electric vehicle to achieve its range up to 200 km. For this calculation, We assumed 200 V a nominal voltage for series connection.

General required value depending upon a single cell -

• Diameter = Ø25.96 +/- 0.5 mm = 26 mm = 0.026 m
• Length = 65.15 +/- 0.5 mm = 65 mm = 0.065 m
• Mass = 76 g = 0.076 kg
• Single-cell Voltage = 3.3 V
• Capacity = 2.6 Ah
• Max Continuous Charge Current (C – rate) = 10 A
• Max Pulse Charge Current (C-rate) = 20 A

Mathematical Battery Calculation -

Single Cell Calculation -

= πr²h
= π × 0.013² × 0.065
= 0.03451 m³

r = radious of cell

h = hight of cell

Basically, the value of the volume of the cell is used to find out the total volume of the battery pack.

Single Cell Energy (Wh) -

= Cell Capacity (Ah) × Cell Voltage (V)
= 2.6 × 3.3
= 8.58 Wh

The value of a single cell is used to calculate the total energy of the battery pack.

Single Cell Energy Density -

Volumetric energy (Wh/m³) = Single Cell Energy / Volume of each cell

= 8.58 Wh / 0.03451 m³
= 248.62 Wh/m³

Gravimetric energy (Wh/kg) = Single Cell Energy / Single Cell Mass

= 8.58 Wh / 0.076 kg
= 112. 89 Wh/kg

Calculation of High Voltage Battery Pack -

For design and calculation of battery pack here, we assume a vehicle to achieve 200 km in 18 kWh battery pack.

So,

Vehicle Range = 200 KM (assumed)

Battery Pack capacity = 18 kWh or 18000 Wh

Now, We will calculate per km average energy = 18000 / 200

= 90 Wh/km

Let’s assume Nominal voltage for series connection = 200 V, Basically, this depends upon the motor power, We can decide the value according to the motor.

Number of cells connected in series -

= Nominal voltage of series connection / Nominal voltage of a single cell
= 200 V / 3.3 V
= 61

61 Cells will be connected in a series connection to provide a proper voltage.

Maxumum Current of battery pack (Ah) -

= Battery Pack Capacity / Nominal voltage of series connection
= 18000 Wh / 200 v
= 90 Ah

Number of cells connected in parallel -

= Maximum Current of battery pack / Nominal Current of single cell
= 90 Ah / 2.6 Ah
= 35

35 cells will be connected in parallel to provide a proper capacity power.

Total number of cells for battery pack -

= No. of cells connected in parallel × No. of cells connected in series
= 61 × 35
= 2135

2135 Cells will be used to make a complete battery pack.

After the total no of cell calculation, We have to calculate the exact Powe getting from the battery pack -

= Total No. of cells connected in battery pack × Nominal Voltage of a single cell × Nominal Current of single cell
= 2135 × 3.3 × 2.6
= 18.3183 kWh

Safety of margin should be there so to design an 18 kWh battery we have to have at least 18.3183 kWh for uninterrupted operations.

Mass of battery pack -

= Total no. of cells for batter pack × Mass of single cell
= 2135 × 0.076 kg
= 162.26 kg

It is the total weight of cells connected in parallel or series.

Total volume of the battery pack -

= Total no. of cells for batter pack × Volume of single cell
= 2135 × 0.03451 m³
= 73.678 m³

This is the volume of 2135 cells that are used to design the battery pack system.

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The above calculation is to find the total number of cells for a full battery pack for an Electric vehicle, also the battery pack weight, current, capacity, and voltage.

A battery pack is a system of multiple components and functions and its design involves the application of knowledge and practice in the electrochemical, electrical, mechanical, thermodynamic, and control fields. The following sections summarize the various stages of a battery pack design, covering specific aspects of the A123 cells which may be unique from other cells.

Mechanical Connection :

Design mechanical interconnects to prevent accidental short circuits. Size these mechanical interconnects to carry the expected maximum rated current for both the maximum time and ambient temperature in which the pack is expected to operate.

Cell Interconnects :

Size cell interconnects for the expected maximum current carrying capability. Improperly sized tabs could heat up excessively, resulting in even higher resistivity. For reliable welded connections at the terminals, A123 Systems recommends either pure nickel tabs or nickel-clad copper tabs. In addition, other alloys afford both weldability, strength, and low resistance, which are all desirable features of a good interconnection. Cell interconnects (tabs) should be neither soldered on the end caps nor attached using extreme heat. A123 Systems recommends tabs be resistant or laser-welded to both ends of the cells. Resistance welding provides a constant current for a specified period of time through a salient feature in the weld tab. These parameters can be specified and regulated for a consistent high-quality weld every time.

The Gap Between Each Cell :

The cell vents, located on the end cap(s) of the cells (on the POSITIVE side of the cell), should not be blocked by any mechanical means. Blocking the cell vent and then sufficiently abusing the cell to build up pressure in the cell prevents the cell from properly venting. An ESS designed to install cells end-to-end needs at least 1 mm of space between the cells to allow the vent to open under fault conditions.

Cell Design and Assembly of Battery Pack -

Single Cell Dimensions :

A Cell is basically consisting of Anode, Cathode, Separator, Positive Cap, Negative Cap, and Gasket -

The arrangement design of single-cell is shown below -

Cylindrical cells:

Cylindrical cells are the most common cell type in use today. This design allows for better automation processes and techniques that increase consistency and lower cost.

Compared to prismatic cells, cylindrical cells can be produced much faster so more KWh per cell can be produced every day equaling lower $ per kWh. The electrodes in a cylindrical cell are wound tightly and encased in a metal casing, This minimizes electrode material from breaking up from the mechanical vibrations, thermal cycling from charging and discharging, and mechanical expansion of the current conductors inside from thermal cycling. Many cells are combined in series and in parallel to increase the voltage and capacity of the battery pack. If one cell goes bad, the impact on the entire pack is low. With prismatic cells, if one cell goes bad it can compromise the whole battery pack. Cylindrical cells radiate heat and control temperature more easily than prismatic cells.

Here we use the lego connector Because In this easy to connected cells, when one cell fails, easy to deconstruct.

Series Strings :

Cells combined in series strings can achieve higher operating voltages by connecting the positive terminal of one cell to the negative terminal of the next cell. Connect strings of series cells using their current collection tabs like that illustrated in Figure -

Parallel Strings :

Cells combined in parallel strings can achieve higher operating capacities by connecting like-polarity terminals of adjacent cells. Connect strings of parallel cells using their current collection tabs like that illustrated in Figure -

Nickel Strips :

Conductive metal strips are spot welded between all the cells of an electric bicycle battery pack. Nickel is the material of choice due to its low relative resistance and ease of spot welding. Steel is cheaper than nickel, which is why many vendors sell nickel-coated steel strips. It brings the cost down, but at the expense of pack health. Because steel has higher resistance, the same size strips will heat up more, wasting energy from your pack. This not only cuts down on your range but also damages your battery by cooking it slowly over time with extra heat.

• The first step in the design of the pack is to determine the configuration of cells, i.e. how many cells overall, how many are in series, and how many are in parallel. This is the foundation of the design process since all other design decisions follow the cell configuration.
• The second step is to design a mechanical structure around the cells to support and protect them. This step requires knowledge of electrical, mechanical and thermodynamic requirements and properties of the cells, application, and the materials used in the pack.
• The third step is to design the protection of the cells, particularly electrical protection. The pack must be protected from inadvertent short circuits internal and external to the pack as well as excessive charging and discharging imposed on its terminals.
• The fourth step is to design a control system that monitors and manages the cells, keeping them from being damaged and maintaining the pack at peak performance.
• Finally, a pack performs best when it is used properly. The final section of this chapter describes how to charge and discharge the battery pack to make it perform its best.

General Steps to Design a practical Battery Pack :

Step 1 -

Design of cell holder (Bottom and Top) :

The main function of a battery holder is to keep cells fixed in place safely and securely also it is used to keep cells in a proper arrangement so that there will be a decrease in mechanical vibration, and have a better thermal runway in between the gaps of the cell. Most of the cell holder is made up of polypropylene or nylon bodies rated for 80–100 °C.

Step 2 -

Inserting cells inside the holder (Bottom and Top) :

Insert the cells inside these connectors with positive and negative poles kept alternatively in adjacent rows.

Step 3 -

Spot welding :

Spot weld with nickel strips to make a series connection between the cells as shown -

After making series connections, connect the strings in parallel. And complete the connection below -

Step 4 -

Install BMS for the battery pack as shown -

Step 5 -

Install cooling structures for air cooling/ liquid cooling as shown -

Step 6 -

Install the battery pack inside the enclosure as shown -

Enclosure for battery packs :

There are different types of enclosures available for the battery pack. A few are explained below. The main features required for EV battery enclosures are :

• Lightweight
• Shock and Vibration-proof
• Fire protection
• Prevent Electromagnetic radiation
• Low cost
• Aluminium sheet battery enclosure

Aluminium sheet battery enclosure -

Aluminium's inherent properties — corrosion resistance and recyclability — make the metal attractive in electric-vehicle manufacturing. The enclosure regulates thermal conductivity by helping to cool and heat the battery cells.

Using aluminium to enclose a lithium-ion battery pack in an EV can reduce weight by about 50 per cent.

Weight savings translates into a longer driving range, a major challenge for the competitiveness of electric vehicles.

The lightweight aluminium translates into a lower cost per weight at mass-production volumes.

Result :

The detailed technical specification of the 2135 cells Battery Pack is as follow -

• Cells connection in Package - 61S / 35P
• Capacity - 91 Ah
• Voltage - 201.3 V
• Weight - 162.26 kg (Only Cells)
• Volume - 73.678 m³ (Only Cells)
• No. of Cells - 2135
• Power - 18318.3 Wh or 18.3183 kWh

CONCLUSION :

Prepared a detailed battery pack drawing along with its enclosure and Stated my assumptions.

REFERENCES :

https://en.wikipedia.org/wiki/Battery_pack

https://a123batteries.com/anr26650m1-b-lithiumwerks-nanophosphate-3-3v-2-5ah-lithium-iron-phosphate-battery/

https://www.powerstream.com/BPD.htm

https://www.novelis.com/automotive/gen2battery-enclosure-solution/

https://www.gardnerweb.com/articles/aluminum-sheet-for-ev-battery-enclosure