Project 1 Mechanical design of battery pack

TASK :

AIM:

Design a mechanical battery pack.

OBJECTIVE:

Battery pack capacity : 18 Kwh

Cell : ANR26650M1-B

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

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 : ANR26650Ml - 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

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 ANR26650MIB 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 lon 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 .

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 o Max Continuous Charge Current ( C - rata ) = 10 A 
•  Max Pulse Charge Current (C-rate)= 20A

Mathematical Battery Calculation :

Single Cell Calculation :

 πr²h

 π x 0.0132 x 0.065 = 0.03451 m³

r = radious of cell

h = hight of cell

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

Single Cell Energy ( Wh ) :

Cell Capacity ( Ah ) x Cell Voltage ( V )

 2.6 x 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 x 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 x Nominal Voltage of a single cell x Nominal Current of single cell

2135 x 3.3 x 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 x 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 x 0.03451 m³ = 73.678 m³

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

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  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.

Cylindrical cells :

Cylindrical cells are the most common cell type in use today .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 chanter describes how to charge and discharge the battery pack 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.

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

Step 4 :

Install BMS for the battery pack :

Step 5 :

Install cooling structures for air cooling / liquid cooling.

Step 6 :

Install the battery pack inside the enclosure.

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 .