Project 1 Mechanical design of battery pack

AIM:

Battery pack capacity: 18 kWh

Cell: ANR26650M1-B

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

Objective:

• In this project, we have to calculate all the necessary parameters for the ANR26650M1-B battery pack design requirements.
• Make a report on all calculated parameters.
• Prepare a detailed battery pack drawing along with its enclosure.

The cell which is given for the design of the battery pack is designed and manufactured by A123 systems. It is a high-performance Nanophasphate Lithium iron phosphate (LiFePO4) battery technology that delivers high power and energy density combined with excellent safety performance and extensive life cycling in a lighter weight, more compact package. The cells manufactured by this company have low-capacity loss and impedance growth over time as well as high usable energy over a wide SOC range, allowing our systems to meet end-of-life power and energy requirements with minimal pack oversizing.

To calculate all the parameters of cell ANR2665031-B, we have to check the datasheet. The following figure shows the basic characteristics of the cell given in the datasheet.

The following are the constant power discharge characteristics of the cell at 23°C temperature. These characteristics are drawn for different power ratings of the cell. If we observe the plot, as the power of the cell increases the voltage and its discharging characteristics are decreasing.

The following figure shows the 2.5C rate Constant current discharge characteristics at different temperatures, if we observe the plot, it can be said that the cell characteristics vary with respect to the temperature changes. So we have to operate the cells within their nominal temperature zone.

The following figure shows the cycle life performance, 100% DOD (Depth Of Discharge), various temperatures, and discharge rates of the cell.

The battery parameters have a significant influence on other components and attributes of the vehicle, like Maximum traction motor torque, Maximum regeneration brake torque, Vehicle range, Vehicle total weight, and Vehicle price. So based on all these parameters we have to design a battery pack. All major aspects of a pure electric vehicle depend on the parameters of the high voltage battery.

A battery pack consists of one or more electrochemical cells which are converting chemical energy into electrical energy during discharging and electrical energy into chemical energy during charging. There are several types of batteries used in hybrid and electric vehicle propulsion systems but we are going to consider only Lithium-ion cells. The main reason is that Li-ion batteries have higher specific energy (Wh/kg) and specific power (W/kg) compared with other types.

Individual battery cells may be grouped in parallel and/or series as modules. Further, battery modules can be connected in parallel and/or series to create a battery pack. Depending on the battery parameters there may be several levels of modularity. The total battery pack voltage is determined by the number of cells connected in series. The current capacity of the battery pack is determined by the number of cells connected in parallel. The following figures give information on series and parallel connections.

Based on the cell parameters provided by the manufacturers, we can calculate the energy content, volume, gravimetric density, and volumetric density for each cell. The following figure shows the dimensions of the ANR2665031-B cell.

Now calculate the electrical and mechanical parameters of the battery pack which is helpful to say the total electrical specification for the battery pack and the calculations which are helpful for the battery pack packing in the module.

By using the data from the datasheet of the ANR2665031-B cell, and making the required assumptions the following parameters are calculated.

From the datasheet, the following parameters are considered for the calculation.

Cell parameters from the datasheet:

Cell dimensions = 26mm×65mm (diameter of cell × height of the cell)

Cell weight = 76g

Nominal cell capacity = 2.5Ah

Nominal voltage = 3.3V

Power = 2600W/kg

Continuous C-Rate = 10

Peak C-Rate = 24

The volume of Cell (For cylindrical) = πr²h

Radius of a cell = Diameter/2 = 26/2 = 13mm = 0.013m

Height of a cell = 65mm = 0.065m

Volume of a Cell = πr²h = 3.14×0.013²×0.065 = 3.45×10^-5 m³

The volume of cell = 3.45×10^-5 m³

Now will calculate the energy of each cell,

Cell energy = Cell capacity × Cell voltage

Cell energy = 2.5 × 3.3 = 8.25

Cell Energy = 8.25Wh

Now we calculate the total energy of a cell by using each cell’s energy,

Battery pack energy density = Cell energy ÷ Volume of a cell

Battery energy = 8.25 ÷ 3.45×10^-5 = 239130.435

Battery pack energy density = 239130.435 Wh/m³

Gravimetric Energy = Cell Energy ÷ Weight of cell

Weight of cell = 76g = 0.076kg

Gravimetric Energy = 8.25÷0.076 = 108.55

Gravimetric Energy = 108.55 Wh/kg

In this challenge, we have to make a battery pack capacity of 18kWh = 18000Wh

Number of cells connected in series = Battery pack voltage ÷ Voltage of each battery cell

Let us assume the battery pack voltage required is 330.

No. of cells connected in series = 330÷3.3 = 100

The number of cells connected in series = 100 cells.

The energy of a string = No. of cells connected in series × Energy of a battery pack

Energy of a string = 100×8.25 = 825

The energy of string in series = 825Wh.

Number of cells connected in Parallel = Battery pack Capacity ÷ Energy of a string in series

No. of cells connected in Parallel = 18000÷825 = 21.81

A number of cells connected in parallel = 22 cells.

Battery pack capacity = Number of cells connected in Parallel × Capacity of a cell

Battery pack capacity = 22×2.5 = 55

Battery pack capacity = 55Ah.

Total number of cells in a battery pack = Number of cells connected in series × Number of cells connected in Parallel

Total number of cells in a battery pack = 100×22 = 2200

Total number of cells in a battery pack = 2200 cells.

Weight of the battery pack = Total number of cells in a battery pack × Weight of a cell

Weight of the battery pack = 2220×0.076 = 167.2

Weight of the battery pack = 167.2 kg.

The volume of the battery pack = Total number of cells in a battery pack × Volume of a cell

Volume of the battery pack = 2200×3.45×10^-5 = 0.0759

The volume of the battery pack = 0.0759 m³.

string peak current = Peak C-Rate of a cell × Cell capacity

String peak current = 24×2.5 = 60

String peak current = 60A.

Battery pack peak current = String peak current × Number of cells connected in Parallel

Battery pack peak current = 60×22 = 1320

Battery pack peak current = 1320A.

Battery pack Peak power = Battery pack peak current × Battery pack voltage

Battery pack Peak power = 1320×330 = 435600W = 435.6kW

Battery pack Peak power = 435.6kW

String continuous current = Continuous C-Rate of a cell × Capacity of a cell

String continuous current = 10×2.5 = 25

String continuous current = 25A.

Battery pack continuous current = String continuous current × Number of cells connected in Parallel

Battery pack continuous current = 25×22 = 550

Battery pack continuous current = 550A

Battery pack continuous power = Battery pack continuous current × Battery pack voltage

Battery pack continuous power = 550×330 = 181500W = 181.5kW

Battery pack continuous power = 181.5kW

Report of Calculated Parameters:

All calculated parameters are listed below in the table.

Parameter Name	Parameter Value
Volume of cell	3.45×10^-5 m³
The volume of the battery pack	0.0759 m3
Weight of the battery pack	167.2 kg
Cell Energy	8.25Wh
Battery pack energy density	239130.435 Wh/m3
Gravimetric Energy	108.55 Wh/kg
Number of cells connected in series	100
Number of cells connected in parallel	22
The energy of a string in series	825Wh
Battery pack capacity	55Ah
Total number of cells in a battery pack	2200
String peak current	60A
Battery pack peak current	1320A
Battery pack peak power	435.6kW
String continuous current	25A
Battery pack continuous current	550A
Battery pack continuous power	181.5kW

Before moving to the enclosure procedure, we have to know the few steps for making a battery pack.

Cell → Battery module → Battery pack.

Cell:

A battery cell is the basic unit of a Li-ion battery that exerts electric energy by charging and discharging. Made by inserting cathode, anode, separator, and electrolyte into a rectangular aluminum case.

Battery Module:

A battery module is a battery assembly put into a frame by combining a fixed number of cells to protect the cells from external shocks, heat, or vibration.

Battery Pack:

The battery pack is the final shape of the battery system installed in an electric vehicle. Composed of modules and various control/protection systems including a BMS (Battery Management System), a cooling system, etc.

Enclosure Procedure:

Battery Holders:

The number of cells is connected in series or parallel according to the battery capacity. After making the required connections we have to assemble the cells in the holders. The cell holder normally consists of a hollow cylindrical brass block with an outside diameter of 50-80mm and an inside diameter of 10-20mm, which matches the outside diameter of the scattering cell. These battery holders are designed to maximize the packaging density of a battery pack. That means we can fit more cells into less space, while still maintaining air gaps between cells. The following figure shows the holding of different battery cells according to their connection type (series and parallel).

Types of battery holders include:

• Plastic - Plastic holders are fabricated from molded plastic.
• Metal - Metal holders similar in form to molded plastic holders, but with flexible metal clips into which the batteries are inserted.
• Battery snaps - Battery snaps are snap terminals, typically for 9V batteries.
• Coin cell - Coin cell battery holders are for round coin cell batteries.
• Component clips - Component chips are flexible clips into which batteries are inserted and held under constant spring tension.
• Battery racks - Battery racks range in size from those that contain a single battery to those that contain multiple batteries. They are usually available in one, two or multiple tiers.
• Battery cabinets - Battery cabinets are basic battery enclosures.
• Single cell - Single cell battery holders hold only one battery.
• Multiple cells - Multiple cell holders hold 2 or more batteries.

Common connection types for the connection of battery holders to the device include:

• Surface mount - Surface mount holders can be mounted on the surface of the device. This can be accomplished through solder tabs or an adhesive base.
• Through-pin PC or socket mount - Through-pin PC or socket mount holders are mounted on a printed circuit board (PCB). PCB mounting is via pins that extend through holes in the board.
• Solder lugs - Battery holders with solder lugs are soldered to the device.
• Snap terminals - A holder with snap terminals can be snapped to the device.
• Wire leads - Batteries can also be connected to the device via wire leads.

Nickle strips:

Using the Nickle strips will connect the cells in parallel or series as per our capacity and voltage requirements. Nickle strip is generally used in battery building. It’s due to its efficiency of usage in spot welding and soldering as well as its high corrosion resistance over time. It has good properties and excellent resistance to many corrosive environments. The following figure shows how the nickel strips are connected to the battery cells. If both ends are connected it is a parallel connection and if only one end is connected it is a series connection, we can observe this in the below image. The second image shows the holders with the connection made by using nickel strips.

Spot Welding:

To connect the Nickle strips will use the spot-welding technique. Spot welding works by applying pressure and heat to the weld area using copper alloy electrodes. These electrodes transport an electrical current through the weld segments. The material melts the parts are fused. At this point, the current is turned off and pressure from the electrodes is upheld. This can be done manually or by using machinery. The following figure shows the spot welding of the battery cells.

Assembly:

In the below figures we can see the detailed process of cell to battery pack assembly and testing process of EV cylindrical batteries.

The most common battery enclosures are made from plastic materials that are resistant to alkaline solutions and have a high impact strength. Metal housings are sometimes used, but metal requires careful design and assembly to avoid shorting of the cells in the battery pack. Aluminum is not recommended for enclosures because if cell leakage does occur, the electrolyte will react with the aluminum.

Final battery pack:

In the following figure, we can see the arrangement of modules of the battery pack which are used in electric vehicles

Conclusion:

For the given cell model of ANR2665031-B, we have calculated all the parameters required to make a battery pack. The parameters which are calculated have been tabulated. And the enclosure procedure is also explained in the respective figures.