Challenge

Project 1 Mechanical design of battery pack

AAshish Pithawe15 Oct 2021

Aim: Design a Battery pack capacity of 18 kWh

Objective: To prepare a ANR26650M1-B cell battery pack of 18kwh for Electric Vehicle.

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

Lithium Iron Phosphate Cell:

A cell refers to a single anode and cathode separated by electrolyte used to produce a voltage and current. A battery can be made up of one or more cells. This is a lithium iron phosphate cell (LiFePO4).

Lithium iron phosphate has the battery technology delivers high power and energy density combined with excellent safety performance and extensive life cycling in a lighter weight, more compact package. This cells have low-capacity loss and impedance growth over time as well as high usable energy over a wide state of charge (SOC) range, allowing our systems to meet end-of-life power and energy requirements with minimal pack oversizing.

Battery:

A battery is a source of electric power consisting of one or more electrochemical cells with external connections for powering electrical devices such as flashlights, mobile phones, and electric cars.

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.

Lithium Iron Phosphate:

Characteristics of Lithium Iron Cell:

Lithium ion phosphate offers good electrochemical performance with low resistance. This is made possible with nano-scale phosphate cathode material. The key benefits are high current rating and long cycle life, besides good thermal stability, enhanced safety and tolerance if abused.

Li-phosphate is more tolerant to full charge conditions and is less stressed than other lithium-ion systems if kept at high voltage for a prolonged time.

As a trade-off, its lower nominal voltage of 3.2V/cell reduces the specific energy below that of cobalt-blended lithium-ion.

With most batteries, cold temperature reduces performance and elevated storage temperature shortens the service life, and Li-phosphate is no exception.

Li-phosphate has a higher self-discharge than other Li-ion batteries, which can cause balancing issues with aging. This can be mitigated by buying high quality cells and/or using sophisticated control electronics, both of which increase the cost of the pack.

Let’s Configure the 18kwh battery pack of lithium iron phosphate cell.

The technical data of the Cell: ANR26650M1-B is given below:

We want to built a 18 kwh battery.

So let’s us assume that the voltage of the battery pack is 300V

We know that P = Volatge*Current, hence we can calculate the Battery pack capacity.

Pack Capacity = Energy Capacity / Pack Voltage

Pack Capacity = (18*10^3)/300 = 60 Ah

No. of the cells in series = Pack Voltage / Cell Voltage = 300/3.3 = 90.90 so nearly 91 cells

No. of the cells in parallel = Pack Capacity / Cell Capacity = 60/2.5 = 24 cells

Total number of the cells in battery pack = (No. of the cells in series) *(No. of the cells in parallel) = 91*24 = 2184 cells

Total Energy of the pack = Pack Voltage * Pack Capacity = 300 * 60 = 18 kwh

So the battery pack having 18 kwh capacity with 91S24P Configuration.

Weight of the battery pack = (Total numbers of the cells in the battery pack) * (cell weight) = 2184 * 76 = 166 kg

Length of the battery pack = (No. of the cells in series) *(Cell Diameter) =

91 * 0.026 = 2.36 m

Width of the battery pack = (No. of the cells in the parallel) *(Cell Diameter) =

24 * 0.026 = 0.624 m

Area of the battery pack = (length of the battery pack) * (breadth of the battery pack) = 2.36 * 0.624 = 1.472 m^2

Volume of the battery pack = (Area of the battery pack) * (Cell Height) =

1.472 * 0.065 = 0.0956m^3 = 95.6 litres

Specific Energy density = Pack Energy Capacity/Weight of the battery pack =

(18 * 10^3)/166 = 108.43 Wh/kg

Volumetric Energy Density = Pack Energy Capacity / Volume of the battery pack = (18*10^3)/95.6 = 188.28 Wh/L

Cost of the battery pack = (Total no. of cells in the battery pack )*(Cost of the each cell) = 2184 * 6.55 $ = 14305.2 $ = 1072890 Rs

Design Overview:

Cell Configuration:

Series Cell Configuration:

In battery positive terminal of the cell is connected to with the negative terminal of the succeeding cell , then we say that the cells are connected in series. Series connections involves connecting 2 or more batteries together to increase the voltage of the battery system keeps the same amp-hour rating.

Parallel cell configuration:

When the positive terminals of the cells are connected together and similarly negative terminal of these cells are connected together in the battery then the cells are said to be connected in parallel. Connecting a battery in parallel is when you connect two or more batteries together to increase the amp-hour capacity with a parallel battery connection the capacity will increase however the battery voltage will remain the same.

Structure:

Electric Design:

This is a electrical design of the battery pack. It prevents against to the short circuit. And it’s delivered the efficient power. It’s prevent from short circuit also.

Thermal Design;

This kind of design improved the pack efficiency. We can avoid thermal accidents.

How to assemble the battery pack?

As shown below they are battery holders. In that circle the battery is fitted. Battery holder will also provide enough spaces between cells to avoid unwanted properties like vibrations.