Project 1 Mechanical design of battery pack

Battery pack capacity: 18 kWh

Cell: ANR26650M1-B 

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

what is a 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. A single AA battery, for example, is one cell.

what is Battery?

Batteries are a collection of one or more cells whose chemical reactions create a flow of electrons in a circuit. All batteries are made up of three basic components: an anode (the '-' side), a cathode (the '+' side), and some kind of electrolyte (a substance that chemically reacts with the anode and cathode).

when the anode and cathode of a battery are connected to a circuit, a chemical reaction takes place between the anode and the electrolyte. This reaction causes electrons to flow through the circuit and back into the cathode where another chemical reaction takes place. When the material in the cathode or anode is consumed or no longer able to be used in the reaction, the battery is unable to produce electricity.

Battery Pack :

Battery packs are any number of individual batteries arranged in series or parallel to provide the required power to electronic devices. The most common type of battery used in battery packs is lithium-ion due to its high power density. The application of battery packs is wide-ranging from phones to lawnmowers to automobiles.

Here we are using Lithium Iron Phosphate cell some considerations.

Calculations :

According to the objective, we should calculate the capacity of 18kwh.

The cell ANR26650M1-B Datasheet :

1. Energy = 18KWh (required)

2. Number of cells(N) = energy/(nominal voltage*capacity)

N=18000/(3.3*2.4)

N=2273

We need a minimum of 2274 cells to get 18kWh energy, now based on that, we need to decide configuration – series and parallel. In series configuration capacity remains the same and voltage will be the sum of individual cell voltage. In a parallel configuration, the voltage remains the same and capacity accumulates.

Let’s need 330V to run our vehicle.

The nominal voltage of one cell = 3.3V

Number of cells required to connected in series connection to produce 330V(Ns) = voltage required/nominal voltage of one cell

Ns=100

100 cells connected in series will generate 330 voltage and the capacity of the whole module will be 2.4Ah.

Now we need to find the number of cells which will be connected in a parallel configuration

Np(cell in parallel) = total cell / Ns

Np=2273/100

Np=22.73

Several cells can’t be infracted, so we take 24 cells for each parallel row, although it is exceeding the required capacity, that is negligible.

Cell architecture = 100Series and 24Parallel = 100S24P

3. Battery capacity 

We know parallel configured cell multiplies individual cell capacity so,

Capacity = parallel cell row*individual cell capacity

Capacity =2.4*24

Capacity = 57.6Ah

4.Current supplied by the battery

We have already calculated battery capacity which is 57.6Ah means it is going to supply 57.6A current per hour. which can be considered as C-rate.

5. Range

Let’s powertrain energy consumption = 129 Wh/Km at 70% powertrain efficiency

Range = Capacity / energy consumed per kilometre

Range = 18000/129

Range = 139Km

So, our vehicle will be able to cover around 139Km with current parameters.

Design Overview

 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.

1. 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.
2. 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. 
3. 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.
4. 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.
5. 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. 

                                                                   

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. A123 Systems recommends either pure nickel tabs or nickel-clad copper tabs for reliable welded connections at the terminals. 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. 

Battery Dimensions :

                                        

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.

                                            

We Using Cylindrical cell :

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 a lower price 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 using the lego connector Because In this easy to connected cells, when one cell fails, easy to deconstruct.

the same way the best electrical and mechanical connection.

                                        

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 Mechanical battery pack overall view from starting to end :

BATTERY PACK Exploded view: