Project 1 Mechanical design of battery pack

To make the battery pack we have to first finalize the nominal voltage and capacity of the pack. Either it will be in terms of volt, mAh/Ah or Wh.

The individual cell : ANR26650M1-B has

Nominal voltage = 3.3 V

Current capacity = 2.5 Ah

So, let us assume that for 18 kwh battery pack we need supply voltage of 200 V and 90Ah.

*Number of cells in series-

cells in series = Total voltage / single cell voltage

cell in series = 200/3.3 = 60

Now, as we know that battery pack capacity(wh) = Total current * Total voltage

18000 =(2.5*cell in parallel)*200

cell in parallel = 18000/(2.5*200)= 36

Module capacity:

The required battery pack combination is 60S36P

For this we decided to choose 5 series and 3 parallel module connection

Each 5S3P configuration Module voltage capacity = 5*3.3 = 16.5 v

Each 5S3P configuration current module capacity = 3*2.5 = 7.5 Ah

So, total capacity of 1 modul is = 16.5*7.5 = 123.75 wh

*Total cells-

Total number of cells = cell in one module * 144

where 18000/124 = 144

Total number of cells = (5*3)*144

Total number of cells = 2160

one cell capacity = 3.3*2.5 = 8.25 Wh

so, total cell capacity=2160*8.25

Total cells capacity =17820 Wh = 17.82 Kwh (almost 18 kwh)

Note : We should always target more kwh then required considering the losses and connection limitations.

Weight of one cell= 76 g = 0.076 kg

so battery pack cel weight = 0.076*2160 = 164.16 kg

Module connection:

For 18 kwh of total 200 V and 90 Ah battery pack capacity we need 18000/123.75 = 145 modules

For simplicity of configuration we took 12*12 = 144 modules of 5s3p

We have connecting the cells in parallel to reach the desired capacity (mAh) and connect such parallel group in series to achieve the nominal voltage (volts).

In this way to achieve 200v battery voltage number of 5s3p configuration in series connected modules are = 200/16.5 = 12.

The 5s3p connection of ANR26650MI- B cell is shown below.

A typical bill of material (BOM) includes the following

*Battery cell

*Nickel or other tabbing

*PCB/PCM/BMS

*PVC shrink wrap

*Insulating paper (battery paper)

*connectors and wires

Step 1: Select the ANR26650M1-B Cells for the battery pack.

Step 2: check the cell voltage

*Before connecting the cells in parallel first check the indiividual cell voltages

*For paralleling the cells t,he voltage of each cell should be near to each other, otherwise a high amount of current will flow from the cell with higher volatage to the cell with lower voltage.

* This can damage the cells and even results in fire on rare occasions.

Step 3: Assemble the ANR26650MI-B Cells.

*From the previous step, it is clear that our batterypack is made up of 3 parallel groups connected in series and each parallel group have 5 cells

*Now we have to arrange the 15 cells properly for making the electrical connection among them and with BMS board.

*Place the first parallel group of cells(5 nos) positive side up, then place the second parallel parallel group negative side up and then finally the last parallel group positive side up.

*We can assemble the cells to make the pack by using hot glue or by using plastic 18650 battery holder. we used plastic the ANR26650M1-B cell holders/ spacers to assemble the 15 cells.

*It provides space between the cells, which allow fresh air to pass and the battery get cooled easily.

Step 4 : choosing the right battery strips and spot weld the Nickel strips.

We have two options two connect the ANR26650M1-B cells together.

1.Soldering

2.Spot welding

The best choise is always spot welding but spot welder is much costlier than a good quality soldering iron.

To make the battery pack we have to connect battery cells together by means of nickel strips or thick wire.

Generally, Nickel strips are widely used for this.

Step 5: Adding the BMS-

A battery management system (BMS) is any electronic system that manages a lithium battery pack and the main functionalities are:

1. Monitors all of the parallel groups in the battery pack and disconnect it from the input power source when fully charged.

2. Balance all the cells voltage equally.

3. Doesnt allow the pack from over-discharged.