Project 1 Mechanical design of battery pack

So now we have a datasheet for this cell. We have to make a battery pack for this 18000 Wh capacity.

First, we have to know the nominal voltage and nominal capacity.

total battery pack capacity: 18000 Wh

Nominal capacity: 2.5Ah

Nominal voltage: 3.3V

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

Now we know that,

supply voltage = Nominal voltage * cell in series

200V = 3.3V * cell in series

cells in series = 60.60 ~ 60.

And,

So the current cell in parallel will take in the calculation because the current would be similar in series and the voltage would be the same in a parallel connection.

18000Wh = (2.5Ah * cell in parallel ) * 200V

cells in parallel = 36.

So the required battery pack is 60s36p.

So the total cells required is 60*36=2160 cells.

For this, we decided to choose 5 series and 3 parallel module connections.

Each 5S3P configuration Module Voltage capacity = 5*3.3 = 16.5 V. (because 5 cells in series voltage should be added.)

Each 5S3P configuration Current Module capacity = 3*2.5 = 7.5 Ah. (because 3 cells in parallel current should be added.)

So, total capacity of 1 module is = 16.5 * 7.5 = 123.75 Wh.

For 18 KWh of a total 200 V and 90 Ah battery pack capacity, we need 18000/123.75 = 145.45 modules.

For Simplicity of configuration, we took 12*12= 144 modules of 5S3P.

1 cell capacity = 3.3V * 2.5Ah = 8.25 Wh.

So, total cell capacity = 2160*8.25= 17820Wh.(almost 18 KWh)

Weight of one cell = 76 g = 0.076kg.

So, Battery pack cell weight = 0.076*2160 = 164.16 kg.

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

In this way, we achieve 200 V battery voltage, number of 5S3P configuration in series connected modules are = 200/16.5 = 12.12 = 12.

For the 90 Ah battery current, now we connected 12(90/7.5 = 12) times such (series of 12 modules of 200 V) in parallel to get 90 A current from this battery pack.

The 5S3P connection of the ANR26650M1-B cell is shown below.

• This 16.5 V lithium-ion battery pack has a 5S3P configuration with 7.5Ah (amp-hours) capacity.

A typical bill of materials (BOM) includes the following:

• Battery cell
• Nickel or other tabbing
• PCB/PCM/BMS
• PVC shrink wrap
• Insulating paper (barley paper)
• Insulating foam
• 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 individual cell voltages.
• For paralleling the cells, the voltage of each cell should be near to each other, otherwise, a high amount of current will flow from the cell with higher voltage to the cell with lower voltage.
• This can damage the cells and even result in fire on rare occasions.

Step 3: Assemble the ANR26650M1-B Cells.

• From the previous step, it is clear that our battery pack is made up of 3 parallel groups connected in series and each parallel group has 5 cells.
• Now we have to arrange the 15 cells properly for making the electrical connection among them and with the BMS board.
• Place the first parallel group of cells (5 nos) positive side up, then place the second 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 a plastic 18650 battery holder. I used plastic the ANR26650M1-B cell holders/spacers to assemble the 15 cells.
• It provides space between the cells, which allows fresh air to pass and the battery gets cooled easily.
• It also makes our battery pack solid and reliable and provides safety anti-vibration to our battery pack.

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 choice 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 the ANR26650M1-B cells together by means of Nickel strips or thick wire.
• Generally, Nickel strips are widely used for this.
• In general, two types of nickel strips are available in the market: nickel-plated steel strips and pure nickel strips.
• Pure nickel is a little bit costlier than nickel-plated steel, but it has much lower resistance. Low resistance means, less heat generation during the charging and discharging, which leads to longer useful battery life.

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 them from the input power source when fully charged.
2. Balance all the cell's voltage equally.
3. Doesn't allow the pack from over-discharged.

The battery pack has all-around exposed nickel strips, to avoid any accidental shorting and Enclosure used.

https://www.instructables.com/DIY-Professional-18650-Battery-Pack/