Project 1 Mechanical design of battery pack

Aim :

We need to design a Battery Pack with ANR26650M1-B cells, which should have the capacity of 18kWh

Requirements :

Cell : ANR26650M1-B

• These ANR26650M1-B cells are High-performance Nanophosphate lithium iron phosphate (LiFePO4) battery technology delivers high power and energy density combined with excellent safety performance and extensive life cycling in a lighter weight, more compact package.
• Our 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.

APPLICATION OF ANR26650M1-B BATTERIES :

TECHNICAL DETAILS OF ANR26650M1-B BATTERIES :

Certain parameters needs to be know before starting our calculation process :

Nominal voltage of cell : 3.3V

Nominal capacity of cell : 2.5Ah

so, inorder to construct a battery pack of 18kWh, we need a cell arrangement of 200V * 90Ah to obtain the 18000wh.

Calculation part :

1. Cell in series :

Inorder to find the requirement of cells that need to be placed in series to get the required Voltage capacity of the battery pack to reach 18kwh.

Total Voltage of the battery pack = Total no of cells in series(Ns) * Nominal voltage of each cell

200v = Ns * 3.3v

Ns = 60 Approx

2. Cells in parallel :

Inorder to find the requirement of cells that need to be placed in parallel to get the required Current capacity of the battery pack to reach 18kwh.

Total current capacity of the battery pack = Total no of cells in parallel (Np) * Nominal current capacity of each cell

90Ah =Np * 2.5v

Np = 36

3. Battery pack capacity confirmation :

Total battery pack capacity = Total voltage of battery pack (V) * Total current capacity of battery pack(Ah)

18000 = 200v * 90v

so, we need to design a 60S36P battery pack

Thus we can split the complete 60S36P battery pack into modules, each modules with a arrangement of 5S3P which is an equal combination of series and parallel arrangement.

Inorder to know the total cell availability :

5S3P --->12 * 12 --->60S36P

Total no of cells = No of cells in 1 module * 144

= 15 * 144

= 2160 cells in our battery pack

Capacity calculation :

Each 5S3P Module Voltage capacity = 5 * 3.3 = 16.5 V

Each 5S3P configuration Current Module capacity = 3 * 2.5 = 7.5 Ah

1 module capacity = (3.3 *2.5) * 15 = 123.75

Capacity of battery pack = 123.75 * 144 = 17820kAh, actually its approximate to 18kWh.

Module connection :

Total capacity of the battery pack is 200V * 90Ah ;

so, here is how you need to arrange the modules

5S3P configuration modules in series are 200/16.5 =12.12

5S3P configuration modules in parallel are 90/7.5 = 12

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

Lets starts with the following steps to design your battery pack ;

1. Choosing of Cell: ANR26650M1-B

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 cells 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.

3. Assemble the cell as mentioned above :

• 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 have 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 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 allow fresh air to pass and the battery get cooled easily.
• It also makes our battery pack solid and reliable and provide safety anti-vibration to our battery pack.

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 (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.
• The pure nickel is little bit costlier than the 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.

5. Adding BMS to your battery pack :

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

• Monitors all of the parallel groups in the battery pack and disconnect it from the input power source when fully charged.
• Balance all the cells voltage equally
• Doesn't allow the pack from over-discharged

After all, use the battery pack enclosure to avoid the exposure of all those nickle strips, to get rid of any external shortening of battery.

eg :