Project 1 Mechanical design of battery pack

PROJECT 1: "MECHANICAL DESIGN OF 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. 

To make the battery pack durable and prone to damages we need to take care mechanical design of the battery. A typical battery pack design procedure is shown below in which blocks in golden yellow are the part of mechanical design of battery pack:

The image below shows the view of battery pack with different parts pointed out. Although the cells here are prismatics the battery pack design wont differ much if cylindrical cells are used instead. Prismatic and pouch cells requires extra protection mechanically as more pressure acts on them. 

Before we design the battery pack we should always remember that first the placement of battery pack is decided in the making of battery pack. We may design the battery pack efficiently but if we are unable to place it at its location like in EVs the whole effort might go in vain. We are assuming here that what we are designing fits and is able to sit comfortably in its location without any compromise in safety of the battery.

Battery pack designing involves following steps:

1. Battery Connection
2. Inner Battery Pack 
3. Environmental Shock Vibration and Acceleration Damping Solution
4. External Case Assembly

Battery Connection

Battery connection is about cell configurations, cells placement pattern, welding and the cell holding.

Cell name: ANR26650M1-B 

Required battery capacity: 18 kWh

Cell specification from cell data sheet:

Let us assume that voltage required by our system is 300 V. We will now configure the cells in series and parallel configuration. We will find the series and parallel as shown below:

• Capacity in Ah = 18000kWh/300V = 60 Ah
• Prallel cells (Decides capacity) = Required capacity/Capacity of one cell = 60/2.5 = 24 cells
• Series cell (Decides voltage) = Required voltage/voltage of a cell = 300/3.3 = 91 cells
• Total cells required = 91 * 24 = 2184 cells.

Cells will be connected in nPmS configuration. That is we first make the module of n parallel cells and then we connect this modules m times. Here n is 24 and and m is 91. We have chosen this configuration because of its simplicity and the cells in this configuartion can perform auto balance to some extent.

There are numerous way to arrange cell in battery pack here we have chosen cubic pattern. Cubic packing is in neat rows. The size of such a pack is nD x mD x H, where n is the number of cells in a row, m is the number of rows, D is the cell diameter, and H is the cell height.

According to this pattern size of our battery pack is (24*25.96)*(91*25.96)*(65.15) = 95891 cubic.meter

Above images how cells look after the congiguration is done. 

–The connection between the cells is a web of a Ni tabs. Laser welding process is used to create the connection between two batteries. The orientation of the polarities of the batteries is established to gain the 24 S 91 P setup in the smallest footprint possible.

–Laser welding creates a fusion connection between the tabs. In a Fusion Bond, either similar or dissimilar materials with similar grain structures are heated to the melting point (liquid state). The subsequent cooling and combination of the materials forms a “nugget” alloy of the two materials with larger grain growth. The bonded materials usually exhibit excellent tensile, peel and shear strengths.

–The design of the webbing needs to be a function of the total resistance and the current traveling through the material.

Inner Battery Pack

If battery pack were made of prismatic or pouch cells they might have needed some extra mechanical design which needs to be taken care of. Below is the image which shows force acting on battery pack which is the result of pressure exerted by the battery weight and expansion and contraction of cells:

But as we are dealing with cylindrical cells here we only need the cell holders.

–The battery pack matrix welded with the tabs to the individual cells is placed into a inner casing.

–The inner casing is made out of a polymer material to create a protective insulated shell around the battery pack. This inner case is flexible and resistant to the environmental and electrical requirements.

–The battery 24P 91S  matrix is fitted inside the inner casing, the cells have to be protected during the assembly from any mechanical and electrical damage.

–The casing is a combined top and bottom shell constrained by spacers placed between the cells. These spacers are pulling together the top and bottom inner casing through counter sink head screws to save space.

–The battery pack with the inner casing forms a solid entity floating inside the outer casing.

–The cells are electrically monitored at all times during the operation of the battery pack; this was made possible by attaching the cell wires which are inserted into the connector pin sockets.

The ultimate shape and dimensions of the battery pack are mostly governed by the cavity which is planned to house it within the intended application. This in turn dictates the possible cell sizes and layouts which can be used. Prismatic cells provide the best space utilisation, however cylindrical cells provide simpler cooling options for high power batteries. The use of pouch cells provides the product designer more freedom in specifying the shape of the battery cavity permitting very compact designs. The orientation of the cells is designed to minimise the interconnections between the cells.

Environmental Shock Vibration and Acceleration Damping Solution

–The inner battery pack is suspended onto “L” shaped mounts, shock and vibration dampening absorbent material

–The shape, size, relative stiffness and hardness of these damping inserts are established based on engineering calculations which are functions of the environmental loads. The key factor in the design is the natural frequency of the assembly.

–There are 8 mounts installed on the edges of the inner pack. The mounts are compressed for optimal vibration damping. The design accommodates this compression factor.

–Below shows a few examples of the random vibration, acceleration and shock requirements.

Acceleration

• Steady state acceleration of 24.0 G for min. 360 sec.
• For the X Axis only (flight direction), perpendicular to the cover, the cover is facing upwards.

External Case Assembly

Design process of the case:

1. The case is designed and tested with the Finite Element Analysis Software for optimization.

2. These figures are showing the case put through the vibration environmental testing and the process flow of design optimization.

3. The case from the original design (due to failure) during the simulation is redesigned, by eliminating the stress risers and weak points on the questionable surface’s areas.

4. The design reaches a “pass” stage after which the optimization is continued to reach the safety factors needed.

5. The design is finalized and ready for detailing.

6. Shock and acceleration simulations are performed as well to pass all the needed environmental requirements

7. After several iterations, a final design is chosen to be built and tested in real shock, vibration and acceleration conditions.

External case assembly:

–The external case assembly is formed from the case, the ring-gasket and the cover.

–The case is a one piece machined aluminium box fitted with a flange on the top side.

–There are two mounting brackets located on the back of the case.

–The ring gasket houses the rubber “O” ring and is sandwiched between the cover and the flange of the case.

–The cover is constrained onto the case with screws and washers. Once the screws are tightened the “O” ring inside the ring-gasket is squeezed between the case and the cover creating a tight seal. The cover is reinforced with aluminium extrusions for strength purposes, similar design is applied onto the bottom of the case.

– On the side of the case is a connector attached which establishes the communication between the battery pack and the exterior. Each battery is monitored for its performance through that connector.

Mechanical design of battery packs leads to safe, and cost-effective structural elements that are lightweight and highly reliable. In this Project we got to know that how battery pack is mechanically designed. Some of the important considerations while designing the battery mechanically are: Material selection, Base plate design for individual cell accommodation, Cell movement constrain and control, Uniform pressure over cell surface, Material cost optimization, Outer case design for overall protection, Bus-bar designing, Packaging constraints. Apart from the design we also need to test it for surety and finalization of design. Here are the General battery pack test standards: Harmonic vibration test - AIS 048: 2009, Shock abuse test - AIS 048: 2009, Random vibration test - SAE J 2380