Project 1 Mechanical design of battery pack

AIM:

To build a battery pack of capacity 18 kWh with the cell ANR26650M1-B. Also prepare a detailed battery pack drawing along with its enclosure.

OBJECTIVE:

The objective of the project is to know how a battery pack for an EV is designed and implemented, the challenges faced and tackling the challenges.

THEORY:

Nano phosphate is a registered trademark for Lithium Iron Phosphate battery (LiFePO4) that maximises the performance of the standard battery. The three main areas where Lithium Nano phosphate has advantages over other traditional Lithium Iron Phosphate batteries. They are Power, Safety and Life.

Power - High power products are able to pulse at discharge rates and deliver superior power by weight or volume in a cost-effective solution. With their low impedance and thermally conductive design, high power cells can be continuously discharged at a high C rate, a marked improvement over other rechargeable battery alternatives, and have consistent power over a wide state of charge (SOC) range.

Safety - Nano Phosphate Lithium-ion battery technology offers stable chemistry, faster charging, consistent output, excellent cycle life and superior cost performance. It provides the foundation for safe systems while meeting the most demanding customer requirements. Multiple layers of protection are employed at the chemistry, cell and system level to achieve an energy storage solution with superior safety and abuse tolerance compared to metal oxide lithium-ion chemistries.

Life - Nano phosphate technology delivers exceptional calendar and cycle life. At low rates our cells can deliver thousands of cycles at 100% depth of discharge (DOD), a feat unmatched by most commercial lithium ion cells. Even when cycled at 10C discharge rates, cells deliver in excess of 1,000 full DOD cycles. Not only does the technology retain its energy, it also retains its power capability. Not all lithium-ion technologies behave the same. The nanoscale structure of Nano phosphate chemistry, for example, enables battery systems with higher power, increased abuse tolerance, longer life and the greater ability to maintain consistent power over a wide range of state-of-charge (SOC) as compared with competing lithium-ion and other chemistries.

Choosing the type of cell

• 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. There are three forms of cells - Prismatic, Pouch & Cylindrical cells. 
• 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.
• Since the given cell is a cylindrical type of cell, we will be using the same. 
• The orientation of the cells is designed to minimise the interconnections between the cells which leads to lesser space consumption.

The data sheet of the ANR26650M1-B cell is shown below.

Design of the battery pack

• The design of the outer package or housing of the battery depends to a great extent on the components it has to accommodate and the physical protection it has to provide for them. 
• Besides the cells, many battery packs now incorporate associated electronic circuits. These may be protection devices and circuits, interconnections and connectors, monitoring circuits, charge controllers, fuel gauges, and indicator lights. Electronics for high-power multi-cell packs also include cell balancing and communications functions.
• In addition to the basic battery support electronics, the battery pack may include other functions such as heaters to extend the lower working temperature or solar cells to keep the battery fully charged. These circuits in turn have their own control circuits. Space, fixing points and methods, and interconnections need to be allocated for all these electronic circuits. For high power - high energy batteries, robust packaging is required for safety reasons. 
• Battery Pack energy capacity is 18KWh for this we are calculating the no. of cells required in series and parallel combination, Ampere hour rating (Ah) and Voltage (V). Each cell is of 2.5 Ah nominal capacity & 3.3 V nominal voltage. So to achieve an 18kWh battery pack, we need both series & parallel combinations of cells. With various permutations & combinations, we estimate the 45S50P combination which makes 2250 cells.
•  For 2250 cells, space required will be high and all the cells in a single matrix will lead to poor thermal management. This is because we are employing the air cooled technique in the battery pack. The ventilators will not be able to cool the inner cells as the air will not reach till the inner cells. 
• The solution for both the space required and the thermal management of the battery pack is to create a module of 5S5P. This will create a 16.5 V 12.5 Ah module. Arranging such 5S5P modules in a 9S10P manner will create a 45S50P array giving 148.5 V 125 Ah. The total capacity of the battery pack will be 18.5 kWh. 
• Here, we have designed the total capacity of the battery pack a bit higher than the target because from the time the pack is manufactured to the time it is started to use by the consumer, there is a considerable amount of time the battery pack is kept idle. In this period, the SOH of the battery pack will be reduced and as a result the usable capacity is also reduced.

PROCEDURE:

• First, the cell is designed. A cylinder of radius 13 mm & height 65 mm is created. 
• Then the cell is replicated to form the 5S5P combination. This creates a module.
• Once the cells are created, a casing underneath is also designed.
• Then the module along with the casing is replicated into 8 rows and 4 columns. These are kept in one level.
• In another level the modules are replicated. 
• For the third level, only 6 rows of modules are replicated. 
• This way, the total numbers of cells are created. 

Dimensions & Particulars of the battery pack:

Length - 1207 mm (1.2 m)

Breadth - 603 mm (0.6 m)

Height - 211 mm (0.2 m)

Total no. of cells - 2250

Combination of cells - 45S50P

Total Voltage rating - 148.5 V

Total Ah rating - 125 Ah

Total capacity - 18562 Wh (18.5 kWh)

CONCLUSION:

This way the cells are created in modules and the modules are connected to form the battery pack.