Project 1 Mechanical design of battery pack

Abstract:

This project is about mechanical design of Lithium-ion battery pack consisting of 18KWh. The cell name used in preparation is ANR26650M1-B, necessary assumptions were considered during building the cell then a module and finally a battery pack was developed.

DESCRIPTION:

Cell : ANR26650M1-B 

Considering various  losses, such as Leakage, shelf life, Temperture effects, ageing etc the required capacity is 18KWh

Required Capacity - 18 KWh battery pack.

A standard car running entirely on the battery pack requries voltage range from 400V-800V. 

Therefore let us assume that our battery pack has 400V Voltage. 

hence, the Required Capacity comes out to be 18,000/400 = 45 Ah

Energy = Voltage * Capacity

           = 400 * 45

           = 18000

           = 18 kWh

We have A123 cell of 3.3V/2.5Ah rating

How to decide number of cells in series and in parallel ?

For 400 V pack

Number of cells in series = 400/3.3 = 122 cells connected in series. 

Also, for 45 Ah capacity

Number of cells in parallel = 45/2.5 = 18 cells to be connected in parallel. 

Total Number of cells required = 121*18 = 2196 Cells

Battery pack construction: 

 A battery pack is a system of multiple components and functions and its design involves the application of knowledge and practice in the electrochemical, electrical, mechanical, thermodynamic, and control fields. The following sections summarize the various stages of a battery pack design, covering specific aspects of the A123 cells which may be unique from other cells.

1. The first step in the design of the pack is to determine the configuration of cells, i.e. how many cells overall, how many are in series, and how many are in parallel. This is the foundation of the design process, since all other design decisions follow from the cell configuration.
2. The second step is to design a mechanical structure around the cells to support and protect them. This step requires knowledge of electrical, mechanical and thermodynamic requirements and properties of the cells, application, and the materials used in the pack. 
3. The third step is to design the protection of the cells, particularly electrical protection. The pack must be protected from inadvertent short circuits internal and external to the pack as well as excessive charging and discharging imposed on its terminals.
4. The fourth step is to design a control system that monitors and manages the cells, keeping them from being damaged and maintaining the pack at peak performance.
5. Finally, a pack performs best when it is used properly. The final section of this chapter describes how to charge and discharge the battery pack to make it perform its best. 

Mechanical connection :

It interconnects to prevent accidental short circuits. These mechanical interconnects carry the expected maximum rated current for both the maximum time and ambient temperature in which the pack is expected to operate. 

Cell interconnects :

Cell interconnects were designed for the expected maximum current carrying capability. Improper sized tabs could heat up excessively, resulting in even higher resistivity. For reliable welded connections at the terminals, A123 Systems recommends either pure nickel tabs or nickel-clad copper tabs. In addition, other alloys afford both the weldability, strength and low resistance, which are all desirable features of a good interconnection. Cell interconnects (tabs) should be neither soldered on the end caps nor attached using extreme heat. A123 Systems recommends tabs be resistance or laser-welded to both ends of the cells. Resistance welding provides a constant current for a specified period of time through a salient feature in the weld tab. These parameters can be specified and regulated for a consistent high-quality weld every time. 

The gap between each cell :

The cell vents, located on the end cap(s) of the cells (on the POSITIVE side of the cell), should not be blocked by any mechanical means. Blocking the cell vent and then sufficiently abusing the cell so as to build up pressure in the cell prevents the cell from properly venting. An ESS was designed to install cells end-to-end and needs at least 1 mm of space between the cells to allow the vent to open under fault conditions.

  

Cell Support:

Secure the cells in place by supporting their outside cases, not their terminal ends. The vibration induced between the terminal ends and the rest of the case has been shown to be detrimental to the life of the cell, causing internal and external cell damage. The intercell terminations must be light enough not to cause vibration-induced damage to the cell.

Thermal Management:

A123 Systems’ cells operate throughout a wide temperature range; however, they are most effective between 10°C and 50°C. The temperature differential between the coolest cells and the hottest cells should be no more than 10°C. Careful attention to thermal management is necessary to keep the cells operating at peak efficiency and avoiding fault conditions. In most cases, this implies a cooling system. There are certain applications - such as HEV vehicles operating in cold climates - where a heater isbeneficial to keeping the cells operating in their optimal range.

Electrical:

When joining cells together, A123 recommends using a Battery Management System responsible for carefully monitoring cell voltage, current, and impedance. The BMS may be implemented as discrete circuitry or through a microcontroller.

Battery pack construction:                                             

Cell dimmensions :

                      

Series strings:

Cells combined in series strings can achieve higher operating voltages by connecting the positive terminal of one cell to the negative terminal of the next cell. Connect strings of series cells using their current collection tabs in a manner similar to that illustrated in figure.

                                                       

Parallel strings :

Cells combined in parallel strings can achieve higher operating capacities by connecting like-polarity terminals of adjacent cells to each other. Connect strings of parallel cells using their current collection tabs in a manner similar to that illustrated in figure .

                                            

Nickel Strips:

Conductive metal strips were spot welded between all the cells of a battery pack. Nickel is used due to its low relative resistance and ease of spot welding. 

Battery pack production line: