Project 1 Mechanical design of battery pack
Aim:
To prepare a detailed Mechanical Design of the Battery pack for the required capacity of 18KWh.
Explanation:
The various stages are involved in the design of the battery pack.
The various design elements to be considered while developing a battery pack are as follows
Electrical Design:
- Determination of capacity, voltage, and current
- High voltage insulation
- Short circuit prevention
- Efficient power density
Thermal Design:
- Improve pack efficiency
- Mitigate thermal accidents
- Increase Pack life
Mechanical Design:
- Safe Structure for extreme condition
- Cost , productivity and reliability
- Ease of assembly and service
- Aesthetics compactness and light weight.
Battery Management System:
- Maintain cell/pack operating limits
- prevent events that don't follow safety limits
- Control thermal system
- Communication and logistics
Battery Pack Construction:
The Batterypack constructed via Cell -> module -> Pack.
As a single unit, a cell performs the primary function of a rechargeable battery. The cell can be in various formats such as Cylindrical cells, Pouch cells, Prismatic cells.
A module is formed by connecting multiple cells providing them with a mechanical support structure and thermal interface and attaching terminals. Modules are designed according to cell format and voltage, current requirements
A pack is formed by connecting the multiple modules with the sensors and a controller and then housing the unit in a case.
Capacity and Voltages:
High battery voltages are achieved by adding more cells in a series chain. The voltage of the battery is the voltage of a single cell multiplied by the number of cells in the chain. This does not increase the amp-hour capacity of the battery, but it increases the WattHour capacity, or the total stored energy, in proportion to the number of cells in the chain.
Battery capacity can increased through adding more parallel cells. This increases the AmpHour capacity as well as the WattHour capacity without increasing the battery voltage. For batteries with parallel chains the capacity of the battery is the capacity of the individual chain multiplied by the number of parallel chains.
Whereas cell voltage is fixed by the cell chemistry, cell capacity depends on the surface area of the electrodes and the volume of the electrolyte, - that is, the physical size of the cell. If at all possible the number of cells in a pack should be minimised to simplify the design and to minimise potential reliability problems.
Pack Design Option:
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. These components are not just the cells, but also protection devices, electronic circuits, interconnections and connectors which must all be specified before the final battery case can be designed. For high power, high energy batteries robust packaging is required for safety reasons.
Cell Configuration:
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 utilization, 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 minimize the interconnections between the cells.
Battery Electronics:
Besides the cells, many battery packs now incorporate associated electronic circuits. These may be protection devices and circuits, monitoring circuits, charge controllers, fuel gauges, and indicator lights. Electronics for high-power multi-cell packs also include cell balancing and communications functions. The packs may also be designed to deliver more than one voltage from the basic cell combination, although applications requiring multiple voltage sources are more likely to make provision for this within the application. 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.
Software:
Sofware is a major component of Lithium batteries, particularly for automotive applications. . Control systems are required to keep the cells within their specified operating range and to protect them from abuse. Fuel gauging needs complex algorithms to estimate the state of charge (SOC). Communications with other vehicle systems are needed for monitoring the battery status and controlling energy flows.
Internal Interconnections:
Low power cells are usually connected together using nickel strips which are welded to the cell terminals or the case. Soldering is not recommended since the soldering process is apt to apply large, uncontrolled amounts of heat to the battery components which may damage the separators or the vents which are normally made of plastic. Modern computer controlled resistance welders allow much more precise control of the welding process, both limiting the amount of heat applied to the battery and localising the heat to a small desired area. Welding also provides a stronger, low resistance joint. The interconnecting strips often have complex shapes and profiles which may be stamped out of flat strip in a progressive die. High power cells may use solid copper bus bars or braided straps. The electronic components are usually mounted on a conventional printed circuit board (PCB). Flexible PCBs may cost more than rigid PCBs but they can be used to reduce the overall product costs. Not only do they save weight and space but they also provide more packaging options and they simplify physical interconnections and assembly operations as well as eliminating the need for connectors. Connectors may in fact be specified to facilitate assembly and disassembly if the design requires that individual battery components need to be changed or serviced but there is usually a cost and reliability penalty associated with such designs.
External Connections:
The type of terminals or connections to the external circuits depend on, the current to be carried, the frequency with which the battery may be connected and disconnected and the design of the design of the circuit to which the battery will be connected.For low power circuits, gold plated contacts are the terminals of choice for connectors which are subject to frequent insertions. Gold is hard wearing, it has low contact resistance and doesn't oxidise. Flying leads with spade terminals or snap on studs are also used for low power applications. Metal tabs are also used on pouch cells.Terminals for high power applications are usually threaded metal studs to ensure a reliable connection. Safety requirements on high voltage batteries may also dictate shrouded terminals to prevent accidental exposure of the operator to dangerous voltages or of the battery to short circuits. Keyed or terminals or connections are also advisable to prevent connection to incorrect chargers or loads.
Thermal Design
Thermal management is a major issue in high-power designs, particularly for automotive applications. As part of the battery system, it may be necessary to provide air or water cooling ducts, pumps or fans and heat exchangers for high temperature working or heaters for operating in low-temperature environments. The layout of the cells should be conducive to managing heat flows within the pack.
Battery Packaging
The battery casing has to provide the mechanical and electrical interfaces to the product it is designed to power as well as to contain all the components outlined above.The simplest and least expensive packaging for small batteries is shrink wrap or vacuum-formed plastic. These solutions are only possible if the battery is intended to be completely enclosed by the finished product. Injection-molded plastics are used to provide more precision packs. For enclosed packs designs using a minimum of materials are based around which a plastic frame holds the components in place thus minimizing the cost, the weight, and the size of the pack. The overall product cost can be further reduced by using insert moldings in which the interconnection strips and the terminals are molded into the plastic parts to eliminate both materials and assembly costs. overmoudling may also be used to encapsulate and protect small components or sub-assemblies.
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Case for 3 AAA Cells |
Case for a Single Prismatic Cell |
Insert Mouldings Showing Cell Interconnecting Strips
In some designs, the battery pack forms part of the outer case of the end product. The colors and textures of the plastic must match the plastics of the rest of the product even though they may come from a completely different supplier. These designs are usually required to incorporate a mechanical latch to hold the battery in place. Again this latch as well as the terminals must interface with plastic parts from a different supplier so high precision and tight tolerances are essential. Batteries for traction applications are usually very large and heavy and subject to large physical forces as well as vibrations so substantial fixings are required to hold the cells in place. This is particularly necessary for batteries made up of pouch cells which are vulnerable to physical damage. Automotive battery packs must also withstand abuse and possible accidental damage so metal casings will normally be specified. The metal pack casing also serves to confine any incendiary event resulting from the failure of a cell or cells within the battery and to provide a measure of protection for the user. At the same time, the case must also protect the cells and the electronics from the harsh operating environments of temperature extremes, water ingress, humidity and vibration in which these batteries work.Usually, the complete pack is replaced when the battery has reached the end of its useful life. In certain circumstances, however, for instance, when the pack incorporates a lot of electronic circuits, it may be desirable to design the pack such that the cells within the pack can be replaced.
Mechanical Design of the Battery Pack:
The Cell is used for the construction of the battery pack is ANR26650M1-B.The required capacity of the battery pack is 18 KWh
Cell Parameters used for the Mechanical Design Process:
Cell voltage = 3.3V
Cell Capacity = 2.5 Ah
Cell weight = 76g
Cell diameter = 26mm
Cell height = 65mm
Calculation:
In case assume the voltage of the battery pack is 400 V.
Pack Capacity = Energy capacity / Pack voltage.
Pack capacity = `(18*10^3) / 400 = 45 Ah.`
No of cells in series = Pack voltage / cell voltage.
No of cells in series = `400/3.3 = 121.212 ~= 121 cells`
No of cells in parallel = pack capacity / cell capacity
No of cells in parallel = `45/2.5 = 18 cells`
Total No of cells in the Battery pack = No of cells in series * no of cells in parallel
Total No of cells in the Battery pack = `121 *18 =2178 cells`
Weight of battery pack = Total no of cells * cell weight
Weight of battery pack =`(2196*76)/1000 = 166.896 Kg ~= 167 Kg`
Length of the Battery pack = No of cells in series * cell diameter
Length of the Battery pack = 122*0.026 =3.172 m
Width of the battery pack =No of cells in parallel * cell diameter
Width of the battery pack = `18* 0.026= .468 m`
Area of the battery pack = Length of the battery pack * Width of the battery pack
Area of the battery pack = 3.172*0.468 = 1.4845 m^2
The volume of the battery pack = Area of the battery pack * Cell Height
Specific Energy Density = Pack Energy capacity / weight of the battery pack
Specific Energy Density =18*10^3 / 167 = 107.78Wh/Kg
Volumetric Energy Density = Pack Energy Capacity / Volumr of the Battery pack
Volumetric Energy Density= 10*10^3/96.5 =186.53Wh/L
