Project 1 Mechanical design of battery pack
Aim :- Mechanical Design of a battery pack.
Objective :- Make a mechanical design of a battery pack with 18kWh capacity using Cell:ANR26650M1-B.
To prepare a detailed battery pack drawing with its enclosure.
Theory:-
Li-ion Cell :
As the name obviously indicates, the Lithium Ion batteries use the Lithium ions to get the job done. Lithium is a very light metal with high energy density, this property enables the battery to be light in weight and provide high current with a small form factor. Energy density is the amount of energy that can be stored in per unit volume of the battery, the higher the energy density the smaller the battery will be. Despite the overwhelming properties of lithium metal, it cannot be used as an electrode directly in the batteries since lithium is highly unstable because of its metallic nature. Hence we use lithium-ions which more or less has the same property of a lithium metal but it is non-metallic and is comparatively safer to use.
Normally the Anode of a Lithium battery is made of Carbon and the Cathode of the battery is made using Cobalt oxide or some other metal oxide. The electrolyte used connecting these two electrodes will be a simple salt solution that contains lithium ions. When discharging the positively charged lithium ions move towards the cathode and bombard it until it becomes positively charged. Now since the cathode is positively charged it attracts negatively charged electrons towards it. These electrons are made to flow though our circuit thus powering the circuit. Similarly while charging, the exact opposite happens. Electrons from the charges flow into the battery and hence the lithium ions move towards the anode making the cathode to lose its positive charge.
Cell : ANR26650M1-B
A123’s high-performance Nanophosphate lithium iron phosphate (LiFePO4) cell technology delivers high power and energy density combined with excellent safety performance and extensive life cycling in a lighter weight, more compact package. This 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 systems to meet end-of-life power and energy requirements with minimal pack oversizing.
Cell Specification,
Components Of the Battery Pack,
1). Cells(Different form factors and chemistries).
2). BMS(Embedded systems to manage battery operations).
3). Connection System(Connector,pigtail,wires).
4). Housing(Plastic,Sheet metal,Shrink).
The primary function of a BMS is to fulfill safety requirements. But there’s more to it. Objectives related to the more efficient usage of battery cells and prolongation of their lifetime are also being increasingly integrated into the design of BMS.
While there is no unique definition of a BMS, it should be designed with a minimal set of requirements Such as-
- It must measure individual cell voltages
- The BMS must measure temperatures at different points as close as possible to the battery
- It must measure currents flowing through it
- The BMS should communicate information to control units and undertake action to ensure the battery will be operated within safety limits
- The BMS should balance battery cells passively or actively
- And, the BMS should provide thermal management
Connection system,
The connection system is what transforms a cell into a battery pack. There must be a connection between the cell and the BMS in order to interface with each other. Nickel strips are the preferred method of connecting a battery cell to the control system. A thin strip of nickel is capable of carrying high amounts of current, is flexible, durable, and can be attached to the cell without the use of excessive heat. These strips provide a safe means of getting the "fuel" out of the "fuel tank" to use it in a safe manner.
Housing,
Lastly, all these components need to be packaged so that the battery pack can be installed into a device. Once all components are properly placed and connected to one another, they are sealed with either shrink wrap or a hard case. The housing ensures that the components remain safely located and provides a clean package for the eventual use by an end user. The type of housing depends on where the battery pack will be located inside the device and if it is intended to be accessed by the end-user or a technician.
Thermal Management,
The performance of EVs is greatly dependent on the battery pack. Temperatures of the cells in a battery pack need to be maintained within its optimum operating temperature range in order to achieve maximum performance, safety and reliability under various operating conditions. Poor thermal management will affect the charging and discharging power, cycle life, cell balancing, capacity and fast charging capability of the battery pack. Hence, a thermal management system is needed in order to enhance the performance and to extend the life cycle of the battery pack.
Calculations:-
Cell Characteristics (For different Conditions).
Cell Calculation,
Cell Volume = `pi*r^2*h`
= `3.14*(0.013)^2 *0.065`
= `0.000034m^3`
Cell Energy = Nominal voltage * Nominal Cell Capacity
= `3.3 * 2.5`
= `8.25Wh`
Cell Energy Density = Cell energy / Cell volume
= `8.25`/`0.000034`
= `24.3 kWh`/`m^3`
Battery Pack Assumptions :
There will be 46 cells in series and 48 cells in parallel.
So the pack will be 46S48P.
All cells will be of ANR26650M1-B type.
Battery Pack Calculations,
Total no of cells = Cells in series * Cells in parallel
= `46*48`
= `2208`Cells
Voltage = Nominal cell voltage * Cells in series
= `3.3*46`
= `151.8 V`
Capacity = Nominal cell capacity * Cells in parallel
= `2.5 * 48`
= 120
Energy = Voltage * Capacity
= `151.8 * 120`
= `18216 Wh`
= `18.216 kWh`
Volume = Cell volume * Total no of cells
= `0.000034 * 2208`
= `0.075 m^3`
Energy density = Battery pack energy / Volume
= `18.216`/`0.075`
= `242.88 kWh`/`m^3`
Weight = Total no of cells * Cell weight
= `2208 * 76`
= `167.808 Kg`
Battery Pack Design
The purpose of a battery pack is to provide a convenient integrated power source for portable applications.The pack may fulfil several functions:-
It enables higher voltage or higher capacity batteries to be built up from low voltage, low capacity cells.It houses a cell or a bank of cells together with the associated interconnections in a single convenient pack.
It accommodates any necessary monitoring and electronic protection devices or circuits within the pack.
It can accommodate additional circuitry such as indicator lights, heaters, cooling ducts and solar panels.
It matches and meshes with the cavity in the product which the battery is intended to power providing both electrical and mechanical interfaces.
It can provide unique electrical and mechanical interfaces to ensure compatibility both of the battery with the intended product and the charger with the battery.
Capacity and Voltage
With a simple series chain of cells, the battery capacity in AmpHours is the same as the capacity of the individual cells since the current flows equally through all the cells in the chain. 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 AmpHour 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. Fewer cells require fewer support electronics. Thus parallel chains should be avoided by specifying the highest capacity cells available.
Pack Design Options
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 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.
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.
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 moulded 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 minimising the cost, the weight and the size of the pack. The overall product cost can be further reduced by using insert mouldings in which the interconnection strips and the terminals are moulded into the plastic parts to eliminate both materials and assembly costs. Overmoulding may also be used to encapsulate and protect small components or sub-assemblies.
In some designs the battery pack forms part of the outer case of the end product. The colours 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. ABS Polymers are the materials typically used for this purpose. 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 from 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.
Pack Cost
Pack cost contains four factors,
1. Design Engineering Costs.
2. Tooling Costs.
3. Prototypes.
4. Production Costs.
Complete EV Battery Pack
Conclusion,
In this project I able to learn,how battery pack calculations were made for the customized battery pack using specific Li-ion cell.Also how the procedure goes for the construction of the complete battery pack.