Project 1 Mechanical design of battery pack
Aim:
To prepare a mechanical design of battery pack using ANR26650M1-B cell model.
Question:
Battery pack capacity: 18 kWh
Cell: ANR26650M1-B
Prepare a detailed battery pack drawing along with its enclosure. State your assumptions.
Theory:
A battery pack is a set of any number of (preferable) 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. The term battery pack is often used in reference to cordless tools, ratio-controlled hobby toys,and battery electric vehicles.
Components of battery packs include the individual batteries or cells, and the interconnects which provide electrical conductivity between them. Rechargeable battery packs often contain a temperature sensor, which the battery charger uses to detec the end of charging. Interconnects are also found in batteries as they are the part which connects each cell,though batteries are most often only arranged in series strings.
When a pack contains groups of cell in parallel there are differing wiring configurations which take into consideration the electrical balance of the circuit. Battery regulators are sometimes used to keep the voltage of each individual cell below its maximum value during charging so as to allow the weaker batteries to become fully charged bringing the whole pack back into balance. Active balancing can also be performed by battery balancer devices which can shuttle energy from strong cell to weaker ones in real time for better balance. A well-balanced pack lasts longer and delivers better performance.
For an inline package , cell are selected and stacked with solder in between them. The cells are pressed together and a current pulse generates heat to solder them together and to weld all connections internal to the cell.
FIgure:1 battery pack
GOVERNING EQUATIONS:
We will use the following formulae and equations to design the battery pack:
1. Total battery pack voltage = Nominal voltage of a cell * number of cell in series.
2. Total battery pack capacity = Nominal cell capacity * Number of cell in parallel.
3. Total battery pack energy = (Total battery pack voltage) * (total battery pack capacity).
4. Battery pack volume = Cell Volume * Total Number of cells.
5. Energy density of battery pack = Total number of cell * Each cell weight.
6. Total weight of the battery pack = total number of cells * each cell weight.
7. Cell volume = π (pi) * redius of the cell * height of the cell.
8. Cell energy = cell voltage * cell capacity
9. Cell energy density = cell energy/ cell volume.
Objectives:
To design a battery pack which has 18kWh capacity, using ANR26659M1-B cell module.
Solution:
First let us look what are the specifications and battery chemistry of ANR26650M1-B cell:
Datasheet:
Figure.2. datasheet
The above datasheet gives the technical specifications of the cell. Here the manufacturer's name is A123 system.
Link for full pdf of cell data: https://www.batteryspace.com/prod-specs/6610.pdf
Figure.3. Cell output graphs.
These are the graphs related to the cell regarding the performance of the battery. This high-performance Nanophosphate lithium iron phosphate (LiFePO4) Battery technology delivers high power and energy density combined with excellent safety performance and extensive life cycling in a lighter weight, more compact package. Our cells have low capacity loss and impedance growth over a wide state of charge (SOC) range, allowing our systems to meet end-of-life power and energy requirement with minimal pack oversizing.
Cell diamentions:
Cell Chemistry:
The lithium iron phosphate battery (LiFePO4 battery) or LFP battery (lithium ferrophosphate), is a type of lithium-ion battery using lithium iron phosphate (LiFePO4) as the cathode material and a graphitic carbon electrode with a metallic bracking as the anode. The energy density of LifePO4 is lower than that of lithium cobalt oxide (LiCoO2), and also has a lower operating voltage. the main drawback of LiFePO4 is its low electrical conductivity. Therefore all the LifePO4 cathodes under consideration are actually LifePO4/C (Composite made with corbon). Because of low cost ,low toxicity,well-defined performance, long-term stability,etc. LiFePO4 is finding a number of roles in vehicle use, utility scale stationary applications, and backup power. LFP batteries are cobalt-free.
Figure.4. LFP cell chemistry
APPLICATIONS OF ANR26650M1-B Cell:
Commercial Solutions:
1. Advanced lead acid replacement batteries for:
2. Dtacenter UPS
3. Telecom backup
4. IT backup
5. Autonomously guided vehicles (AGVs)
6. Industrial robotics and material handling equipments
7. Medical devices.
Government Solutions:
1. Military vehicles
2. Military power grids
3. Soldier power
4. Directed energy
Grid Solutions:
1. Frequency regulation
2. Renewables intergration
3. Reserve capacity
4. Transmission and distribution
Transportation Solution:
Hybrid, Plug-in hybrid and electric Vehicle battery system for:
1. Commercial vehicles
2. Off-highway vehicles
3. Passenger vehicles.
BATTERY PACK DESIGN:
The purpose of a battery pack is to provide a convenient intergrated power source for portable applications. The advantages of using custom designs are outlined in the section on benefits of custom packs. The pack may fulfil several functions,
1. It enables higher voltages of higher capacity batteries to be built up from low voltage,low capacity cells.
2. It houses a cell or a bank of cells together with the associated interconnections in a single convenient pack.
3. It accommodates any necessary monitoring and electronics protection devices or circuits within the pack.
4. It can accommondate addtional circuitry such as indicator lights, heater, cooling ducts and solar panels.
5. It matches and meshes with the cavity in the product which the battery is intended to power providing both electrical and mechanical interfaces.
6. It can provide unique electrical and mechanical interface to ensure compatibility both of the battery with the intended product and the charger with the battery.
Following factors are considered while we design a battery pack:
a) Capacity and voltage:
With a simple series chain of cells, the battery capacity in amp hours is the same as the capacity of the individual cells since the current flows equally through all the cell 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 cell in the chain. This does not increase the ampHour capacity of the battery,but it increase the wattHour capacity, or the total stored energy ,in proportion to the number of cells in the chain.
Battery capacity can increase 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 cell in a pack should be minimised to simplify the design and to minimise potential reliability problems. Fewer cell require fewer support electronics. Thus, parallel chains should be avoided by specifying the highest capacity cells available.
NOTE: Cells with different capacities or cell chemistries should not be mixed in a single battery pack.
b) Cell Configurations:
The ultimate shape and dimension of the battery pack are mostly governed by the cavity which is planned to house it within the intended application. This it turn dictates the possible cell sizes and layouts which can be used. Prismatic cells provide the best space utilisation, however cylindrical cells provides simpler cooling options for high power batteries. The use of pouch cell 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.
c) Battery Electronics:
Besides the cells many battery packs now incorporate associated electronics circuits. These may be protection devices and circuits, monitoring circuits,change controllers, fuel gauges, and indicators 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 source 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 function 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 electronics circuits.
d) Software:
Software is a major components of lithium batteries, particularly for automative applications. Control system 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.
e) Internal Interconnections:
Low power cells are usually connected together using nickel strips which are welded to the cell terminal 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 stranger, area, low resitance 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 electronics 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 reduced the overall product costs. Not only do they save weight and space but they also provide more packaging option and they simplify physical interconnection and assembly operations as well as eliminating the need for connectors. Connectors may in fact be specified to facilities 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.
f) External Connections:
The types of terminals or connections to the external circuit depends on the current to be carried the frequency with which the battery may be connected and disconnected and the design of the circuit to which the battery will be connected. For low power circuit 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 danderous voltages or of the battery to short circuits. Keyed or terminals or connections are also advisable to prevent connection to incorrect chargers or loads.
g) Thermal Design:
Thermal Management is a major issuse in high power designs, particularly for automotive applications. See details in the thermal management section. 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 environmets. The layout of the cell should be conductive to managing heat flows within the pack.
h) Battery Packaging:
The battery casing has to proving 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 vaccum formed palstic. 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 moulding in which the interconnection strips and the terminals are moulded into the plastics parts to eliminate both materials and assembly costs. Over moulding 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 tolerance are essential. As 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 force 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 cell which are to physical damage.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 incorporate a lot of electronics circuit, it may be desirable to design the pack such that the cells within the pack can be replaced.
If the design required provision for replacement of the cells the casing of the battery pack must be designed to clip or screw together. Normally the parts of the plastic housing will be ultrasonically welded together both for security and for low cost as well as to prevent unauthorised tampering with the cells and the electronics.
i) Other Considerations:
Thermal effects need to be taken into account and,tolerance must allow for potential swelling of the cell. Some lithium pouch cells may swell as much as 10% or more over the lifetime of the cell. For this reason,plotting is not recommended. In low power designs groups of pouch cells may be shrink wrapped but for higher power applications plastic or metal frames may be used both to provide physical protection of the cells as well as to allow for swelling.
The battery pack should nor normally be airtight or sealed since many batteries release hydrogen or oxygen during operation which could cause bursting of the pack or an explosing if the gases are allowed to accumulate. Lithium cells do not emit gases under normal circumstances, but in the case of failure and thermal breakdown , inflammable gases may be vented by the cells. Some form of ventilation or purging should be provided to avoid these problems.
Tolerances should also allow for the use of alternative cells from other manufacturers. While the cells may be "Standard" sizes, there Could still be differences between cells from different vendors.
High power batteries may need special ventilating or channels between the cells to permit forced air or liquid cooling.
j) Identification:
The pack design must incorporate some from to identification to indicate the manufacture's name,the cell chemistry, the voltage and the capacity as well as the country of manufacture. Most manufacturers will also include a date stamp and or serial number to assits traceability in case of problems. This information is usually provided on a label but it may also be printed directly on the battery casing.
Finally, our battery pack design:
From the above data sheet of the cell, we know that,
Cell Radius = 0.013 m
Cell height = 0.065 m
Cell capacity = 2.5/2.4 Ah
weight of cell = 76 g
Nominal Voltage = 3.3 V
Power = 2600 W/kg
Therefore,
Cell Volume = π * cell radius * Cell height
= (3.14*0.013*0.065)
cell volume = 2.653*(10^-3)m^3
cell energy = cell voltage * cell capacity
= (3.3*2.5)
cell energy = 8.25kW
Now for battery pack we have been given that the capacity should be 18kWh,
so,
Total battery pack energy = 18000 Wh
Now let's consider,
Total battery pack voltage = 150 V
therefore,
Total battery pack capacity = (total battery pack energy)/(total battery pack voltage) = 1800/150
Total battery pack capacity = 120Ah
So, we need a pack that has 120Ah battery capacity.
Now the no.of cells needed to produce this amount to energy are as follow:
No.of cells in parallel = ( Toatal battery pack capacity)/ (Nominal cell capacity) = 120/2.5
No.of cells in parallel = 48 cells
No.of cell in series = (total battery pack voltage)/(nominal cell voltage) = 150/3.3 = 45.45
No.of cells in series = 46 cells
So,our battery pack will a combination of 46s 48p cells.
therefore,
Total number of cells = 48*46=2208 cells
Total number of cells = 2208 cells
Therefore ,our battery pack will contain total of 2208 ANR26650M1-b cells which will produce 18000Wh of energy.
Now to calculate dimensions of the battery pack:
length of the battery pack = (No.of cell in series*cell diameter) = 46*0.026 = 1.196 m
length of the battery pack = 1.196m
width of the battery pack = (No.of cell in parallel)*(cell diameter) = 48*0.026 = 1.248m
Width of the battery pack = 1.248m
Height of the battery pack = height of the cell = 0.065m
Height of battery pack = 0.065m
therefore,
Area of battery pack = (Length of battery pack)*(width of the battery pack)= 1.196*1.248
Area of battery pack = 1.5 m^2
Volume of battery pack = (Area of battery pack)*(Height of battery pack) = 1.5*0.065
Volume of battery pack: 0.0975 m^3
Total weight of the battery pack = (total no.of cells)*(each cell weight) = 2208*0.076
Total weight of the battery pack = 167.808kg
So, using the above dimensions we will build a case for the battery pack.
Cost of each cell = $7
Cost of battery pack = (total no.of cells)*(cost of each cells) 2208*7
Cost of battery pack = $15,456
So, our battery pack that produces 18KWh costs $15,456.
Thus , we have designed our battery pack considering above calculations and assumptions.
Detailed battery pack drawing along with its enclosure.
Battery holder, spacer for all cell cylinderical Li-ion.
Nickel Strips
Nickel strip is generally used in battery building. Its due to its efficiency of usage in spot welding and soldering as well its high corrosion resistance over time. It has good properties and excellent resistance to many corrosive environments. The pure nickel will never get rust after it applied to battery packs. It won't affect the current transfer between batteries.
Pure nickel strips are the material you want, but many vendors swap out it for nickel-plated steel strips, which are poorer and have greater resistance - not suitable for most battery projects.
100% pure nickel strip. verify all of our strips to assure that you are getting the high-quality pure nickel. It also solder incredibly quickly, so you can weld them into your existing busbar connection to raise the supply of current you can pull through the busbar. The benefits are nearly limitless.
Battery Enclosure:
Design Benefits:
1. Up to 50 percent lighter than an equivalent steel design.
2. Best -in-class mass energy density at more than 160 watt-hours per kilogram, an extremely efficient design for OEMs and Tier 1 battery pack manufacturers.
3. Accommodates all battery cell types and is designed
4. Power packs such as Pick-up trucks,sports utility vehicles and crossovers.
5. Most cost-effective vehicles when compared to aluminium extrusion and casting-intense designs.
6. Low price premium on $/kg saved compared to equivalent steel designs at substantially lower weight.
Conclusion:
Thus, we have designed a battery pack successfully which has 18kWh capacity and consists of ANR26650M1-B cells.