Project 1 Mechanical design of battery pack

Aim:

To prepare a mechanical design of battery pack using ANR26650M1-B cell model.

Theory:

A battery pack is a set of any number of 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, radio-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 detect the end of charging. Interconnects sre 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 cells in parallel there are differing wiring configuration which taj=ke into consideration the electrical balance of the circuit. Battery regulator 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 into balance. Active balancing can also be performed by battery balancer devices which can shuttle energy from strong cells to weaker ones in real time for better balance. A well -balanced pack lasts longer and delivers better performance.

For an inline package, cells 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.

Governing equations:

We will use the following and equations to design the battery pack:

1. Total battery pack voltage = nominal voltage of a cell* number of cells in series
2. Total battery pack capacity = nominal cell capacity* number of cells in parallel
3. Total battery pack energy = ( total battery pack voltage)*(toatal battery pack capacity)
4. Battery pack volume= cell volume* total number of cells
5. Energy density of battery pack = total number of cells * each cell weight
6. Total weight of the battery pack = total number of cells * each cell weight
7. Cell volume =π* radius of the cell * height if the cell
8. Cell energy = cell voltage * cell capacity
9. Cell energy density = cell energy / cell volume

Objective

To design a battery pack which has 18 kWh capacity, using ANR26650M1-B cell module.

Solution

First let us look what are the specs and battery chemistry of ANR26650M1-B cell:

Datasheet:

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, over time as well as high usable energy over a wide state of charge (SOC) range, allowing our systems to meet end-of-lift power and energy requirements with minimal pack oversizing.

Cell chemistry:

The lithium iron phosphate battery (LiFePO4 battery) or LFP battery (lithium ferrophosphate), is a type of lithum-ion battery using lithium iron phosphate (LiFePO4) as the cathode material, and a graphitic carbon electrode with a metallic backing 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 carbon). 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 battery are cobalt-free.

Applications of ANR26650M1-B cell:

1. Adcanced lead acid replacement batteries for:
2. Datacenter UPS
3. Telecom backup
4. IT backup
5. Autonomously guided vehicles (AGVs)
6. Industrial robotics and material handling equipment
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 integration
3. Reserve capacity
4. Transmission and distribution

Transportation solutions

Hybrid, plug-in hybrid and electric vehicle battery systems 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 integrated power source for portable application. 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 voltage or higher capacity batteries to be built up from low voltage, low capacity cells.
2. It houses a cell or a bank cells together with the associated interconnections in a single convenient pack.
3. It accommodates any necessary monitoring and electronic protection devices or circuits within the pack.
4. It can accommodate additional circuitry such as indicator lights, heaters 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 interfaces 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:

1. Capacity and voltage:

With a simple aeries chain of cells, the battery capacity in Amp Hours is the same as the capacity of the individual 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 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 Amp Hour capacity as well as the watt hour 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 thecell chemistry, cell capacity depends on the surfsce 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.

Note: cells with different capacities or cell chemistries should not be mixed in a single battery pack.

1. 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.

1. 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.

1. Software

Software is a major component of lithium batteries , particularly for automotive applications. Control systems are required to keep the cells within their specified operating rang 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.

1. 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 resistances welders allow much more precise control of the welding process, both limits the amount of heat applied to the battery and localising the heat to a small desired area. Welding also provides a stronger, low resistances jpint. The interconnecting strips 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.

1. External connections

The type of terminals or connections to the external circuits depends on, the current to 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 oxidies. Flying leads with spade terminals or snap on studs are also used for low power applications. Metals 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 or loads.

1. Thermal design

Thermal management is a major issue 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 operation in low temperature environments. The layout of the cells should be conducive to managing heat flows within the pack.

1. Battery packaging

The battery casing has to provide the mechanical and electrical interfaces to the products 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 shrinks wraps or vacuum formed plastic. There solutions are only possible if the battery is intended to be completely enclosed by the finished product.

Injection moulded plastics are used to provided 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 plastic 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 tolerances are essential. ABS polymers are the materials typically used for this purpose.

Batteries for traction application are usually very large and heavy and subject to large physical forces as well as vibrations so substantial fixing 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 casing will normally be specified. the metals pack casing also serves to confine any incendiary event resulting from the failure of a cell or cells within the battery and to 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.

If the design requires 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.

1. Other considerations:

Thermal effects need to be taken into account and, tolerances must allow for potential swelling of the cells. Some lithium pouch cells may swell as much as 10% or more over the lifetime of the cell. For this reason, potting is not recommended. In low power designs groups of pouch cells may be shrink wrapped but for higher power application plastic or metals frames may be used both to provide physical protection of the cells as well as to allow for swelling.

The battery pack should not normally be airtight or sealed since many batteries release hydrogen or oxygen during operation which could cause bursting of the pack or an explosion 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 manufactures. While the cells may be standard sizes, there could still be differences between cells from different vendors.

High power batteries may need special ventilation or channels between the cells tp permit forced air or liquid cooling.

1. Identification:

The pack design must incorporate some form of identification to indicate the manufactures names, the cell chemistry, the voltage and the capacity as well as the country of manufacture. Most manufacturers will also include a data stamp and or serial number to assist traceability in case of problems. This information is usually provided on a label but it may also be printed directly on to 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 Heightπ×Cell radius×Cell Height

= 3.14 *0.013*0.065

Cell Volume =2.653×(10−3)m32.653×(10-3)m3

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) = 18000/150

Total Battery pack Capacity = 120 Ah

So, we need a pack that has 120Ah battery capacity.

Now the no. of cells needed to produce this amount to Energy are as follows:

No. of Cells in Parallel = (Total Battery Pack Capacity)/ (Nominal Cell Capacity) = 120/2.5

No. of Cells in Parallel = 48 cells

No. of Cells 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 Cells in series * Cell Diameter) = 46 * 0.026 = 1.196 m

Length of the Battery Pack = 1.196 m

Width of the Battery Pack = (No. of Cell in parallel) * (Cell Diameter) = 48 * 0.026 = 1.248 m

Width of the Battery Pack = 1.248 m

Height of the Battery pack = Height of the cell = 0.065 m

Height of Battery Pack = 0.065 m

Therefore,

Area of Battery Pack = (Length of Battery Pack) * (Width of the Battery Pack) = 1.196 * 1.248

Area of Battery Pack = 1.5 m2m2

Volume of Battery Pack = (Area of Battery Pack) * (Height of Battery Pack) = 1.5 * 0.065

Volume of Battery Pack: 0.0975 m3m3

Total Weight of the Battery Pack = (Total No. of Cells) * (Each Cell Weight) = 2208 * 0.076

Total Weight of the Battery Pack = 167.808 kg

So, using the above dimensions we will build a case for the battery pack.

Cost of Each cell = $6

Cost of Battery Pack = (Total no. of cells) * (Cost of Each Cell) = 2208 * 6

Cost of Battery Pack = $13,248

So, our battery pack that produces 18kWh costs $13,248.

Thus, we have designed our battery pack considering above calculations and assumptions.

CONCLUSION:

Thus, we have designed a Battery Pack successfully which has 18kWh capacity and consists of ANR26650M1-B cells.