Project 1 Mechanical design of battery pack

AIM : To develop the mechanical design of a battery pack based on the required energy capacity.

OBJECTIVES :

• To design a battery pack whose energy capacity is equal to 18 kWh.
• The battery pack should be developed from lithium-ion cells whose specification is ANR26650M1-B.
• While designing the battery pack relevant assumptions can be made and the same be stated in the report.

THEORY :

Stages of Battery Pack Design

The various design elements to be considered while developing a battery pack are as follows :

1. Electrical Design

• Determination of capacity, voltage and current.
• High voltage isolation.
• Short circuit prevention.
• Efficient power delivery.

2. Thermal Design

• Improve pack efficiency.
• Mitigate thermal accidents.
• Increase pack life.

3. Mechanical Design

• Safe structure for extreme conditions.
• Cost, productivity and reliability.
• Ease of assembly and service.
• Aesthetics, compactness and light weight.

4. Battery Management System (BMS) Design

• Maintain cell / pack operating limits.
• Prevent events compromising safety.
• Control thermal systems.
• Communication and logistics.

Battery Pack Construction

1. Capacity and Voltage Requirements

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

2. 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 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.

3. 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.Space, fixing points and methods and interconnections need to be allocated for all these electronic circuits.

4. Software (BMS)

Sofware is a major component of lithium-ion battery packs, particularly for automotive applications. The software is implemented using the Battery Management System (BMS). 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.

5. Internal Inter-connections

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 bound 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.

6. 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 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 of the battery or to short circuits. Keyed terminals or connections are also advisable to prevent connection to incorrect chargers or loads.

7. 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.

8. Battery Pack Casing or Housing

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 pack 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. These designs are usually required to incorporate a mechanical latch to hold the battery in place. This latch as well as the terminals must interface with plastic parts from a different supplier so high precision and tight tolerances are essential. Acrylonitrile butadiene styrene (ABS) thermoplastics 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.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.

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 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 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 manufacturers. 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 to permit forced air or liquid cooling.

Battery Pack Assembly Line

MECHANICAL DESIGN PROCESS :

Required Energy Capacity for the pack = 18 kWh

Cell Datasheet

Cell used for forming the pack = ANR26650M1B

Cell Parameters used for the Mechanical Design Process

Cell Voltage = 3.3 V

Cell Capacity = 2.5 Ah

Cell Weight = 76 g

Cell Diameter = 26 mm

Cell Height = 65 mm

Cost of each cell = $ 6.55

Mechanical Construction

Let us assume that the voltage of the battery pack (Pack Voltage) = 400 V

We know that P = VI, hence we can calculate the Pack Capacity as follows:

Pack Capacity = Energy Capacity / Pack Voltage

Pack Capacity = (18*10³) / 400 = 45 Ah

No. of cells in series = Pack Voltage / Cell Voltage = 400 / 3.3 = 121.21 ≈ 122 cells

No. of cells in parallel = Pack Capacity / Cell Capacity = 45 / 2.5 = 18 cells

Total no. of cells in the battery pack = (No. of cells in series) * (No. of cells in parallel)

= 122 * 18 = 2196 cells

Weight of battery pack = (Total no. of cells in the battery pack) * (Cell Weight)

= (2196 * 76) / 1000 = 166.89 kg ≈ 167 kg

Length of battery pack = (No. of cells in series) * (Cell Diameter)

= 122 * 0.026 = 3.172 m

Width of battery pack = (No. of cells in parallel) * (Cell Diameter)

= 18 * 0.026 = 0.468 m

Area of battery pack = (Length of battery pack) * (Width of battery pack)

= 3.172 * 0.468 = 1.4845 m²

Volume of battery pack = (Area of battery pack) * (Cell Height)

= 1.4845 * 0.065 = 0.0965 m³ = 96.5 litres

Specific Energy Density = Pack Energy Capacity / Weight of battery pack

= (18 * 10³) / 167 = 107.78 Wh/kg

Volumetric Energy Density = Pack Energy Capacity / Volume of battery pack

= (18 * 10³) / 96.5 = 186.53 Wh/L

Cost of battery pack = (Total no. of cells in the battery pack) * (Cost of each cell)

= 2196 * 6.55 = $ 14,383.8 = Rs 10,45,544.76