Project 1 Mechanical design of battery pack

AIM:

Prepare a detailed battery pack drawing along with its enclosure. State your assumptions
Hint - Battery pack capacity: 18 kWh, Cell: ANR26650M1-B

INTRODUCTION:

As electric vehicles are increasing in the market is gradually increasing, there is considerable growth in the production of lithium-ion batteries. As it is the electrical source for the EV.

A lithium-ion battery or Li-ion battery is a type of rechargeable battery in which lithium ions move from the negative electrodes through an electrolyte to the positive electrode during discharge, and back when charging. Li-ion batteries use an intercalated lithium compound as the material at the positive electrode and typically graphite at the negative electrode.

Lithium-ion batteries are made use of various different electrodes. The most common material use is lithium cobalt oxide and graphite which are most commonly found in portable electronic devices such as cellphones and laptops. Lithium manganese oxide and lithium iron phosphate are also used as cathode materials.

Battery Pack :

A battery pack is a set of any number of (preferably) 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 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 cells 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 cells to weaker ones in real time for better balance. A well-balanced pack lasts longer and delivers better performance.

Features: The ANR26650M1-B is the next generation A123 system's pioneering 26650 cylindrical cell, now with greater power & energy density & lower impedence. This versatile lithium ion cell is suitable for wide variety of applications & system designs. Proven performance in the toughest conditions, combining durability, reliability, & safety, the ANR26650M1-B cell offers an excellent combination of price-performance.

How does a lithium-ion battery works?

• Most Li-ion batteries share a similar design consisting of a metal oxide positive electrode (cathode) coated onto an aluminum current collector, a negative electrode (anode) made from carbon/graphite coated on a copper current collector, a separator and electrolyte made of lithium salt in an organic solvent
• While the battery is discharging and providing an electric current, the electrolyte carries positively charged Lithium-ions from the anode to the cathode and vice versa through the separator
• The movement of the Lithium-ions creates free electrons in the anode which creates a charge at the positive current collector
• The electrical current then flows from the current collector through a device being powered (cell phone, computer, etc.) to the negative current collector. The separator blocks the flow of electrons inside the battery
• During charging , an external electrical power source (the charging circuit) applies an over-voltage (a higher voltage than the battery produces, of the same polarity), forcing a charging current to flow within the battery from the positive to the negative electrode, for example in the reverse direction of a discharge current under normal conditions
• The Lithium-ions then migrate from the positive to the negative electrode, where they become embedded in the porous electrode material in a process known as intercalation

Primary and secondary cells

• Primary batteries, or cells, are not rechargeable, and must be discarded once their charge is exhausted. By contrast, secondary types can be recharged using an external electric charger
• Today, most attention is given to secondary types, particularly Li-ion batteries, because of their widespread application in cell phones and electric vehicles
• However, primaries still play an important role, especially when charging is impractical or impossible, such as in military combat, rescue missions and forest-fire services
• Regulated under IEC 60086, primary batteries also service pacemakers in heart patients, tire pressure gauges in vehicles, smart meters, intelligent drill bits in mining, animal-tracking, remote light beacons, as well as wristwatches, remote controls, electric keys and children's toys

Lithium-ion battery applications

Lithium-ion batteries are popular because of their high energy density and other properties – and as the technology improves and prices reduce, they are proliferating in many applications. Here are some examples for Li-ion battery applications:

• Portable power packs: Li-ion batteries are lightweight and more compact than other battery types, which makes them convenient to carry around within cell phones, laptops and other portable personal electronic devices
• Uninterruptible Power Supplies (UPSs): Li-ion batteries provide emergency back-up power during power loss or fluctuation events. Office equipment like computers, as well as IT servers and complete data centers, must be protected from power interruptions to prevent data loss. Back-up power is also needed in the medical and health care industries to guarantee consistent power supply to life-saving medical equipment
• Electric vehicles: The automotive industry is creating a demand for Li-ion battery packs to provide power sources for electric, hybrid or plug-in hybrid electric vehicles. As Li-ion batteries can store large amounts of energy and can be recharged many times, they offer good charging capacity and long lifespans
• Marine vehicles: Li-ion batteries are emerging as an alternative to gasoline and lead-acid batteries in powering work or tug boats and leisure craft like speed boats and yachts. Li-ion batteries provide quiet and efficient power sources and can also be used to provide electricity to appliances inside the boat while it is in dock
• Personal mobility: Lithium-ion batteries are used in wheelchairs, bikes, scooters and other mobility aids for individuals with disability or mobility restrictions. Unlike cadmium and lead batteries, lithium-ion batteries contain no chemicals that may further harm a person's health
• Renewable energy storage: Li-ion batteries are also used for storing energy from solar panels and wind turbines as they can be charged quickly. They are lighter, more compact and can hold higher amounts of energy than lead-acid batteries. Within battery energy storage systems, they fulfil a valuable role in balancing supply and demand, and in grid stabilization

Types of lithium-ion batteries

There are many types of lithium-ion batteries, which are

1. Lithium Cobalt Oxide (LCO) - LiCoO2">LiCoO2LiCoO2
2. Lithium Manganese Oxide (LMO) - LiMn2O4">LiMn2O4LiMn2O4
3. Lithium Nickel Manganese Cobalt Oxide (LNMC) - LiNiMnCoO2">LiNiMnCoO2LiNiMnCoO2
4. Lithium Nickel Cobalt Aluminum Oxide (LNCA) - LiNiCoAlO2">LiNiCoAlO2LiNiCoAlO2
5. Lithium Iron Phosphate (LFP) - LiFePO4">LiFePO4LiFePO4
6. Lithium Titanate Oxide (LTO) - LiTiO3">LiTiO3LiTiO3

From above mentioned battery types, three types of lithium batteries are particularly suitable for an EV battery, namely LiMn2O4">LiMn2O4LiMn2O4, LiFePO4">LiFePO4LiFePO4 and LiNiMnCoO2">LiNiMnCoO2LiNiMnCoO2

Here we going to discuss about the required battery of 'Lithium Iron Phosphate (LFP)'

LITHIUM IRON PHOSPHATE (LFP)

• In 1996, the University of Texas (and other contributors) discovered phosphate as cathode material for rechargeable lithium batteries
• Li-phosphate offers good electrochemical performance with low resistance. This is made possible with nano-scale phosphate cathode material
• The key benefits are high current rating and long cycle life, besides good thermal stability, enhanced safety and tolerance if abused
• Li-phosphate is more tolerant to full charge conditions and is less stressed than other lithium-ion systems if kept at high voltage for a prolonged time
• As a trade-off, its lower nominal voltage of 3.2V/cell reduces the specific energy below that of cobalt-based lithium-ion
• With most batteries, cold temperature reduces performance and elevated storage temperature shortens the service life, and Li-phosphate is no exception
• Li-phosphate has a higher self-discharge than other Li-ion batteries, which can cause balancing issues with aging
• Cleanliness in manufacturing is of importance for longevity. There is no tolerance for moisture, lest the battery will only deliver 50 cycles. Figure 9 summarizes the attributes of Li-phosphate
• Li-phosphate is often used to replace the lead acid starter battery. Four cells in series produce 12.80V, a similar voltage to six 2V lead acid cells in series
• Vehicles charge lead acid to 14.40V (2.40V/cell) and maintain a topping charge. With four Li-phosphate cells in series, each cell tops at 3.60V, which is the correct full-charge voltage
• At this point, the charge should be disconnected but the topping charge continues while driving. Li-phosphate is tolerant to some overcharge; however, keeping the voltage at 14.40V for a prolonged time, as most vehicles do on a long drive, could stress Li-phosphate. Cold temperature operation starting could also be an issue with Li-phosphate as a starter battery

Battery specifications:

Cell Characterstics/Data:

The following figure shows the 2.5C rate Constant current discharge characteristics at different temperatures, If we observe the plot it can be said that the cell characteristics vary with respect to the temperature changes. So we have to operate the cells within their nominal temperature zone.

Testing of a cell:

The battery parameters have a significant influence on other components and attributes of the vehicle, like Maximum traction motor torque, Maximum regeneration brake torque, Vehicle range, Vehicle total weight, and Vehicle price. So based on all these parameters we have to design a battery pack. All major aspects of a pure electric vehicle depend on the parameters of the high voltage battery.

A battery pack consists of one or more electrochemical cells which are converting chemical energy into electrical energy during discharging and electrical energy into chemical energy during charging. There are several types of batteries used in hybrid and electric vehicle propulsion systems but we are going to consider only Lithium-ion cells. The main reason is that Li-ion batteries have higher specific energy (Wh/kg) and specific power (W/kg) compared with other types. The following plot shows the Ragone chart of different cells. The Li-ion shows its best characteristics compared to other cells.

The voltage level of the battery determines the maximum electrical power which can be delivered continuously. As power is the product of voltage and current. The higher the current, the bigger the diameter of the high voltage wire and the higher the thermal losses. For this reason, the current should be limited to a maximum, and the nominal power obtained by having a higher voltage. For our modeling of the pack, considering the nominal voltage of 264V. All high voltage battery packs are made up of battery cells arranged in strings and modules. A battery cell can be regarded as the smallest division of the voltage.

Individual battery cells may be grouped in parallel and/or series as modules. Further, battery modules can be connected in parallel and/or series to create a battery pack. Depending on the battery parameters there may be several levels of modularity. The total battery pack voltage is determined by the number of cells connected in series. The current capacity of the battery pack is determined by the number of cells connected in parallel.

Dimension of a cell:

Cell applications:

• Energy storage
• Frequency regulation
• Uninterruptible Power Supplies
• Communication technologies
• Aviation/Aerospace
• Electrified mobility devices
• Industrial equipment
• Medical devices
• Consumer Products.

Cell Calculations:

This calculation is to determine how much space of volume the cell will take in the to have the capacity of 18kw.

Cell energy = Nominal volatge * nominal cell capacity

∴Cell energy=3.3*2.5

=8.25Wh

∴So each cell has the energy of 8.25 wh.

Cell Volume = π *r^2*h

∴Cell Volume=3.14*0.013*0.13*0.065

=0.0344929 m^3

So, now we have to calculate how much Watt per volume the cell can produce:

Cell energy density = Cell energy/Cell volume

∴Cell energy density = 8.25/0.0344929 Wh/m^3

=239.17 wh/m^3.

Battery pack Calculations:

To get 18 kWh as a output power we have to consider series and parallel cells to be connected to create the battery pack module of the required capacity.For design and calculation of battery pack here, i assume a vehicle to achieve 200km in 18 KWh battery pack also i assume 145.2 nominal voltage for series connection.

Assumptions,

Vehicle Range = 200 Km

Battery Pack Capacity = 18KWh

Now,

a) We will calculate per Km average energy requirement = 18000 (Wh)/200 (Km)

= 90Wh/Km

∴Battery Pack Total Energy = Average energy * km

= 90Wh/km * 200km

= 18000 Wh

Let's assume nominal voltage for series connection= 145.2 V basically this depends upon the motor power,we can decide the value according to the motor.

b) Number of cell connected in series = Nominal voltage of series connection/nominal voltage of a single cell

= 145.2V/ 3.3V

= 44 cells

44 cells will be connected in a series connection to provide a proper voltage.

c) Energy for battery pack (Wh) = Number of cell connected in series * Single cell energy

= 44* 8.25Wh

= 363 Wh

d) Number of cell connected in parallel = Battery pack total energy (Wh) /energy for battery pack (Wh)

= 18120.96 Wh/363 Wh

= 50 cells

50 cells will be connected in a parallel to provide a proper capacity power.

e) Battery Pack Total Energy (Wh) = No. of cell connected in parallel * energy for battery pack

= 50* 363 Wh

= 18.150 KWh

f) Battery Pack Capacity (Ah) = No. of cell connected in parallel * single cell capacity (Ah)

= 50*2.5 Ah

= 125 Ah

The value of battery pack capacity is used for the total power required by the vehicle.

g) Total Number of Cell for Battery Pack = No. of cell connected in parallel * number of cell connected in series

= 50*44

=2200 cells

2200 cells will be used to make a complete battery pack.

h) Mass of Battery Pack = Total number of cell for battery pack * mass of single cell

= 2200 * 0.076 kg

= 167.2 kg

167.2 kg its the total weight of cell connected in parallel and series.

i) Total Volume of the Battery Pack = Total number of cell for battery pack * volume of single cell

= 2200 * 0.0345 m^3

= 75.9 m^3

75.9 m^3 its the volume occupied by total cell used in battey pack.

j) Cell Peak Current(A) = C rate(max pulse charge current)* single cell capacity

= 20* 2.5

= 50A

k) Battery Pack Peak Current(A) = cell peak current(A) * no. of cell connected in parallel

=50*50

= 2500A

l) battery pack peak power(W) = Battery pack peak current(A) * voltage of series connection

= 2500 A * 145.2

= 363 KW

m) cell continuous current(A) = C rate (max continuous charge current) * single cell capacity

= 10* 2.5

= 25 A

n) battery pack continuous current(A) = cell continuous current(A) * no. of cell connected in parallel

= 25*50

= 1250 A

o) battery pack continuous power(W) = Battery pack continuous current(A) * voltage of series connection

= 1250 A * 145.2

= 181.5 KW

Now we know the weight and volume of the battery pack of 18kWh.So, we will consturct the battery pack.

General Steps to Design a practical Battery Pack :

Assembly of the battery pack

Component of the battery pack

1. 2200 cells
2. Nickel strip
3. PVC heat sink film
4. spot welder
5. Connected
6. Cell holder
7. Solder

Step 1 -

Design of cell holder (Bottom and Top) :

The main function of a battery holder is to keep cells fixed in place safely and securely also it is used to keep cells in a proper arrangement so that there will be a decrease in mechanical vibration, and have a better thermal runway in between the gaps of the cell. Most of the cell holder is made up of polypropylene or nylon bodies rated for 80–100 °C.

Step 2 -

Inserting cells inside the holder (Bottom and Top) :

Insert the cells inside these connectors with positive and negative poles kept alternatively in adjacent rows.

Step 3 -

Spot welding :

Spot weld with nickel strips to make a series connection between the cells as shown -

After making series connections, connect the strings in parallel. And complete the connection below -

Step 4 -

Install BMS for the battery pack as shown -

Step 5 -

Install cooling structures for air cooling/ liquid cooling as shown -

Step 6 -

Install the battery pack inside the enclosure as shown -

Enclosure for battery packs :

There are different types of enclosures available for the battery pack. A few are explained below. The main features required for EV battery enclosures are :

• Lightweight
• Shock and Vibration-proof
• Fire protection
• Prevent Electromagnetic radiation
• Low cost
• Aluminium sheet battery enclosure

Aluminium sheet battery enclosure -

Aluminium's inherent properties — corrosion resistance and recyclability — make the metal attractive in electric-vehicle manufacturing. The enclosure regulates thermal conductivity by helping to cool and heat the battery cells.

Using aluminium to enclose a lithium-ion battery pack in an EV can reduce weight by about 50 per cent.

Weight savings translates into a longer driving range, a major challenge for the competitiveness of electric vehicles.

The lightweight aluminium translates into a lower cost per weight at mass-production volumes.

Battery enclosure and thermal managment of battery and BMS:

• The battery encolsure is the final protection of the battery.
• So the battery enclose must be very strong and light in weight.
• In the case of any accident the batteries has the tendentcy of catch fire if it get leaked during the car crash,so the battery modle is enclose in a very strong encolsure due to the safety.
• The weight should be less so we can get the best pssible range.
• In this metal housing there is a temperature managment system provided to cool and heat the battries when there is a need to get the best performace of the battries.
• There a several numbers of models connected in series and parallel to build a entire battry pack.

BMS: A battery management system (BMS) is an electronic system that manages a lithium battery pack and the main functionalities are:

1. Monitors all of the parallel groups in the battery pack and disconnect it from the input power source when fully charged.
2. Balance all the cells voltage equally.
3. Doesn't allow the pack from over-discharged.

Final Overview of battery pack enclosure: In the following figure we can see the arrangement of modules of the battery pack which are used in electric vehicles and according to the company vehicle designs the arrangement may be varied.

So, the enclosure procedure, the electrical and the mechanical parameters of the battery pack are also calculated of the ANR26650M1-B is done with the relevent images.

Conclusion:Thus we have studied how to model a battery pack for 18kw battery.

REFERENCES :

https://en.wikipedia.org/wiki/Battery_pack

https://a123batteries.com/anr26650m1-b-lithiumwerks-nanophosphate-3-3v-2-5ah-lithium-iron-phosphate-battery/

https://www.powerstream.com/BPD.htm