Project 1 Mechanical design of battery pack

Aim: Mechanical Design of battery pack

Battery pack capacity:18 KWh

Cell: ANR26650M1-B

Prepare a detailed battery pack drawing along with its enclosure. State your assumptions (3.3 Volt, 2.5Ah, Lithium iron Phosphate [LiFePO4] Battery Lithium ion cell, Nanophosphate® (LiFePO4) Technology {Formerly A123 Systems)

EV battery pack design

The high voltage battery it is one of the most important components of an electric vehicle (EV).

The battery parameters have a significant influence on other components and attributes of the vehicle,like:

. High nominal voltage

. High Ah capacity

. Low mass

. Maximum traction motor torque

.Maximum regeneration brake torque

. Vehile total weight

. Vehicle price

. Vehicle range

When designing a battery pack you will always be asked to benchmark it. For this there are a nimber of key metrics:

. Safety

. Wh/kg- Pack Gravimetric Energy Density

. Wh/litre

. W/kg

.$/kwh

. Cell to Pack mass ratio

. Cell to pack volume ratio

. Durability-% of pack capacity available after 10 years

. A battery consists of one or more electrochemical cells (battery cells) which are converting chemical energy into electrical energy (during discharging) and electrical energy into chemical within a battery and the chemical reactions during discharging-charging events define the chemistry of a battery.

. Individual battery cells are grouped together into a single mechanical and electrical unit called a battery module. The modules are electrical connected to form a battery pack.

. There are several types of batteries (chemistry) used in hybrid and electric ehicle 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.

Mechanical design

The mechanical design of the cell has to hold all of the elements of the cell together and provide a leak proof container over the lifetime. The mechanics of a cell are quite dependent on cell format (button, cylindrical, pouch, prismatic). The fundamental requirements are:

. Maintain sealing of the cell active contents

. locate the positive and negative tabs

. controlled failure during venting

. controlled failure during thermal runway

For the design of the 2019 Porsche Taycan the battery housing is a load-bearing component of the body structure:

. Truss-design battery frame with multiple subdivisions

. 28 bolts fix the battery to the body structure

. battery contributes ~ 10% of the vehicle’s overall stiffness

Thermal cycling of components can have a significant impact on the overall mechanical design.

• Safety and reliability are the two key challenges for largescale electrification of road transport sector. Current Li-ion battery packs are prone to failure due to reasons such as continuous transmission of mechanical vibrations, exposure to high impact forces and, thermal runaway. Robust mechanical design and battery packaging can provide greater degree of protection against all of these.

Turbo S Performance Battery Plus

This is the largest battery capacity on offer in 2019 and highest power.

Specification:

. Total energy = 93.4kwh

. Usable energy = 83.7kwh

. Usable Soc= 90%

. Peak discharge power

. 460kw_10s

. Continuous power = 270 kw

The 270kw continuous discharge power is based on the charge power and hence the overall system thermal and electrical limits for extended current demand and heating.

. Nominal voltage = 723 V

. Capacity = 129.2 Ah

. voltage range = 610 V to 843 V

. Weight [kg] = 630kg

Cells = 355 kg

. Volume pack = litres

. pack dimensions [m] =

. number of cells = 396

. 198s

. 2p

. Charge time = 5 % to 80% in 22.5 minutes

. 270 kw peak charge power

. Modules: 33 modules with 12 cells in each module

. cooling system

. Water -glycol system that can heat and cool the battery

. Cooling system sits under the main battery structural tub ensuring any coolant leaks cannot enter the pack

. Heat pump based system that allows heat from HV components to be stored in the battery pack

. Cell target temperature is changed based on driving style selection

. Battery is preconditioned to a certain temperature level when the vehicle is connected to the mains for charging.

Cell Parameters (ANR26650 M1-B)

. In order to choose what battery cells our pack will have; we’ll analyse several battery cells models available on the market.

. Here, we choose the ANR26650M1-B battery model for our battery design.

ANR26650M1-B Cell (Data sheet)

Manufacturer:	LithiumWerks
Mfg PN:	300732-006
Chemistry:	Lithium Iron Phosphate (LiFePO4)
Voltage:	3.3 Volts
Capacity @ 25°C:	2.56 Ah
Energy @ 25°C:	8.25 Wh
Energy @ 25°C:	8.25 Wh
Specific Power:	>4000W/kg | (@ 25°C, 2 sec pulse)
Impedance:	>6mΩ | (1kHz AC) Typical)
Cycle Life:	> 4000 Cycles | (@ 1C/1C, 100% DOD)
Dimensions
Height (max):	65.15 (+/- 0.5 mm)
Diameter (max):	25.96 (+/- 0.5mm)
Mass:	39.5 (+/- 1.0 g)
Warranty:	DOA
Discounted pricing when you buy: 5 - 24 and pay only $6.55 each 25 - 99 and pay only $6.00 each 100 - 999 and pay only $5.54 each 1000 or above and pay only $5.14 each Charge current and voltage calculation examples Example 1. If a cell group has 3 cells in parallel (3p), and the recommended charge current per cell is 20A, then the charge current for this group is 60A: (3 cells, parallel) x 20A = 60A Example 2. If a cell string has 10 cells in series (10s), and the recommended charge voltage per cell is 3.6V, then the end of charge voltage for the string is 36V: (10 cells, series) x 3.6V = 36V

Graph:

Battery voltage and current during recharge

. High-performance Nano phosphate® 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.

. 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.

Battery Pack Calculation

Based on the cell parameters provided by the manufactures, we can calculate on the energy content, volume, gravimetric density and volumetric density for each cell.

Manufacturer A123-Systems

Type Cylindrical

Model ANR26650M1-B

Source [5]

Length[m] 0.065

Diameter[m] 0.026

Height[m] 0

Width[m] 0

Thickness[m] 0

Mass [kg] 0.07

Capacity [Ah] 2.5

Voltage[V] 3.3

C-rate (cont.) 10

C-rate(peak) 24

Cylinder Cell primary Applications:

. Portable high power devices

. Stationary battery backup systems

. Grid stabilization energy storage systems

. Commercial truck and bus hybrid electric vehicles (HEVs)

1. Volume of each cell = π*r*r*h

= 3.14*0.013*0.13*0.065

= 0.0344929m^3

8.25*0.0344929Watt per volume we have to calculate:

Cell Energy Density = Cell Energy /Cell Volume

= 8.25/0.0344929^3

= 239.17 wh/m^3

Now we have to assume that for 18kwh capacity to design 30 cells in series and parallel to obtain the battery pack in order too obtain the required capacity of about 18kwh.

Let us assume 58 cells connected in series and 38 cells connected in parallel

Total number of cells connected are 58*38= 2204 Cells

Voltage of pack = Nominal voltage*Cells connected in series

= 3.3 *58

= 191.4 V

Current capacity of the pack= Rated Capacity* cells connected in parallel

= 2.5 Ah *38

= 95 Ah

Total energy of the battery pack = 191.4*95

= 18.184kw

Now we know the total number of cells connected in the battery pack, we have to calculate the en

Density, Volume, and weight of the battery pack

= Cell volume * Total number of cells connected

= 0.0344929*2204

= 76.022m^3

Total weight of the battery = Total number of cells* Weight of each cell

= 2204 *76

= 167.505 kg is the total weight of the battery

Now we know the total weight of the battery and volume battery pack of 18kw

Volume of each cell,

Vc= π.(Dc)24.Lc

Where,

Lc- Battery cell length [m]

Dc- Battery cell diameter [m]

Vc=( π*0.0262/4)*0.065=0.034510 m³

2. Battery cell energy, E_bc[Wh]

Ec=Cc.Uc

Where,

C_c – battery cell capacity [Ah]

U_c – battery cell voltage [V]

E_c = 2.5*3.3=8.25 Wh

3. Battery cell energy density

Volumetric energy density, U_v [Wh/m³]

U_V=E_cV_c

U_v = 8.25/0.034510 = 108.55 Wh/m³

Gravimetric energy density, U_G [Wh/kg]

U_G= E_cm_c

Where,

M_c – battery cell mass [kg]

U_G=8.25/0.076 = 239.06 Wh/ka

. With the above cell parameters and core requirements for the battery (nominal voltage, average energy consumption and vehicle range), we calculate the main parameters of the high voltage battery.

High voltage battery

1.The Battery pack total energy E_b [Wh]

. The required battery pack total energy E_b [Wh] is calculated as the product between the average energy consumption E_gvg [Wh/km] and vehicle range D_v [Km].

. Here we’ll design the high voltage battery pack of 18Kwh for a vehicle range of 250 km.

There for, E_b = 18Kwh D_v = 200km

E_avg = E_b/D_v= 18000/200 = 90 Wh/km

2. The battery pack Voltage ,U_b [V]

. The battery pack voltage is considered to be 300V high voltage.

U_b = 300V

We are going to consider that the battery pack is made up only form several strings connected in parallel.

4. The number of battery cells connected in series N_s

. N_s in a string is calculated by dividing the nominal battery pack voltage U_b [V] to the voltage of each battery cell U_c [V]. The number of strings must be an integer._s

N_s = U_bU_c

N_s = 300/3.3 = 90.90 =91 S

5. The energy content of a string E_s [ Wh]

. E_b is equal with the product between the number of battery cells connected in series N_s [-] and the energy of a battery cell E_c [Wh].

E_s=N_s . E_c

E_s = 91*8.25= 750.75 Wh

6. The number of battery cells connected in parallel, N_p

. The total number of string of the battery pack N_p [-] is calculated by dividing the battery pack total energy E_b [Wh] to the energy of a string E_s [Wh]. The number of strings must be an integer. Therefore, the result of the calculation is rounded to the higher integer.

N_p=E_bE_s

N_p = 18000/750.75 = 23.9 = 24 P

We can recalculate the battery pack total energy E_b[Wh] as the product between number of strings N_p[-] and the energy content of each string E_s[Wh].

E_b = 24*750.75= 18.02 KWh

7. The battery pack capacity C_b [Ah]

C_b is calculated as the product between the number of strings N_s [-] and the capacity of the battery cell C_c[Ah].

C_b = N_s.C_c

C_b = 24*2.5 = 60 Ah

8. The total number of cells of the battery pack N_b

. The total number of cells of the battery pack is calculate as the product between the number of strings N_p [-] and the number of cells in a string N_s [-].

N_b = N_P. N_s

N_b = 91*24= 2184 no of cells

The size and mass of the high voltage battery are very important parameter to consider when designing a battery electric vehicle (BEV). In this example we are going to calculate the volume of the battery pack considering only the battery cells. In reality there are other factors to consider, like: electronic circuits, cooling circuit, battery casing, wiring, etc.

9. The battery pack mass (cells only) m_b [kg]

. The battery pack mass (cells only) is the product between the total number of cells N_b [-] and the mass of each battery cell m_c[kg].

M_b=N_b.m_c

M_b = 2184*0.076 = 165.98 kg

10. The volume of the battery pack (cells only) V_b [m³]

. The volume of the battery pack (Cells only) is the product between the total number of cells N_b [-] and the mass of each battery cell V_c [m³].

. This volume is only used to estimate the final volume of the battery pack, since it does not take into account the auxiliary components/ systems of the battery.

V_b=Nb.Vc

V_b= 2184*0.034510=75.37 m³

. The volume can be also calculated function of number of strings and number of cells in a string. This calculation method is more appropriate for cylindrical cell, since the colume occupied by a cylindrical cell must take into account the air gap between cells.

11. The string peak current I_spc[A]

. The string peak current is the product between the peak C- rate of the battery cell C – rate_cp [h^-1] and the battery cell capacity C_b[Ah].

I_sp= Create_cp.C_b

I_sp= 24*2.5=60A

12. The battery pack peak current I_bp[A]

. The battery pack current is the product between the string peak current I_spc[A] and the number of strings of the battery pack N_p [-].

I_bp=I_sp.N_p

I_bp=60*24=1440A

13. The battery pack peak power P_bp[W]

. The battery pack peak power P_bp [W] is the product between battery pack peak current I_bp [A] and the battery pack voltage U_b [V]

P_bp=I_bp.U_b

P_bp= 1400*300=432kw

14. The string continuous current I_sc [A]

. The string continuous current the product between the continuous C-rate of the battery cell C- rate_bc [h^-1] and the battery cell capacity C_b [Ah].

I_sc = Crate_bc.C_b

I_sc = 10*2.5 = 25 A

15. The battery pack continuous current I_bc[A]

. The battery pack continuous current is the product between the string continuous current I_sc[A] and the number of strings of the battery pack N_p.

I_bp=I_sc.N_p

I_bc = 25*24 = 600A

16. The battery pack continuous power P_bc [W]

. The battery pack continuous power is the product between battery pack continuous power is the product between battery pack continuous current I_bc [A] and the battery pack voltage U_b[v].

P_bc=I_bc.U_b

P_bc=600*300= 180KW

The mass and volume are calculated only at cell level, taking into account the cell dimensions and mass. The battery pack which will be in the vehicle will have additional components (wires, electronic components, soldering, case, etc.), which will increase both the final volume and mass.

The Results:

Manufacturer A123- Systems

No of cells in string [-] 91

String energy [Wh] 750.75

NO of strings [-] 24

Bp energy [kwh] 180.2

BP capacity [Ah] 60

No total cells [-] 2184

BP mass [kg]* 165.98

BP volume[ [I] 7.37

BP peak current [A] 1440

BP peak power [kw] 432

BP continuous current [A] 900

BP continuous power [kw] 180

Battery Module:

. The battery requirements is 18.02 Kwh battery pack, the required configuration of the is 91S24P, i.e., 91 series, and 2184 cells connected in parallel.

. To build such a large grid, we need to slip the battery packs into the battery module, which reduce the complexity of monitoring and optimizing battery management and minimizing battery damage.

So, we are Splitting the 91S24P 2184 cells into no of 7S4P Cell modules

No of cells in battery pack, 91*24= 2184 cells

No of cells in battery Module, 7*4= 28 cells

No of Modules = 2184/28 = 78 no of modules

Modules Configuration = 13*6 = 78

Cell configuration in a module = 7S4P

No of Modules in the Batery Pack = 78

Module Configuration in the Battery pack = 13*6

Cell Dimensions:

Length 65.15+0.05 mm

Diameter 25.85±0.03 mm

Weight 0.076 kg

Cell Holder:

. Each Cell holder can hold up to 4 battery cells, and can hold the battery pack with more holders connecting together, holder improves the stability of the battery back and reduces vibrations.

. By connecting more holders to each other we can make the battery gride according to the required battery back configuration.

Here we are going to make 7S4P battery, therefore a battery holder can use to hold 4cells of parallel connected cells and 7 such holders can be implement to achieve 7S4P battery module.

The holder is designed to have adequte air gap between eaach cell and can be used to implement thermal management strips or plates.

Dimensions:

Length =198.66mm

Breadth = 113.52mm

Height = 69.71mm

Weight = 0.076*28 = 2.13 kg (cells only)

Electric Connection:

Nickel Strips/Tabs:

Conductive metal strips are spot welded between all the cells of an electric bicycle battery pack. Nickel is the material of choice due to its low relative resistance and ease of spot welding.

. Pure nickel strip is ideal for use in the building of batteries. As the alloy has high corrosion resistance, can be efficiently spot welded and sensitivity, it allows battery sources to perform well, even in highly corrosive environments.

. Resistance spot welding, micro TIG welding, and laser welding processes all enable high quality volume production. The selection of one technology over another is usually made based on the application’s specific requirements and the alignment of the technology to these needs.

We use a nickel tab to connect the cells to make a battery pack. It has 4 markings, which can be spot welded between the battery and the trip using any spot weld technique.

. The thickness of the strip is 0.3 mm, length and width are 38.7mm and 10.7mm.

. Connections are made by connecting paralle and in series.

BMS (Battery Management System)

. A Batty Management System (BMS) is said to be the brain of a battery pack. The BMS is a set of electronics that monitors and manages all of the battery’s perforfmance. Most importantly, it keeps the battery from operating outside of its safety margins

. If it detects any unsafe conditions, the BMS shuts the battery down to protect the lithium-ion cells and the user. The BMS also monitors the remaining charge in the battery.

. Therefore, the BMS component of the battery pack is essential, and each module in the battery pack requires a dedicated BMS to monitor and protect the cell set. Therefore, BMS need to be given enough space in the battery module design and connections are made with BMS to perform the required functions.

BTMS (Battery Thermal Management System)

. For battery thermal modelling, a reliable thermal model of the battery is required. Starting from the heat generated from the battery, the thermal management system can be modelled by applying the correct boundary conditions. Battery thermal models are realized by considering the electrical/ electrochemical characteristics and the thermal behaviour of the battery. Heat generation inside the batteries is governed by complex electrochemical reactions depending on temperature and state of the battery. Coupled, uncoupled or partially coupled models of the battery can be realized.

. There enough gap between the cells in our design to implement thermal strip or fins.

Battery pack Enclosure:

. Lithium-ion Battery pack which is comprised of assembly of battery modules is the main source of power transmission for electric vehicles. During the actual operation of electric vehicle, the battery packs and its enclosure is subjected to harsh environmental conditions such as the external vibrations and shocks due to varying road slopes. This will result in stresses and deformations of different degrees.

. The vehicle safety heavily depends on the safety of battery pack which in turn is dependent on its mechanical features, such as the ability to resist deformation and vibration shocks. In addition, lighter weight vehicle is preferred because it can increase the range of vehicle and the life cycle of a battery pack.

. Enclosure must be design in a manner with lot of structural analysis and lot of optimisation processes.

. A wall thickness as of 3 mm, which requires a gap of 1.5mm, is recommended as the battery compartment is flush with the wall surface. Adjust the required gap or screw length for larger wall thickness.

Identification:

. The pack design must incorporate some from of identification to indicate the manufacturer’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 assist traceability in case of problems. This information is usually provided on a label but may also be printed directly on to the battery casing.

Module Configuration in Battery Pack = 13*6

Length of module = 198.66mm + BMS (PCB) = 200mm

Battery Pack Length = 13*200mm = 3600mm +2*(3+1.5) = 3610mm = 3.61m

Battery Pack Width = 6*114 =684mm + 2*(3+1.5) = 695mm = 0.695m

Battery Pack Height = 70mm + 2*(3+1.5) = 80mm = 0.08m

Conclusion:

. From the single-cell technical data sheet, I have designed a battery pack here for the electric vehicle with a total power of 18 Kwh. The total number of cells in the 91S/ 24P configuration is 2184 cells. And, the nominal voltage from the 91 series connection of the battery is 300V, and the capacity we get from the configuration of 24 parallel cells is 60 Ah. The vehicle uses all the design to achieve a range of 200km, which is an assumption to calculate the battery pack. The entire battery pack is divided into 78 modules, of which there are 28 cells in each block, and this configuration is designed to accommodate the 915/24P cell configuration.

References:

1. https://www.batteryspace.com/prod-specs/6610.pdf

1. https://x-engineer.org/ev-design-battery-calcution/

3. https://mpoweruk.com/pack_construction.htm

4. https://www.hbmprenscia.com/products/veicle-electrification/mechanical-durability