Project 1 Mechanical design of battery pack
Abstract :-we will be developing the battery pack for an EV of rating 18KWh by using cell of ANR26650MI and we will be seeing its electrical aspect such as its DOD , SOC , Operating current , nominal voltage an d its temperature dependency along with its aging factor. Lastly we will we having a look on its geometrical structure such as size ,width , weight, length ,and also its enclosure model.
OBJECTIVE:
1) Calculating all Electrical parameters for 18 kWh Battery Pack.
2) Calculating all Physical dimensions for Battery Pack.
3) Consider perfect enclosure for dimensions that we calculated for the Battery Pack.
Design flow:-
1)calculation of the load:-based on the motor rating we will be deciding the total capacity of the battery. The battery C rating should match up with the load (motor) that is given and the battery parameter is also decided by the vehicle top speed and the acceleration needed.
2)deciding no of series and parallel connection :- based on the rating given we will be deciding the number of series and parallel cell connection that we will need.
3) After calculation of series and parallel combination we have to calculate the proper Geometry and topology consideration for assembling batteries.
4) After Assembly we need to find a proper enclosure for the battery pack for thermal consideration and other physical damages.
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 energy (during charging). The type of elements contained within a battery and the chemical reactions during discharging-charging events define the chemistry of a battery.
A battery cell consists of five major components: electrodes – anode and cathode, separators, terminals, electrolytes, and a case or enclosure. For automotive applications, there are different types of cells used Individual battery cells are grouped into a single mechanical and electrical unit called a battery module. The modules are electrically connected to form a battery pack.
the energy needed for propulsion, the high voltage battery must supply the energy for the vehicle’s auxiliary devices Eaux [Wh/km], like 12 V electrical system, heating, cooling, etc. Also, we have to consider the efficiency of the powertrain ηp [-] during the conversion from electrical energy to mechanical energy.
Assumption:-
Let us assume 300 V voltage is required to run the propulsion and auxiliary units of electric vehicle.
Range we can assume 250km for the given battery capacity
Mechanical design and calculation of battery pack:
Specifications of the cell ANR26650M1B :
Nominal Ratings
Voltage - 3.3 V
Capacity @ 23 ºC Typ (Min) - 2.6 Ah (2.5)
Energy @ 23 ºC - 8.25 Wh
Specific Power - 2600 W/kg
Impedance (1KHz AC) Typ - 6 mΩ
Cycle Life at 1C/1C, 100% DOD - > 4000 cycle
Calculation of Energy consumption:
Range = Capacity/Energy consumption per km
Energy consumption per km = Capacity/Range of vehicle
= 18000 wh/250 km
= 72 wh/km
Therefore as per our assumptions of 250km range for 18kwh battery pack capacity the energy consumption of vehicle is 72wh/km which is 0.072kwh/km.
Calculation of No. of cells:
Total no of cells:
Here, single cell voltage is 3.3V and a capacity of 2.5Ah. Energy provided from single cell is give 8.25Wh.
Total energy = 18000 Wh (as per battery pack)
Number of cells (N) = Total energy / (Nominal voltage*capacity)
= 18000 / (3.3*2.5)
= 2181.8
Total no of cells = 2182 no of cells
No. of cells in series:
The required voltage is 350V, the nominal voltage of the cell is 3.3V so to attain the required voltage the no of cells are connected in series to increase the voltage.
Total voltage of battery pack = Nominal voltage of cell * No.of cells in series
No. of cells in series = Total voltage of battery pack / Nominal voltage of cell
= 350V/3.3V
= 106.06
No. of cells in series = 106 nos.
No. of cells in parallel:
The given battery pack capacity is 18 kwh, now we have 106 number of cells connected in series but when connected in series it will increase only the voltage the capacity will be the same 2.5 Ah.
To get the complete battery pack capacity 18 kwh, we need to add the cells in parallel to increase and attain the battery capacity.
No of cells in parallel = Total no of cells / No. of cells in series
= 2181 / 106
= 20.58
No of cells in parallel = 21 cells in parallel
Or
Single module energy = ( no of cells in series * cell energy)
= (106 * 8.25)
Single module energy = 874.5 wh
how many similar modules should be connected in parallel to achieve 18KWh battery pack.
No of modules in parallel = 18000 Wh / 874.5 Wh
=20.58
No of modules in parallel = 21
So as per calculations 21 no of cells are connected in parallel
Amp hour capacity of battery pack:
21 no of cells are connected in parallel, therefore the capacity of battery pack is
Total Amp hour capacity = No of cells in parallel x Ah of single cell
= 21 x 2.5Ah
Total capacity of battery pack = 52.5 Ah.
Current Supplied:
Battery pack can provide 52.5 A of current continuously for one hour for discharge fully.
Cell configuration:
The cell configuration we arrived is 106S21P
106 no of cells in series and 21 no of cells in parallel
Total no of cells = 106 x 21 = 2226 Nos.
Total energy capacity after calculation:
Total energy = Total no of cells * energy of single cell
Total energy = 2226 * 8.25 = 18364.5 Wh
Total energy we are getting( 18364.5 Wh )is more than the given capacity (18000 Wh).
Battery pack configuration:
As per the calculation, the final configuration of our battery pack is
18.3 Kwh, 52.5 Ah, 106S21P
Cell volume:
The volume of each cell is calculated as:
- cylindrical cells, Vcc[m3]
Vcc=πr^2 H
Vcc = (πD^2/4) x H
D – Diameter of cell – 25.96mm
H – height of the cell – 65.15mm
V = (π x 25.96^2 / 4) x 65.15
V = 3.45 x10^-5 m^3
The battery cell energy Ebc [Wh] is calculated as:
Ebc= battery cell capacity x battery cell voltage
= 2.5 x 3.3
Ebc= 8.25 Wh
volumetric energy density, uV [Wh/L]
uV=Ebc / Vcc(pc)
= (8.25/3.45x10^-5)x1000
uv = 239.130 Wh/L
gravimetric energy density, uG [Wh/kg]
uG=Ebc/mbc
= 8.25/0.076
uG = 108.55 Wh/kg
mbc = battery cell mass
battery pack mass (cells only) mbp [kg]
total number of cells Ncb
mass of each battery cell mbc [kg].
mbp =Ncb*mbc
= 2226 x 0.076 kg
battery pack mass (mbp) = 169.176 Kg
volume of the battery pack (cells only) Vbp [m3]
total number of cells Ncb
Volume of each battery cell Vcc(pc) [m3]
Vbp =Ncb x Vcc(pc)
= 2226 x 3.45x10^-5
volume of the battery pack (Vbp) = 0.0767 m^3
Dimensions of Battery pack Enclosure:
Dimension of cell:
Length of the battery pack:
106 cell are connected in series,based on our selected cell holder 2 cells can be placed in holder and 53 such holders are connected to kept 106 cells.
length of battery pack (L) = (106/2) x length of holder
= 53 x 55 mm
L = 2915 mm
Breadth of battery pack:
21 cells are connected in paralle,based on our cell holder dimension,the width is 27.5mm for single holder
Width of battery pack (W) = 21 x width of single holder
= 21 x 27.5
W = 577.5 mm + space for wrapping,insulation,cover(15mm)
W = 592 mm
Height of battery pack:
Height of single cell is 65.15 mm
Total height of the battery pack = 65.15 + space for wrapping,insulation,cover,strip(15mm)
H = 100 mm
Volume of battery pack:
LxWxH = 2930x592x100 = 0.1735 m^3
Weight of battery pack:
Cell weight = 76g
Battery pack weight (BPw) = (cell weight x no of cells ) + weigth of cover and accessories
= (0.076 x 2226) + 6 kg
BPw = 175 kg