Mechanical design of battery pack
Battery pack capacity: 18 kWh
Cell: ANR26650M1-B
The high voltage battery it is one of the most important component of a battery electric vehicle (BEV). 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
- vehicle price
A battery cell consists of five major components: electrodes – anode and cathode, separators, terminals, electrolyte and a case or enclosure.
Individual battery cells are grouped together into a single mechanical and electrical unit called a battery module. The modules are electrically connected to form a battery pack.
Battery pack consists of number of series and parallel combination. Large battery pack consists of large number of modules.
Series connection is used to obtain required voltage and parallel connection is used to obtain required current capacity.
In order to ensure safe and reliable operation of battery pack the design should follow certain technical criterias
- Mechanical vibration should be less
- controlling on high impact force
- Better thermal runaway for cooling
- Serviceability and recyclability
Technical specifications of ANR26650MB
Calculations as per the assumptions : 18kWh battery pack capacity
18kWh is the energy required to achieve vehicle it's proper range. It depends on the voltage parameter and ampere-hour or capacity(Ah).
The design is for high voltage battery pack of electric vehicle in order to achieve range upto 250km.
Assumed value for nominal voltage for series connection is 145.2V
Diameter = 26mm = 0.026m
Length = 15/65mm = 0.065m
Mass = 76g = 0.076kg
single cell voltage = 3.3V
capacity = 6Ah
Max continuous charge current ( C-rate) = 10A
Max pulse charge current (C-rate) =20A
Mathematical battery calculation
single cell calculation, volume of cell =πr2h">πr2h= 3.14*0.013^2 * 0.065 =0.03451m^3
πr2h">Single cell energy (Wh) = Cell capacity(Ah) x Cell voltage(V)
πr2h"> = 2.5*3.3 = 8.25Wh
πr2h">Single cell energy density
πr2h">volumetric energy (Wh/m3)= single cell energy/ volume of each cell
πr2h"> = 8.25Wh/0.03451m3
πr2h"> = 235714Wh/m3
πr2h">Gravimetric energy (Wh/kg) = single cell energy/ single cell mass
πr2h"> = 8.25Wh/0.076kg = 108.55Wh/kg
πr2h">Total number of the battery pack
πr2h">Ebp = 18000Wh
πr2h">Number of cells connected in series
πr2h">It is calculated by dividing the nominal battery pack voltage by the voltage of each battery cell. The number of strings must be an integer.
πr2h">Ns= (Ubp/Ubc)
πr2h"> By assuming the battery pack voltage to be 330v and the nominal voltage of the battery as per the datasheet as 3.3V.
πr2h">Nbc=Ubp/Ubc = 100
πr2h">Total number of cells in series is 100
πr2h">Engergy content of a string
πr2h">The product between the number of battery cells connected in series and the energy of a battery cell
πr2h">Ncs = Ubp/Ubc
πr2h">Ebs=Ncs.Ebc = 100 x 8.25 = 825Wh
πr2h">Number of strings in the batteryπr2h"> pack
πr2h">It is calculated by dividing the battery pack's total energy Ebp[Wh] by the energy content of the string Ebs[Wh].
πr2h">Nsb= Ebp/Ebs = 18000/825 = 21.81
πr2h"> All the 22 strings are connected in parallel.
πr2h">Battery pack capacity
πr2h">It is calculated as the product between the number of strings Nsb and the capacity of the battery cell Cbc[Ah]
πr2h">Cbp=Nsb.Cbc
πr2h"> =22*2.5 = 55Ah
πr2h">Total number of cells of the battery pack
πr2h">It is calculated as the product between the number of stringNsb and the number of cells in a string Ncs
πr2h">Ncb = Nsb*Ncs
πr2h"> = 22*100 = 2200
πr2h">Total number of cells required for the design pack is 2200
πr2h">Mass of the battery pack
πr2h">It is the product of the total number of cells Ncb and the mass of each battery cell Mbc
πr2h">Mbp = Ncb* Mbc = 2200*0.076 = 167.2Kg
πr2h">Volume of the battery pack
πr2h">It is the product of the total number of cells Ncb and the mass of each battery cell Vcc(pc)[m3]
πr2h">It is used to estimate the final volume of the battery pack since it does not take into account the auxiliary components of the battery
πr2h">Vbp = Ncb* Vcc(pc)
πr2h">Vbp = 2200* 3.5*10^-5 = 77000mm^3
πr2h">String peak current
πr2h">It is the product between the peak C-rate of the battery cell C-ratebcp[h^-1] and the battery cell capacity Cbc[Ah]
πr2h">Ispc = C-ratebcp*Cbc = 24*2.5 = 60A
πr2h">Battery pack peak current
πr2h">It is the product between the string peak current Ispc[A] and the number of strings of the battery pack Nsb
πr2h">Ibpp = Ispc*Nsb = 60*22 = 1320A
πr2h">Battery pack peak power
πr2h">It is the product between battery peak current Ibpp and the battery pack voltageUbp
πr2h">Pbpp = Ibpp*Ubp = 1320*330 = 435600W = 435.6kW
πr2h">String continuous current
πr2h">It is the product between the continuous C-rate of the battery cell C-ratebcc[h^-1] and the battery cell capacity Cbc[Ah]
πr2h">Iscc =C-ratebcc.Cbc = 10*2.5 = 25A
πr2h">Battery pack contimuous current
πr2h">It is the product between the string continuous currentIscc[A] and the number of strings of the battery pack Nsb
πr2h">Ibpc =Iscc.Nsb
πr2h"> = 25*22 = 550A
πr2h">Battery pack continuous power
πr2h">It is the product between battery pack continuous current Ibpc[A] and the battery pack voltage Ubp[V]
πr2h">Pbpc =Ibpc*Ubp = 550 x 330 = 181500w
πr2h">The battery pack continuous power will be 181.5kW
πr2h">Assembly of the battery pack
πr2h">Step1: Snap the square cell holder together
πr2h">Step2: Place the cells in the holder matrix. Place all the cells in the holder and place the second holder in the place.
πr2h">Step3: Connect the cells with nickel strips using spot welding process. Series connection will be made by connecting positive terminal of the cell to the negatove terminal of another cell. Parallel connection is made by connecting the same terminals.
πr2h">Step4: Connect the heat sink to the battery pack.
πr2h">Step5: Connect the balancing wire pin to the battery management system
πr2h">Step6: Place the enclosure for the battery pack
πr2h">
πr2h">Cell holder
πr2h">
πr2h">cell cover
πr2h">
πr2h">This is the final battery model.
πr2h">CONCLUSION
πr2h">From the single cell data, battery pack data have been formulated for electric vehicle application which is having total energy of 18kWh.
πr2h">The steps involved in the design and development of the battery pack have been mentioned.
πr2h">References:https:https://x-engineer.org/ev-design-battery-calculation/
πr2h"> https://datasheetspdf.com/pdf/1424845/A123Systems/ANR26650M1-B/1