Mechanical design of battery pack

CELL Parameters (ANR26650M1-B )

Chemistry:

Lithium Iron Phosphate (LiFePO4)

Cathode: Lithium Iron Phosphate(LiFePO₄)

Anode: Graphite

ANR26650M1-B Specifications
Nominal Ratings
Nominal capacity: 2.5Ah at 0.5C rate
Minimum capacity: 2.4Ah at 0.5C rate
Nominal voltage: 3.3V
Energy: 8.25 Wh
Specific Power: 2600 W/kg
Impedance (1KHz AC Typical) 6 mΩ
Cycle Life at 1C/1C, 100% DOD: > 4000 cycles
Cell Type: 26650 Lithium Ion Power Cylindrical Cell

Discharging
Max continuous discharge current: 25A (10C)
Max pulse discharge current (10 sec): 60A (24C)
Discharge cut-off voltage: 2.0V
Minimum Voltage: 2 Volts
Temperature: -30ºC to 55ºC
AC impedance at 1KHz: 6mΩ

Charging
Recommended Standard Charge: 2.5 Amps
Max Charge Rate: 10 Amps
Max Pulse Charge Current <50% SOC (10s): 25 A
Float Voltage: 3.45 Volts
Recommended charge V & Cut-off Current: 3.6 Volts, taper to 125mA
Temperature
(reduce charging current to 250mA when under 0ºC): 0ºC to 55ºC

Mechanical

Diameter: Ø25.96 +/- 0.5 mm (1.0")
Length: 65.15 +/- 0.5 mm (2.6")
Mass: 76 g

• High-performance Nanophosphate® 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 Capacity : 18 kWh

1. The Battery pack Voltage, Ub [V]

• The battery pack voltage is considered to be 96 V.

Ub = 96 V

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

2. 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

N_s = 96/3.3 = 29.09 = 30 S

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

• Es 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 = 30 * 8.25 = 247.5 Wh

4. The number of battery cells connected in Parallel, Np

• The total number of strings of the battery pack Np [-] is calculated by dividing the battery pack total energy E_b [Wh] to the energy content 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 = 18000/247.5 = 72.7

For a creating battery modules, we take a number with higher no of factors

N_P = 75

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

E_b = 75*247.5 = 18.5 KWh

5. The battery pack capacity C_b [Ah]

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

C_b = 75*2.5 = 187.5Ah

7. The total number of cells of the battery pack Nb

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

Nb = 75*30 = 2250 no of cells

8. 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 = 2250 * 0.076 = 171 kg

9. 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 = 2250 * 0.034510 = 77.65 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 volume occupied by a cylindrical cell must take into account the air gap between cells.

10. The string peak current I_spc [A]

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

I_sp = 24*2.5 = 60 A

11. The battery pack nomical current I_bp [A]

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

I_bp = 60 * 75 = 4500 A

12. The battery pack nominal 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 = 4500 * 96 = 432 KW

13. The string continuous current I_sc [A]

• The string continuous current is 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 = 10 * 2.5 = 25 A

14. 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_bc = 25 * 75 = 1875 A

15. The battery pack continuous power P_bc [W]

• The 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 = 1875 * 96 = 180 KW

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 [-]	30
String energy [Wh]	247.5
No of strings [-]	75
BP energy [kWh]	18.5
BP capacity [Ah]	187.5
No total cells [-]	2250
BP mass [kg]*	171
BP volume [l]*	77.65
BP peak current [A]	4500
BP peak power [kW]	432
BP continuous current [A]	1875
BP continuous power [kW]	180

BATTERY MODULE

• The Battery Requirements is 18.5 KWh battery pack, the required configuration of the pack is 30S75P, i.e., 30 series, and 75 parallel connections.
• 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 30S75P (2250 cells) into no of 5S5P Cell Modules

No of Cells in Battery Pack, 30 * 75 = 2250 cells

No of Cells in a Battery Module, 5 * 5 = 25 cells

No of Modules = 2250/25 = 90 no of Modules

Modules Configuration = 6 x 15 = 90

Cell Configuration in a module = 5S5P

no of Modules in the Battery Pack = 90

Module Configuration in the Battery pack = 6 x 15

Module Isometric View

Module Top View

Module Bottom View

Connections (Module Top Surface)

Connections (Module Bottom Surface)

Module :

Length = 28.38 * 5 = 141.9 mm

Breadth = 28.38 * 5 = 141.9 mm

Height = 69.71mm

Weight = 0.076*25 = 1.9 kg (cells only)

6 Modules in Series

Top View

Bottom View

Battery Pack

Battery Pack :

Length = 28.38 * 75 = 2128.5 mm

Breadth = 28.38 * 30 = 851.4 mm

Height = 69.71mm

Weight = 0.076*2250 = 171 kg (cells only)

Link for Battery Pack (Solidworks 2021) : https://drive.google.com/file/d/1IgqA7s56GNb-aFfKeu2ViXCEyNzZM8LI/view?usp=sharing

Reference:

https://lithiumwerksbatteries.com/