Project 1 Mechanical design of battery pack
The battery pack being designed is made by using the cell ANR26650M1-B.
The data sheet for the same is obtained here:
https://datasheetspdf.com/pdf/1424845/A123Systems/ANR26650M1-B/1
The cell has a lithium iron phosphate (LiFepO4) chemisty. The following table gives the overiew of specifications of the cell.
GEOMETRY OF THE CELL:
Diameter of cell = 26 mm
height of cell = 65 mm
volume occupied by cell = `pi * D^2 /4 *h` = 34510.39mm^3
Mass of the cell = 0.076 kgs.
The cell's rated capacity is at 2.5 Ah for a rated current of 0.5 C = 1.2 A.
With a nominal cell voltage of 3.3 volts and current capacity of 2.5 Ah , the cell energy capcaity is the product of these two parameters = 3.3* 2.5 = 8.25 watt-hr.
The required battery energy capcaity is 18 kwh.
lets assume the reqd voltage for the application of this battery pack is 132 volts nominal.
Using these numbers, we can arrange the battery pack using multi modular arrangement or as a single block.
Let's assume we are packing the battery using individual 8s4p modules. Each module is a matrix of 8*4 rows connected in parallel with each row having 8 cells in series.
The voltage of each module will be 8*3.3 =26.4 V.
The cacpaity of each module is 4*2.5 = 10 Ah.
To get a voltage of 132 volts, 5 of these modules will be connected in series (132/26.4 = 5 ).
To get a pack energy of the battery = 18000 wh, the above row of 5 modules connected in series should be connected in parallel for 14 times. ( 18000/132/10 ~=14).
therefore, we have 5*14 = 70 modules of 8s4p with which the battery pack is made. The battery now becomes 40s56p configuration
Mass of each module is now 8*4*0.076 = 2.432 kgs (only cell mass)
Total no. of cells used in the battery pack is 70 *32 = 2240 cells
Mass of battery pack( only cells) = 2240 * 0.076 = 170.24 kgs
The total nominal energy obtained by this battery pack is 40* 3.3 *56 *2.5 = 18480 kwh.~= 18kwh.
The additional energy can be used to compensate for the losses in the battery due to external environmental conditions. The margin can be designed according to the type of application, and in a controlled environment, this margin remains small.
The division is made for safety reasons as 132 V is a high voltage when handling and it is safer to divide it into modules which will help during repairing or handling the battery.
---Making the battery pack---
The battery pack should be made out of the materials that isolates the cells inside it from external conditions. The pack should have excellent mechanical properties , should be impact resistant, heat resistant, crack resistant, across various temperature ranges. The pack should be stiff to ensure the compactness.
Cell holders are used to keep the cells inplace in a module. The main function of it is to keep the cells fixed in place safely and securely and in a proper arrangement to decrease vibration, increase temperature distribution , by providing gaps between cells. The material used to make these holders are polypropelene or nylon.
Proper tolerances to fit cells in the holes and gaping between two cells is provided by these cell holders as shown in the figure.
An arranges 8s4p module would look like this
the cell cyclinders are joined as shown:
To connect the cells, nickel strips are used as they provide good balance between having low resistance and cost. Spot welding is the technique used to connect the nickel tabs to the cell.
5 such modules are conencted in series by connecting the negative to the psoitve terminals of consectuive modules. 14 of the above configurations are taken and positive terminals of all those are connected as one and negative terminals of all them are connected separately as one giving us the battery pack.
The entire pack is covered by using alumuinium or plastic to keep the pack storng and light. BMS and other control hardware is placed close to this battery pack.
Using the above configuration the volume for each module (just cells) is 8*26*4*26*65 = 1406090mm^3.
Total volume of battery assuming these modules are connected tightly = 98426300 mm^3
The volumetric efficiency of each module is, (32* 34510.39)/1406090 = 78.53%
CONCLUSION :
The battery pack has a voltage of 132 volts and 18kwh , which bring us to a design of 40s56p battery made by using 8s4p modular configuration where 5 of them are arrranged in series and 14 such arranegemnts are connected in parallel.