Project 1
Aim: Design a battery pack for a car.
Objective:
- Design the battery pack configuration.
- Draw the BMS topology for this battery pack.
Solution:
To design the battery pack we need cells which are connected in series and parallel to meet the requirment.
The required condition = 220v and 50Ah.
Given
Nominal voltage = 3.6v
Capacity = 3000mAh = 3Ah
To meet the requirment of 120V and 150Kw, we need to find the number of cells connected in series and parallel.
First convert the battery capacity 150Kw to Ah
Amper hour = Watt hour / Voltage
Amper hour = 150*1000 / 120 = 1,250 Ah
In Series:
Number of cell is Series = Required Voltage / Nominal Voltage of the cell
Number of cell is Series = 120 / 3.6
Number of cell is Series = 33.3
We have to round off to lower value and higher value.
Case 1: Rounding off 33.3 to 33
1) Nominal battery voltage = Cell nominal voltage * number of cells in series
= 3.6 * 33
Nominal battery voltage = 118.8 V
Assume:
Cell maximum voltage = 4.2 V
Cell minimum voltage = 2.7 V
2) maximum battery voltage = Cell max voltage * number of cells in series
= 4.2 * 33
maximum battery voltage = 138.6
3)minimum battery voltage = Cell min voltage * number of cells in series
= 2.7 * 33
= 89.1
Case 2: Rounding off 33.3 to 34
1) Nominal battery voltage = Cell nominal voltage * number of cells in series
= 3.6 * 34
Nominal battery voltage = 122.4 V
Assume:
Cell maximum voltage = 4.2 V
Cell minimum voltage = 2.7 V
2) maximum battery voltage = Cell max voltage * number of cells in series
= 4.2 * 33
maximum battery voltage = 138.6
3)minimum battery voltage = Cell min voltage * number of cells in series
= 2.7 * 33
= 89.1
Rounding off the number of cells in series
we can pick case 1 which is 33 cells in series
Output total volatge will be 118.8V
Number of cells in Parallel:
Number of cells in parallel = required capacity / cell capacity
Number of cells in parallel = 1,250 / 3 = 416.6
We have to round off to lower value and higher value
Case 1: Rounding off 416.6 to 416
1) Nominal battery Capacity = Cell Capacity * number of cells in parallel
= 3 * 416
Nominal battery capacity = 1248 Ah
Case 2: Rounding off 416.6 to 417
1) Nominal battery Capacity = Cell Capacity * number of cells in parallel
= 3 * 417
Nominal battery capacity = 1251 Ah
Its alway recommanded to to discharge the battery for continouos current in 1C.
Continuous current Requirement = 1250Ah,
So we will choose Case 2
We can round off to higher value = 417
The number of cells in Parallel = 417
The Output Capacity will be 1,251Ah
The Current can be limitted to requied value bu using current limitters.
Now the battery pack is made of 33S417P pack so that it can meet the required output of 120v,1250Ah or 120V 5Kw
As the number of cells in series and parallel are very high which makes manufacturing difficult. So, we need to divide it into sub battery packs and remake the connections to get the requried output
As the BMS Supports the max 18 cells in series so we need to divide the 33 sereis cells to 18 cells series sub battery pack which leads to 2 series sub connections one with 18 and another with 15
To make it compact parallel connectons we need to make 52 cells in parallel for each sub pack which leads to 8 parallel connections
Total result in 2 sub battery packs in sereis one with 18 cells in series and another with 15 cells in series and 8 sub battery pack in parallel each with 52 cells in parallel and any one with 53 to meet the requirment 33S417P.
One sub battery pack configuration is 18S52P and another sub battery pack configuration is 15S52P which results in 33S52P pack with 8 copies. these 8 33S52P are connected in parallel (One of the pack with 33S53P) to make 33S417P.
Basic connections block diagram:
Back line indicates the series connection
Blue Line indicates the Parallel connection
Each 18S52P pack is connected in series with the each 15S52P pack results in 33S52P
All 8 33S52P(18S52P+15S52P) are connected in parallel results in 33S417P
Note that last pack in parallel connection contains as 18S35P series with 15S53P pack, one parallel connection is increased in that sub pack to meet the Requirement of 33S147P
Battery Management Sytem (BMS):
BMS Topologi used in this battery pack is Master and Slave BMS Topologi. Because the total battery pack is made of sub battery packs so controlling and manufacturing will be easy.
Master and Slave BMS which has number of slave controllers are 16 and a Master controller which can handel 16 slaves.
It consisting of input sensors of volatge,current and temperature. The voltage sensor analog signal is converted to digital signal by AFE, the current sennsor analog signal is converted to digital signal by the MCU or we can connect the ADC converter between the Current sensor and MCU, the temperature sensor analog signal is converted to digital signal by the MCU or we can connect the ADC converter between the Temperature sensor and MCU. The Realy module control the Realy which make or brack the circuit by the command given be the MCU. The CAN and RS485 are Communication protocalls to the Outside world.
The Blocks inside the red line are the BMS blocks. It isdesigned with the three 18 cell Slave controller which is AFE, it converts the anolog signal from voltage sensor in the battery to Digital signal. The 3 Slaves are connectes in seres as shown, the salve-1 will collect the data feom module-1 and send to slave-2, slave -2 will collect the data from module-2 , now the salve -2 consisting the data of slave-1 and 2 and it sends the both the data to slave-3, the save-3 will collect the data from module-3, now the slave-3 consisting the data of slave-1,2 and 3, like continues for all 16 slaves and at last all the data is send to Master controller which is MCU. It control the battery pack according to the protocals which is given to MCU.
Note that all the slave have a capacity of 18 cell series connection even though half of the sub battery packs have only 15 series cell besuse if we want to upgrade the capacity we can do by increasing Ah of battery pack i.e., by incresing the number of series cells. It makes easy to upgrade the battery pack but in a limitted way.
We can also use the combination of 8 Master and slave BMS i.e., 2 slaves 18S52Pa and 15S52P and a master controller of 8 numbers. For all master control will have one single controller or all the 8 master controllers can directly connected to the MCU. But its costly and complicated process.
Conclusion:
The battery pack was made to design as 33S147P divided into sub battery packs which is combination of 18S52P series with 15S52P, this combination is paralled with 8 copies as shown above toatl results in 33S147P with capacity of 150Kw with 120V.
To charge and discharge the Total battery pack Master and Slave BMS topology is used which is efficient in all ways like cost, design,control and installation.