Project 1

1.a) In general to design a battery pack,

We should find number of cells connected in series and parallel.

Assuming,

Minimum cell voltage = 2.7 V

Maximum cell voltage = 4.2 V

Given,

Nominal cell voltage = 3.6 V

Nominal cell capacity = 3500 mAh = 3.5 Ah

Continuous current = 150000 / 120 = 1250 A

Total voltage = 120 V

The number of cells connected in series is found by the following formula,

No of cells in series = Total voltage/Nominal cell voltage

= 120/3.6 = 33.3

Since the obtained no of cells is in decimal, we need to round off it. To round off the following steps are followed:

Case 1: Rounding off to the lower side ie., to 33,

• Nominal battery voltage = Cell nominal voltage *33 = 118.8 V
• Maximum Battery voltage = Cell maximum voltage *33 = 138.6 V
• Minimum Bttery voltage = Cell minimum voltage *33 = 89.1 V

Case 2: Rounding off to the higher side ie., to 34,

• Nominal battery voltage = Cell nominal voltage *34 = 122.4 V
• Maximum Battery voltage = Cell maximum voltage *34 = 142.8 V
• Minimum Bttery voltage = Cell minimum voltage *34 = 91.8 V

Now comparing case 1 and case 2 with the given data, it is rounded off to 34 cells.

Hence the no of cells connected in series is 34.

The number of cells connected in parallel is found by the following formula,

No of cells in parallel = Continuous current /Cell nominal capacity

= 1250 / 3.5 = 357.1

Since the obtained no of cells is in decimal, we need to round off it. To round off the following steps are followed:

Case 1: Rounding off to the lower side ie., to 357,

Battery capacity = No of cells in parallel * Nominal cell capacity

= 357 * 3.5 = 1249.5 Ah

Case 2: Rounding off to the higer side ie., to 358,

Battery capacity = No of cells in parallel * Nominal cell capacity

= 358 * 3.5 = 1253 Ah

Now comparing case 1 and case 2 with the given data, it is rounded off to cells.

Hence the no of cells connected in parallel is 358.

Hence the battery pack configuration will be 34S358P.

Since the battery pack has is more than 18 cells in series, the BMS topology selected will be Master and slave topology.

1.b) The BMS topology for the above battery pack design is as follows:

2) To build a 3S2P battery pack configuration with generic battery block using Matlab/Simulink the folllowing steps are followed:

From the given configuration we can conclude that 3 cells should be connected in series and 2 cells should be connected in parallel. Therefore the total no of cells in the battery pack

= No of cells in series *? No of cells in parallel

= 3*2

= 6 cells

From the above datasheet the following are considered:

Nominal cell voltage = 3.7 V

Nominal cell capacity = 2.6 Ah

Maximum cell voltage = 4.2 V

The Simulink design for the given configuration is as follows:

The results obtained are as follows:

Nominal voltage of the battery pack

= No of cells in series * Nominal cell voltage

= 3 * 3.7 = 11.1 V

Rated capacity of the battery pack

= No of cells in parallel * Rated cell capacity

= 2 * 2.6 = 5.2 Ah

The model is for an 11.1 V, 5.2 Ah battery.

The above graph represents the cell SOC %, voltage, current.

Overall voltage of the battery pack

= Max cell voltage * no of cells in series

= 4.2 * 3

= 12.6 V

The above graph shows the overall voltage of the battery pack and the value is verified as shown in the calculation.