Project

Modelling of Battery Pack

OBJECTIVES:

Battery configuration:

The lithium-ion datasheet used for this project is:

Data used to configure the battery:

According to the above data, `V_exp` and `Q_exp` can be determined by looking at the discharge characteristic curve:

3S4P SIMULINK MODEL:

3S4P refers to 3-series-4-parallel: as is evident, 3 cells are connected in series (+ve terminal connected to -ve terminal) while the remaining are connected in parallel (+ve terminal connected to +ve and -ve terminal connected to -ve)
The fully charged voltage of one cell is 4.2V, hence 3 cells connected in series would mean a total voltage of around 12.6V, which is evident from the voltage measurement
The maximum capacity of each cell is 2.6Ah so 4 cells connected in parallel means a total capacity of 2.6*4 = 10.4Ah

How to ensure that the total pack is providing a capacity of 10.4Ah?

The battery is configured to discharge a nominal current of 0.52A, which means if the current is assumed to be 0.52A on average, an approximation on how long this battery will last can be estimated.
10.4Ah/0.52A = 20 hours, however this is assuming the discharge current is 0.52A throughout, which cannot be the case all the time as the drive cycle requirements demand a higher current at times. Higher current discharge brings in Peukert's effect and the capacity of the battery is reduced.
So I ran the UDDS drive cycle for 20 hours to find out when the SOC will become 0. Approximation for when SOC would deplete to 0% was around 18 hours.

Evidently, SOC depleted to 0% before 63000 seconds, which is about 17.5 hours. Hence, it can be concluded that this pack was indeed constructed in the correct way to provide a capacity of 10.4Ah
There might be an easier way to confirm that the cells have been connected correctly to give the expected capacity, but as of now I'm unaware of it.

While the battery completely depletes from 100 to 0%, the voltage levels have gone from 12.6V to about 8.5V
Note how this voltage discharge curve matches exactly with a Li-ion discharge curve; starting off with an exponential zone, into the nominal linear zone, and then the steep zone, where in the final 5% SOC the voltage has drastically reduced from 12.4V to 8.5V

4S3P SIMULINK MODEL:

As opposed to the first model, 4 cells are connected in series and 3 in parallel. Same total number of cells, but arranged differently, giving different voltages and capacities
The fully charged voltage of one cell is 4.2V, hence 4 cells connected in series would mean a total voltage of around 16.8V, which is evident from the voltage measurement
The maximum capacity of each cell is 2.6Ah so 3 cells connected in parallel means a total capacity of 2.6*3 = 7.8Ah

Error check:

To make sure this modeled battery pack is providing a capacity of 7.8Ah, the same procedure was repeated, where this time the UDDS drive cycle was run for 7.8Ah/0.52A = 15 hours to see the change in SOC. Approximation for when SOC would deplete to 0% was around 13 hours.

Evidently, SOC depleted to 0% before 47000 seconds, which is about 13 hours. Hence, it can be concluded that this pack was indeed constructed in the correct way to provide a capacity of 7.8Ah

While the battery completely depletes from 100 to 0%, the voltage levels have gone from 16.8V to about 12.7V

COMPARISON OF BOTH MODELS: 3S4P vs 4S3P

The UDDS drive cycle (current vs time) was run (1369 seconds) for both battery packs:

3S4P BATTERY PACK:

4S3P BATTERY PACK:

	3S4P	4S3P
SOC	Decreases to 97.82% Has lower DOD because capacity is higher (10.4Ah)	Decreases to 97.1% Has higher DOD because capacity is lower (7.8Ah)
Current	Right before 200 sec, the drive cycle demands a current of about 8A, but this configuration can discharge a peak current of 2A The entire current curve follows the drive cycle (which is a current vs time graph). However, from the comparison, it is evident that the drive cycle requirements exceed what the battery pack can provide	This configuration at the same point can discharge a peak current of about 2.7A The same thing is happening here, except that the battery pack can discharge a little more current due to greater voltage. Hence, current amplitudes are higher than the 3S4P pack
Voltage	Lower voltage due to only 3 cells connected in series Everytime there is an increase in current demand, the voltage decreases as the pack needs to discharge higher current. Everytime the current goes negative i.e. during steep deceleration of the vehicle, it means the battery is being charged and that increases the voltage. Hence, the graphs of current and voltage are totally opposite of each other at all points.	Higher voltage due to 4 cells connected in series Same voltage theory applies as stated in 3S4P

Ship work like this
in your own course.