Project 1

Mechanical design of battery pack
Objective : To design a 250W Lithium Ion battery pack for an e-cycle.
Design calculations :
Mechanical Design:
1. The application selected for the use of battery pack is an e-cycle.
2. According to ARAI, the max power transmitted from the controller to the motor should be 250W
incase of low speed electric cycles.
3. Considering the restriction on power requirement, a 280Wh pack design was selected.
4. So, being a application with max discharge current being 1C, NMC 18650 cells were selected
for the application.
5. The pack should be lightweight, with excellent operational characteristics, hence cell holders
of material ABS plastic have been selected.
6. So, according to the power, the pack voltage was designed to be 36 V, which was achievable
by using 10 cells in series. And the capacity being 7.8Ah, was achieved by using 3 cells of 2.6
Ah in parallel.
7. The cells were later connected to each other with help of 0.5mm* 10mm nickel strip.
8. The nickel strip is usually spot welded to the cells for cost effectiveness and ease of operation.
Electrical Design :
1. BMS was decided to place at one of the end of the pack, keeping temperature sensor in the
core of the pack.
2. BMS would protect the pack from over currents, over voltage, voltage imbalance and
temperature imbalance.
3. BMS should be rigid and not dislocate during vibration, hence proper packaging has been
done.
4. Natural cooling mechanism hs been selected for cooling of the cells.
Following are the CAD developed in PTC Creo
1. 18650 cell :
2. Cell holders :

Complete assembly of battery pack :
Isometric view :
Top view :
Side View :
Simulation of a BMS
Objective : TO simulate voltage imbalance in a battery pack using a closed loop BMS model.
Description :
Lithium-ion battery packs are the widely used energy storage systems in electric vehicles. The battery
management system (BMS) is responsible for safe operation, performance, and battery life under
diverse charge-discharge and environmental conditions. When designing a BMS the cell voltage and
temperature, state of charge, state of health and the charging profile has to be controlled and
maintained.
Also, it indicates whether there has been a fault, for example that any cell has reached an overtemperature condition or under-voltage condition (Under-voltage occurs when the average voltage of
a system drops below intended levels. It is usually caused by undersized or overloaded utility).
Modelling :
Following is the BMS closed loop model which consists of a BMS ECU(electrical control unit), and a
plant model.The control unit with its various monitoring and control algorithms is connected to a block
with the representation of the battery pack and associated circuitry and peripherals.
A battery pack with 6 cells in series have been used to study the voltage, current, temperature and
SOC behaviour.
The configuration is 6S-1P.
Following is the BMS ECU model :
State Machine defines the main operating state of the BMS. The state machine has four parallel states
(parallel meaning that they can be active at the same time):
a.) The first one defines the variable BMS_State which indicates standby, driving, charging, and fault.
Charging comprises constant current and constant voltage stages.
b.) The second state switches the fault state ON in case a current, voltage, or temperature value
reaches an unsafe level.
c.) The third and fourth states define the contactor ON and OFF switching sequence for the charger
and inverter. This is needed to avoid an excessively large current inrush at the beginning of the
charging stage.
Next, a harness dashboard has been inserted and the voltage has been set.
Further, we select balacing, to check the under voltage.
Lastly, the model is simulated and results are obtained.
Results :
As we can see,
During the charging of the battery after finishing the drive cycle. There is an imbalance in the voltage
as well as an imbalance in the temperatures too.
The 1st cell has higher temperature than the 6th cell. Due to which in run time, the 1st cell will degrade
faster and hence cause a thermal run away in the whole battery pack.
The cell voltage seem to be in imbalance during charging, but settle down to a single value as the
charging is stopped and current is 0. Hence the balancing of the pack works successfully.
Thermal modeling of battery pack
Objective : To study the behavior of EV battery cooling system under three conditions :
1. FTP75 drivecycle
2. Continuous constant current charge cycle
3. Pulse charging current cycle
Description :
This study shows an Electric Vehicle (EV) battery cooling system. The battery packs are located on
top of a cold plate which consists of cooling channels to direct the cooling liquid flow below the battery
packs. The heat absorbed by the cooling liquid is transported to the Heating-Cooling Unit.
The Heating-Cooling Unit consists of three branches to switch operating modes to cool and heat the
battery. The Heater represents an electrical heater for fast heating of the batteries under low
temperature conditions. The Radiator uses air-cooling and/or heating when the batteries are operated
stably. The Refrigerant system is used for cooling the overheated batteries. The refrigeration cycle is
represented by the amount of heat flow extracted from the cooling liquid.
The system is simulated under either FTP-75 drive cycle or fast charge scenarios with different
environment temperatures.
Modelling :
Following is the complete model for the study.
It consists of a battery connected to a cold plate via thermo sensors.
A heat exchanger pump, that can circulate the fluid to cool/heat the liquid into heating-cooling unit.
A heat-cool unit to vary the temperature of the liquid and typically maintain it in the range of 22-25
degree celcius.
Environment to act as a heat sink and release the extra energy to the environment.
1. Battery :
It consists of 4 battery packs modules each with 20 cells connected in series with pack voltage of
72V/pack and capacity of 28Ah/pack.
Hence the nominal voltage of complete pack is 288V and 28Ah.
Thermo sensors are connected to the battery pack modules to sense the temperature.
2. Heating – cooling unit :
There are three types of mechanisms used for cold, normal and hot conditions.
The heater gets activated when the battery temperatures fall below the operational limit of the pack,
and the battery is heated, generally this case occurs in cold snowy regions.
The Radiator is used to circulate the liquid during the normal temperature conditions of around 25
degree celcius, and during the normal driving conditions.
The refrigerant system is activated during generation of excess heat and cool down the battery packs,
this can typically be seen during aggressive driving, fast charging or pulsating charging.
Simulation :
Case 1 :
FTP75 drive cycle : A FTP 75 drive cycle is generated using the signal block and the current generated
in turn is fed to the battery pack. And the system is simulated, for the response.
Result :
As we can see as the vehicle is driven, the temperature of the system rises, due to which the pump
gets activated and the refrigerant cooling system is used to cool down the liquid and lower the
temperature of battery pack.
As soon as the temperature is lowered, refrigerant is cutoff, and pump is stopped until the temperature
rises again and so on.
Following is the heat energy distribution and cooling distribution :
Case 2 :
Continuous current charge cycle :
A continuous constant current of 100A is applied to the system and response is checked.
Here we can see that, during a constant current, the pump is continuously on to dissipate the heat by
circulating refrigerant liquid.
Following is the heat energy distribution and cooling distribution :
Case 3 : Pulse charge current cycle.
Here a pulse current from 150 to 100 has been induced to check the system response.
Here, there are fluctuations in the operation of the refrigerant action, being a pulse signal.
Following is the heat energy distribution and cooling distribution :
Conclusion :
The cooling system can cool the pack dring a specific current consumption of 100A, when the current
is drawn more than 100A, the heat generated is more than the heat dessipated. Hence the battery
pack heats and cannot maintain the optimal temperature of 22-25 degree celcius.
Hence, its necessary to limit the C rating during discharging and avoiding fast charging / chargers of
high power to keep the current drawn in limit.
Steady state flow simulation over a throttle body in a channel
using converge
Objective :
To perform a steady-state simulation of flow across a throttle body in a channel using Converge
CFD and post-process the simulation using Paraview.
Geometry set-up :
The working fluid is atmospheric air. The inlet of the elbow is subjected to a total pressure of 1.5
bar and the outlet static pressure is 1 bar. The air enters from the top and exits from the bottom.
The throttle body is a separate body and does not intersect with the elbow surface at any point. It
is in open position, with no deflection.
Fig 1 : Isometric view of geometry
Mesh generation with embedding :
The grid was set up with fixed embedding on the throttle body boundary with a scale of 2 and of
embedding 2. So scale determines the size of mesh and the embedded layers setup the number of
layers. A scale of 2 signifies that the size of the mesh will be 8 times smaller than the original
mesh. and the embedded layer 2 means that there will be 2 layers of mesh near the throttle body.
Fig 2 : Base mesh with embedding
Fig 3 : Emedded region near the throttle body
Post processed results :
1. Velocity contour
Fig 4 : velocity contour
Here we can see that, the maximum velocity is near the edge of the throttle body, and minimum
velocity is at the back side of the throttle, where there occurs drag of the flow.
2. Pressure contour
Fig 5 : Pressure contour
Here we can see that the pressure results are opposite to velocity results, the maximum pressure is
at the front side of the throttle and minimum pressure is at the edge of the throttle body.
3. Cell count
Fig 6 : Cell count
The cell count observed is 16,000
4. Mass flow rate
Fig 7 : Mass flow rate
Here we can observe that, initially there's a change in the rate at the throttle body, and after 4000
cycles approximately, it reaches steady state. The inlet and the outlet mass flow rate are
same(0.065kg/s), only it differs in sign. These means that the total mass flow rate has been
conserved.
4. Velocity plot
Fig 8: Velocity plot
Here we can observe that, the inlet velocity is around 195m/s and outlet is 220m/s, which implies
increase in velocity. This increase is due to the change in the pressure and the pressure plot
validates it.
5. Total pressure
Fig 9 :Total Pressure
The total pressure in the channel, at inlet is 1.5 bar, and it reaches steady state at 1.35 bar at outlet.
So, There's a pressure drop of 0.15 bar. Total pressure = Dynamic pressure + static pressure. So,
if the static pressure drop is more than total pressure, it means the dynamic pressure drop is
positive.
LHS = - 0.15 bar... (Total pressure drop)
RHS = -X - (-Y)... ( Static pressure drop) + Dynamic pressure drop
6. Static Pressure
Fig 10 :Static Pressure
The static pressure at inlet is 1 bar and it is 1.2 bar at outlet, signifies an increase of 0.2 bar. Hence it
is a pressure increase, which validates, the total pressure data. Hence,
-Total pressure drop = -0.2 + Y
-1.5 = -0.2 + Y
Y = 0.05 (positive)
This signifies that dynamic pressure has increased and is reponsible for velocity increase at outlet too.