Project 2 Thermal modeling of battery pack

THERMAL BATTERY MODELLING:

With the functional superiority of Lithium batteries over most of its other counterparts, it is undoubtedly a subject of extensive study. Thermal issues with these batteries, like having a high potential for thermal runaway and explosion under high temperature, always threaten the operational safety. Owing to their narrow operating temperature range, an accurate prediction of battery temperature is absolutely essential so as to maintain the longevity, performance and safety of these Li-ion batteries. Studies have shown that, under typical conditions (such as a HEV drive cycle), the cells may experience a temperature difference between the core and the surface of 10°C or more.

Since it is not feasible to measure the temperature of the battery core during runtime, the battery management systems typically use thermal models to predict the core temperature. Existing models range from high fidelity thermal models to reduced order models capturing the lumped thermal dynamics of the cell. However, the high fidelity models due to their computational complexity are not suitable for onboard application.

Effect of heat in Battery pack:

When temperatures increase this affects the chemical reactions that occur inside a battery. As the temperature of the battery increases the chemical reactions inside the battery also quicken. At higher temperatures one of the effects on lithium-ion batteries’ is greater performance and increased storage capacity of the battery. It found that an increase in temperature from 77 degrees Fahrenheit to 113 degrees Fahrenheit led to a 20% increase in maximum storage capacity. However there is a side effect to this increased performance, the lifecycle of the battery is decreased over time. In that same study, it was found that when the battery is charged at 113 degrees versus 77 the lifecycle degradation was much more significant at the higher temperature. For the first 200 cycles the battery performance only degraded 3.3% at 77 degrees; at 113 degrees the performance decreased by 6.7%. That’s more than double the amount of degradation! Based on the greater degradation at higher temperatures, the battery lifecycle can be severely diminished due to consistent exposure to extreme heat. While heat exposure does temporarily increase battery capacity the damage that it does to the lifecycle can cause long term problems and prolonged heat exposure should be avoided.

THERMAL MODEL OF 10 SERIES CELL BATTERY PACK :

As there are 10 battery (Table based) connected in series. In these 10, first 4 and last 4 has good ventilation for air flow for cooling. But the middle cells 5 and 6 are not provided with ventilation so that there is less chance for the temperature to get out. So proper cooling (air (or) liquid)is required to avoid thermal run away. 

SUBSYSTEM 1 & SUBSYTEM 3:

  The 4 li-ion cells are connected in series with temperature around 298.15K and as initial we are giving discharge current so initial SOC is 1 or 100%.  Similarly SOC and aging port is ON so that Temperature port and SOC port is Alloted.

SUBSYTEM 2:

Here cell 5 and 6 are connected in series such that they have different properties compared to other series cells. They will require a proper heat removal system like circulation tube of liquid or air through it.

  As mentioned above the capacity of these 2 cells are reduced by 4% due to thermal effect. Similarlly terminal resistance is increases by 5% as cells has positive temperature coefficient (PTC) which means as temperature increases resistance also increases parallely. If the temperature of other cell is 298.15K then here the temperature is increased by 15% as they has the thermal effect of other cell from 1-4 and 6-10 on 5and6 cells through conduction and convection.

THERMAL SYSTEM:

There will both conductive and convective heat transfer to the battery or cells.

CONVECTIVE HEAT:

The movement of fluid molecules from higher temperature regions to lower temperature regions.

As the temperature of the liquid increases, the liquid’s volume also has to increase by the same factor and this effect is known as displacement. The equation to calculate the rate of convection is as follows:

Here the parameter that decided are area of contact of heat and heat transfer coefficient. As in case of middle cells both these parameters are bit higher as it is surrounded with other cells. The parameter values for cell 1-4 and 6-10 is as follows:

 So Area of contact is 1m^2 which is less in case of cells which is available in the edges. Also heat transfer coefficient is bit low as 20. In case of cell 5 and 6 area of contact is 10m^2 and also heat transfer coffient is bit higher as 25. As here 5 and 6 is surrounded with other cell so that the heat of these cells will also affect others cells Similarly as it is in the middle of series it has more area of contact too.

 CONDUCTIVE HEAT TRANSFER:

The process in which heat flows from objects with higher temperature to objects with lower temperature.An area of higher kinetic energy transfers thermal energy towards the lower kinetic energy area. High-speed particles clash with particles moving at a slow speed, as a result, slow speed particles increase their kinetic energy. This is a typical form of heat transfer and takes place through physical contact. Conduction is also known as thermal conduction or heat conduction.

We should provide conductive heat transfer block which has the parameter of thermal conductivity , area etc. We will take default value here.

AMBIENT TEMPERATURE SOURCE:

Here we will provide a temperature source which will provide the required temperature to the cells to know to know the thermal effect  and also change in SOC rate. We should not between the convctive heat transfer block as that may lead to obstructing the heat flow through it. So we shlould go for a referance.

TEMPERATURE SENSOR:

This shows the sensor with referance.

Here we should use a thermal sensor to sense the temperature such that  with respect to the reference from the source. Also the output of these block is put into the scope to the veiw the thermal variation.

CONTROLLED CURRENT SOURCE:

Controlled source is fed into the 10 series cells set up which is a current source. Also initillay the battery is in discharge mode so we will consider the arrow toward downwards.

Effect of ageing on battery degrad:

As Q_m gradually decreases with an increasing number of cycles as expected. The degradation mechanisms for this irreversible capacity loss with cycle aging are found to be related to one or more of the following, namely structural changes of the insertion electrode, electrolyte decomposition, active materials dissolution, phase changes in the insertion electrode and passive film formation over electrodes and the current collector surface.

Effect of aging on the electrolyte:

As temperature increases from 25 °C to 55 °C, the diffusivity of active Li-ions in the electrolyte increases and the Li-ion concentrations that flows through the electrolyte also increases due to the increase in Q_m as a result of the enhanced electrochemical reduction-oxidation (redox) at anode and cathode at elevated temperature, thus a decrease in the resistance of the electrolyte is expected when the cell is initially cycled.

Initially we will use controlled current source input as which is UDDS cycle with run time of 1369 seconds. This UDDS cycle is uploaded in the signal builder.

 AT T=295K

SOC of the battery pack 1 and 3 will reduce together as they are at the edges. But the SOC of battey pack 2 will reduce at faster ratethan others as seen above. Also the temperature is also fluctuating because of varying drive cycle of both acceleration and deacceleration (positive and negative current). Negative current accounts to regenrative charging of battery pack. So the temperature of battery pack 2 is rising at a faster rate as its discharge is faster than other battery pack. This increase in temperature will make the adjacent cell to thermal run away. This phenomena can be avoided by provideing proper cooling system.Initially the temperature is almost the same, but later on the due to increase in heat transfer coefficient and les area of contact it causes increase in temperature steeply.

 AT 315k:

The temperature is also fluctuating due of varying drive cycle of both acceleration and deacceleration (positive and negative current). Negative current accounts to regenrative charging of battery pack. So the temperature of battery pack 2 is rising at a faster rate as its discharge is faster than other battery pack. This increase in temperature will make the adjacent cell to thermal run away. This phenomena can be avoided by provideing proper cooling system.

Now if we consider a constant current of 4.2A as input current which is 1C rating

Here we have given 1C which should take 3600 seconds but due to its thermal property it gets discharged bit faster to 3393 seconds. That is the battery pack 2 will discharge at a faster rate than the other pack 1 and 3 so the SoC of 1 and 3 are not reached to 0 which means full capacity is not used in 1 and 3 battery pack.Also temperature of all the battery packs are increasing but temperature of pack 2 increases more linearlly, but pack 1 and3 increases exponentially.

Now we can go with cyclic input:

The cyclic input block is given with frequency =1/600 ao total time is for 600 seconds also we will give 4.2A that will take complete charge and discharge by 600 seconds.phase shift is zero so that there is no phase shift between any two curve.

Here also the temperature is high for battery pack 2 which is cyclic form due to continuous charging and discharging. With 4.2A current the discharge was taken upto 0.96 which again start to increase charge upto 1. This process is continued.