Battery Thermal Model Testing, Working, and Parameter

Battery thermal model testing is all about safety. Many accidents have been caused when batteries overheat and then catching fire, exploding, or releasing of poisonous gases. This is a very common thing in lithium-ion batteries as compared to the others.

Back in 2018, a Tesla driver died after his car burst into flames. This accident was caused as a result of thermal runaway in the battery. Thermal runaway refers to an unstoppable chain reaction in a battery that causes a rapid rise of temperature within milliseconds. When this happens, all the energy that is stored in the battery is suddenly released.

The temperature of around 400° is created hence releasing gases and lastly, fire erupts. Convectional means can hardly extinguish battery fires. The risk is first identified when the battery temperature reaches 60°C and it becomes very dangerous at 100°C.

Lithium-ion batteries are very powerful but the question for safety has been raised several times. They can cause explosions that can lead to loss of life and damage to property. All these shortcomings have forced the industries to work on ways that will make them safer to be used by the general public. This is where the battery thermal model kicks in. A thermal model is a technique used to calculate the cell temperature of a battery.

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How do you test the battery thermal model?

Temperature is a very important factor when it comes to the safety, efficiency, and performance of lithium battery cells. Furthermore, its capacity is also influenced by the charging and discharging process apart from the environmental temperature alone.

Since it is difficult to measure the temperature of a battery cell, the nearby poles are measure and the result is reported by the BMS. However, it’s not accurate to predict the temperature of the battery based on data from the poles alone. Therefore, a thermal model is used to calculate and estimate the real temperature of the cell.

You can test a battery thermal model by using an RC-network to represent the thermal description used. The only challenge here is to identify the quantities of the same parameters to the R network. Therefore we are going to discuss into details how the thermal model works, and how to identify heat capacity parameters, electric contact resistances, and thermal resistances in the battery model poles.

This is an important topic for discussion because there has been a surge of electric vehicles, e-bikes, scooters, and hoverboards that largely depend on the lithium-ion batteries. Furthermore, they are the best option since they provide an optimal performance such as low discharge during storage, high energy density, and low memory effect. Not forgetting that they offer a high charge cycle.

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How does the battery thermal model work?

Although lithium batteries have a functional superiority over other batteries, they face many thermal issues. For example, explosion after high temperatures and thermal runaway are among the safety concerns while using lithium-ion batteries.

Because their safe temperature operation is very narrow there is a dire need to have an accurate prediction. Doing so helps largely when it comes to safety and maintaining battery longevity. Some studies show that the cells may experience a temperature difference of 10°C between the cells under certain conditions such as the HEV drive cycle.

During battery runtime, it's not easy to get the correct temperature readings. So the battery management systems make use of thermal models that help in predicting the core temperature. There are different types of models that exist such as reduced-order models and high fidelity thermal models. These two models are very essential when it comes to capturing the thermal dynamics of a lithium-ion battery. Reduced-order models are more suitable for onboard applications compared to the high fidelity models.

Keen observations are supposed to be made in order to determine the right thermal battery model to be used. Heat conduction is the main factor to be considered inside the core and the poles. Furthermore, the electro-active region of a cell has a heat conductivity of about 0.8 W/K.m so the core has heat conduction of 40?[W/K·m2]. If the propylene case 5mm thick then the heat conductivity will be 0.15?[W/K·m]. The poles and the core have the highest heat contribution hence the calculation is based on the ratio of both the inside and the outside.

What is the battery thermal model parameter identification?

Thermal model parameter identification is a procedure that is used to determine the thermal model of the battery substitution parameter. This must be based on the input and output signal i and T respectively. The current load is I while T represents the temperature for Aluminum or copper. To minimize the error between the model and the real process the model parameter is optimized.

In parameter identification, high degree parameter optimization algorithms are applied in toolboxes and development. The following requirements have to be fulfilled by the made in order to apply a parameter identification approach

1. Sustainability of the model- It ensures measurement can be carried out

2. Control- The model has to be controllable to enable state stimulation and identification

3. Observable- The model should be observable to allow the reconstruction of states from the input and output measurements.

Kalman criteria are used to prove the properties of observability and controllability. After parameter identification, thermal model analysis has to be done. This involves comparing the actual process with the theoretical model.

Summary

Lithium-ion batteries are with no doubts the most powerful. They can store more power, have a longer life cycle, and are very efficient. However, there are still many thermal concerns associated with these batteries. When they overheat they become dangerous because they release poisonous gases or burst into flames. Battery thermal model testing is used to calculate and estimate the most accurate temperature of the cells. At 40°C, batteries are said to be overheated and dangerous when the temperature amounts to 100°C. Battery management systems cannot report accurate temperature without thermal model testing because there is a big difference in temperature between the cells and the poles.