Conclusiones del capítulo

An adiabatic environment is one where heat is neither lost nor added to the system. Hence, the cell itself should always be at the same temperature as its surrounding. If the cell temperature rises, the temperature of its surrounding should also rise by the same amount. Shown in Figure 6-1, an extended volume accelerating rate calorimeter (ARC) made by Thermal Hazard Technology (THT) was used for this purpose. In the ARC, the temperature of the cell is continuously monitored and the temperature of its surrounding is changed to match it. This is done by a feedback control loop and a series of thermocouples and electric heaters. However, it should be noted that a perfect adiabatic environment is never obtained. Therefore, the quality of an adiabatic calorimeter relates to its ability to maintain the temperature difference between the sample under test and its environment as close to zero as possible [181].

Figure 6-2 shows a block diagram of the ARC system showing its components and explaining its operation. The electronic support unit (ESU) contains all the electronics for running and controlling the system. During operation the thermocouple signals from the calorimeter vessel are sent to the ESU. The accurate performance of the thermocouples is crucial for the operation of the ARC since they are used for temperature control. There are three thermocouples in the calorimeter vessel; side, base and top. The ESU compares the temperatures from these 3 thermocouples to

the temperature from the thermocouple on the cell. According to this, the heating power is adjusted so that the difference in temperature between the cell thermocouple and vessel thermocouples is almost 0 °C. The efficient operation of this system is the basis of adiabatic operation.

Figure 6-1: Photograph of the extended volume accelerating rate calorimeter used in the adiabatic overcharge test [181]

Figure 6-2: A block diagram of the accelerating rate calorimeter system showing its components and operation [181]

6.4.2 Experimental Procedure

The cells were divided into two groups. Group one consisted of cells overcharged in the ARC and group two were overcharged in ambient conditions i.e. non-adiabatic environment. All Cells were preconditioned to a fully charged state using the procedure in Section 4.5.2.1.

In order to overcharge the cells in an adiabatic environment, a Maccor cycler was used in combination with the ARC. Before putting the cell in the calorimeter, the current carrying wires were soldered to the cell tabs to ensure good contact. Inside the calorimeter vessel cells were attached to a metal mounting frame/rig by an aluminium tape. The metal was put above a thermal brick to provide thermal insulation between the bottom of the calorimeter and the cell for accurate temperature monitoring. The ARC thermocouple (N-Type) used to track the cell’s temperature was securely attached to the centre of the cell’s surface by an aluminium tape. Another two cycler

thermocouples (T-Type) were attached to the centre of the cell, this provided the synchronisation of the temperature data with the voltage and current. The calorimeter vessel is equipped with input ports to connect between the cell and the cycler. The maximum voltage limit of the cycler was 8.0 V. Figure 6-3 shows the set up inside the ARC vessel.

The start temperature at which the overcharge started was set to 20 °C and the wait step time was set to 5 min. The temperature rate sensitivity was 0.02 °C /min. The temperature step was set to 0 °C, so that the calorimeter did not heat the cell and only tracked the cell temperature under adiabatic conditions. The maximum heating capability of the calorimeter used was 15 °C /min. Table 6-1 summarises the main test parameters and their description.

Figure 6-3: Overcharge test set-up inside calorimeter vessel

Table 6-1: Main ARC set-up parameters for overcharge test

Test Parameter Value Description

Start Temperature (°C) 20 Temperature at which the cell is stabilised at the start of the test

Temperature Rate Sensitivity (°C /min)

0.02 Self-heating rate above which self-heating activity is detected

Temperature Step (°C) 0 Temperature by which the cell is raised after failure to detect self-heating

Wait Time (min) 5 Period of time during which calorimeter thermal transients are allowed to settle

For comparison, the second group of cells were overcharged in ambient conditions outside the calorimeter. The test temperature was 20 °C, the same as the starting temperature of the adiabatic overcharge and both groups were overcharged with the same regime. Different C-rates were used to overcharge the cells. Cells in adiabatic environments were overcharged with 0.13 C, 0.33 C and 1.3 C and cell charged in ambient conditions were overcharged with 0.13 C and 1.0 C. The overcharge regime started with a 30-minute rest step followed by a constant current charge until the voltage limit of the cycler was reached (8.0 V), then extended rest period to monitor the behaviour of the cell after the overcharge was terminated. It should be highlighted that the nonmatching C-rates, was due to the limited access to the test facilities. Table 6-2 summaries the overcharge steps and a screenshot of the programme is shown in Section (0 of the Appendix.

Table 6-2: Steps of the overcharge regime

Step No. Step Description Condition

1 Rest 30 min

2 Constant current charge 7.9 V

3 Rest 24 hr

4 End

6.5 Results and Discussion