2.1. Temperature
Battery capacity tests were performed at different temperatures to determine the effect of the temperature on the battery and to establish the boundaries of the fuzzy logic controller. Aspilsan brand 18650 cylindrical Li-ion (NMC) batteries were used in the experiment phase (
Figure 1). The reason for choosing these batteries was to support Türkiye’s domestic and national battery production. In addition, the possibility of TOGG— the country’s domestic EV—using these batteries in the future is also considered. In this context, these batteries are used in this study to encourage domestic battery production and support the domestic EV industry. The data obtained in this context will provide an important contribution to EV manufacturers who will use this brand of battery.
To monitor the behavior of the battery at different temperatures, battery capacity experiments were performed at 10 °C, 20 °C, 30 °C, 40 °C, 50 °C, and 60 °C using a water-circulating cooling bath (
Figure 2) and drying oven (
Figure 3). Due to the change observed between 40 °C and 50 °C, battery capacity experiments were performed at 43 °C and 45 °C to more accurately investigate this range.
Discharging at 1C at eight different temperature values was carried out with the DL24P discharge device, and the results are given in
Figure 4. Different batteries were used at each temperature, and the tests were started after the batteries were discharged at 1C and charged at 1C as they left the factory.
Figure 4 shows that the highest capacity value was observed at 43 °C. Considering battery heating during the use of EVs, the battery operating temperature was determined to be between 0 °C and 45 °C. The reason for setting the battery temperature in this range is that if the battery goes out of the ideal operating range, the charging current to be obtained from the energy obtained from regenerative braking will be directed to the SC instead of the battery. Using batteries for charging at low or high temperatures causes permanent capacity losses in the battery [
17]. Considering the results obtained experimentally, although the Aspilsan company states that this model battery can operate up to 60 °C in the catalog of use, the temperature is limited to 45 °C to prevent the temperature increase caused by regenerative braking in the battery within the scope of this study.
2.2. Battery Charging Current
A factor affecting both the battery capacity and battery life is the determination of the values between which the battery charging current will be. The charging or discharging rates in the batteries are symbolized by C, and this rate is defined as, for example, the ability of a 1 Ah battery to maintain a current of 1 A for 1 h. Discharge processes at different values of the Li-ion battery were performed experimentally. In this way, the limits of the discharge current to be used in the fuzzy logic controller were determined. The batteries used in the experiment are offered to the user at a 30% discharge rate from the factory. Operating between 4.2 V and 2.5 V voltage values, these batteries were first discharged at 1 C and then charged at 1 C. Discharging at different C ratios was performed at 43 °C with a DL24P discharger capable of discharging between 0.1 A and 30 A. The battery charging current experiments were performed at 43 °C, and the highest capacity was obtained at this temperature (
Figure 5).
In this test setup, discharges were performed at C/10, C/5, C/3, C/2, C, 2 C, 3 C, and 5 C ratios. The Aspilsan 18650 NMC battery used in this study has a capacity of 2900 mAh. This capacity value of the battery means that it can meet 2.9 A for 1 h. While charging and discharging processes have a positive effect on the battery’s capacity at low C rates, at high C rates, some of the energy is converted into heat and causes capacity loss on the battery. The evolution of the capacity as a function of the discharge rate is given in
Figure 6.
The battery capacity decreased by around 5% of its capacity when the C ratios increased from C/10 to 5 C, while the heating increased from 43.2 to 93.6 °C (
Figure 7). The breaking point of the heat increases is observed above 0.5 C. At high C ratios, some of the energy during the charge/discharge process is converted into heat energy, decreasing the battery capacity.
In a fuzzy logic controller, the battery parameters used in an EV and the proposed SC parameters are needed to determine the constraints. The EV battery and motor parameters used to determine the constraints are given in
Table 1.
There are 95 series-connected battery cells and 48 parallel-connected battery cells in the EV battery pack. With a motor power of 160 kW, the maximum current to be drawn from the battery cell of the EV is 8.7 A. The maximum current value to be drawn from the battery cell corresponds to the 3 C ratio. Formulas (1)–(4) are calculated for transferring the power gained from regenerative braking to the HESS. (1) is the moment of inertia, (2) is the aerodynamic drag resistance force, (3) is the rolling resistance force, and (4) is the power transferred to the HESS.
where
is the rim weight (kg),
is the rim radius (m),
is the wheel weight (kg), and
is the tire sidewall thickness (m).
where
is the air density (kg·m
3),
is the drag coefficient,
is the vehicle front surface area, and
is the speed (km/h).
where
is the rolling coefficient,
is the weight coefficient on the front axle,
is the total weight (kg), and
is the gravity acceleration (m/s
2).
where
is the motor-generator efficiency ratio,
is the transfer rate,
is the transfer efficiency,
is the axle efficiency,
is the variable acceleration (m/s
2),
is the regenerative braking coefficient,
is the coefficient of braking friction,
is the motor-generator moment of inertia (
kg.m2), and
is the power transferred to the hybrid storage system (kW).
Based on these formulas, the energy to be obtained from regenerative braking can be calculated; however, since this energy varies continuously depending on the road condition in EVs, certain scenarios should be determined first. In the first scenario, it is assumed that the vehicle is traveling at 120 km/h and brakes for 12 s after noticing the red lights. Because there is no slope in this scenario, the energy that the EV will obtain from regenerative braking can be calculated by calculating the kinetic energy. First, using the aerodynamic drag resistance force (2), we calculated that the vehicle consumes 5.25 kWh of energy against the drag resistance. The rolling resistance (3) for dry ground is calculated as 15.22 kWh. The kinetic energy of the EV at 120 km/h is 0.316 kWh. In addition to this kinetic energy, the kinetic energy of wheel rotation must be added. The tire size of the EV used in the study is 235/50/19, and the total tire weight is 30 kg. Based on this data, the total rotational kinetic energy for the four wheels is 0.25 kWh. Thus, the total regenerative braking energy will be 0.566 kWh. The regenerative braking energy for EVs with 80% drivetrain efficiency [
48] would be 0.452 kWh. A total of 1627.2 kJ of energy would be stored within the vehicle’s total braking time of 12 s. This will result in a peak power of 108.480 kW. With 80% powertrain efficiency in the vehicle, the peak power value will be 86.784 kW. If this power is divided first by the total battery voltage of 400 V and then by 48 parallel-connected battery cells, a charging current value of 4.52 A per cell will be found. This value corresponds to 1.558 C (4.52/2.9 = 1.558) for a 2900 mAh capacity battery.
In the second scenario, a vehicle traveling at 50 km/h is assumed to be driving in the city. In this drive, 0.0756 kWh of energy will be stored during a full stop, and 0.065 kWh of energy will be recovered from regenerative braking with 80% powertrain efficiency. This complete stop will require 5 s of braking. Repeated 100 times for 100 km of city driving, a total of 6.5 kWh of energy will be stored. For an EV with 16.7 kWh energy consumption per 100 km, this energy will provide a gain of 38.9 km. In this scenario, 197.9 kJ of energy will generate a charging current of 2.061 A per battery cell. This value corresponds to 0.710 C for a 2900 mAh capacity battery.
In the last scenario, the regenerative braking energy that will occur when the vehicle descends from 650 m above sea level to sea level is calculated. Considering the total road length of 15 km, the road slope will be 4.33%. Assuming that the vehicle descends this slope at a speed of 80 km/h, 0.26 kWh of energy will be gained. If this process is repeated five times along the road, a total of 1.3 kWh of energy will be recovered from regenerative braking. In this scenario, a charging current of 2.27 A per battery cell will be generated. This corresponds to a value of 1.277 C for a 2900 mAh capacity battery. In all these scenarios, the vehicle weight and the driver were calculated. Changing the weight of the vehicle causes the regenerative braking energy to change. In this case, the charging current value of the energy gained at different loads to the battery cell is given in
Table 2. The weight of a passenger is assumed to be 80 kg in the calculations.
In the calculations made under different road conditions and different loads from the three scenarios developed, the charging current gained by regenerative braking to the EV battery cell reached a high charging current value with a C value of 2.275. Low C rates during battery charging increase the battery capacity and battery life. In this context, the SC will be preferred primarily for energy storage from regenerative braking. As soon as the instantaneous average consumption exceeds 16.9 kWh, the SC discharges the load by feeding the electric motor. Thus, the SC will remain empty continuously. When the SC is full, if the temperature and SoC values are suitable for the battery, charging by limiting the charging current value by 0.5 C will improve the battery life. If the SC is full of the battery and is not suitable for energy storage, safe deceleration or stopping will be achieved by mechanical braking. In the current applications to preserve battery life, the efficiency of the energy to be stored is lower than that of the method used in this study because of the lack of a HESS.
Considering all these scenarios, the SC capacity was calculated to optimally store the energy gained from regenerative braking. The capacity of a Maxwell brand BCAP0350 model 350 F SC is 0.354 Wh. The SC pack voltage is 400 V, equivalent to the battery pack voltage, and consists of 150 series connections. To store the energy from regenerative braking and to direct the peak current value during braking to the SC, three parallel connections are considered appropriate. In this case, the total capacity of 450 SCs is 159.3 Wh.
When using two SC packs connected in parallel, a capacity of 106.2 Wh is obtained. This capacity value can store 23.49% of the braking of Scenario 1 with only the driver in the vehicle. This may cause us to recharge the battery with high current values without being able to sufficiently transfer the high current values at the beginning of the braking to the SC. Therefore, the number of SCs is set to 450. Thus, at maximum regenerative braking, 35.24% of the energy will be transferred to the SC, and high current values will be prevented from damaging the battery. The parameters of the Maxwell brand BCAP0350 model 350 F SC are given in
Table 3.
The continuous maximum current value of the SC is 170 A. In all calculated scenarios, a maximum charging current of 92.38 A will be obtained for 150 series and three parallel-connected SC cells. In this case, safe storage will be provided without exceeding the maximum current of the SCs.