4.1 System Requirements Analysis
In the process of setting up the charging control system, this study mainly selects the controller required for the system from the perspective of safety and reliability. After careful consideration, the final choice of the main controller is a series of single-chip microcontrollers. When designing the system, this article also considers the corresponding performance of the system, mainly including the following points.
1) High security
Charging an electric vehicle requires not only a short time, but also safety. During the charging process, there are certain differences in the type and brand of battery used by different brands of electric vehicles, so it is easy to encounter different situations such as battery capacity, resistance, and voltage. For different battery packs, the impact of this situation is different, and it will have no impact on some batteries with a relatively small number of single battery packs, but it will have a significant impact on batteries with a relatively large number of single battery packs, and may even become a potential hazard factor.
2) Generality
Many countries and different regions have opened up the electric vehicle industry, but they are in the early stages of development. The batteries and voltages used by different brands and models of electric vehicles vary, but to ensure the safety of vehicle charging, it is necessary to build charging piles in different places. If the charging standards for charging piles are not unified, setting charging piles everywhere will cause a lot of waste, so it is necessary to study and design suitable for all brands The charging pile used by the model car.
3) Fast charging
Generally, owners of shared cars will choose to go to a charging station or use a charging pile to charge, but the charging time is too long and efficiency is not seen, which not only hinders the development of the shared car industry and charging technology, but also affects the car experience of owners. In combination with the time spent filling gas at gas stations and people's travel needs, it should be ensured that at least 60% of the charging process is completed within half an hour to improve people's charging experience.
4) Simplify operation
For most car owners, if the charging equipment at the charging station or charging pile is relatively convenient and simple to operate, then the car owner will also choose this site. However, if the operation process is too cumbersome and even requires professional guidance, the charging experience of car owners will be reduced, and the site will also cost a lot of training costs. This type of charging station will gradually be eliminated from the market.
First, the automatic billing function. The system will record the power consumption based on the current change of the ammeter, and calculate the charging cost based on the set charging standard. Second, the function of autonomous control of electrical energy. The system will automatically detect the information on the user's charging card. If the balance in the user's card is insufficient to pay for this charging charge, the system will remind the user to recharge. After the user completes the recharge, the system will continue charging for them. Third, the rectifier circuit function. The system will convert the AC power connected from the national grid into direct power for the vehicle owner to charge. Fourth, information display function. The system's screen will display real-time information such as changes in current, cumulative charging costs, and consumed charging duration. Fifth, self protection function. If the system detects an abnormality in the device of the charging pile or in the temperature of the battery, the system will enter the protection mode and give an alarm.
4.2 Overall system and hardware and software design
In the process of selecting the overall hardware and software of the system, safety and reliability remain the basic principles to be followed, in addition to considering the convenience of the device. Therefore, the combination method adopted in this article is a modular structure. The system modules mainly include a charging controller module, a module for human-computer interaction, a real-time communication module, a module for printing charging bills, and a module for settling charging transactions.
The AC ammeter can provide real-time statistics of the total amount of electrical energy used by the system to the control center, and can also allocate AC power to the rectifier circuit to meet the charging requirements of the battery pack. During the charging process, the system will detect the input current in real time, determine the voltage and current magnitude of the current, and transmit these information to the control center of the system. In addition, the real-time communication module of the system will provide alarm information based on battery voltage changes, temperature changes, and system protection functions in real time, and transmit it to the control center of the system. Once the battery fails, the communication module will immediately start a contactor to cut off the charging power supply, thereby protecting the charging pile and the equipment being charged.
4.3 Control strategy analysis
The battery selected in this article is a lead-acid battery. When the battery is discharged, there will be a "sulfur formation reaction" between the negative electrode plate and the positive electrode plate, and the "sulfur formation reaction" between the positive electrode plate can be expressed by formula (15):
$$Pb{O}_{2}+3{H}^{+}+HS{O}_{4}^{-}+2e\rightleftarrows PbS{O}_{4}+2{H}_{2}O$$
15
The "sulfur formation reaction" occurring on the positive electrode plate can be expressed by formula (16):
$$Pb+HS{O}_{4}^{-}\rightleftarrows PbS{O}_{4}+2e+{H}^{+}$$
16
The overall situation of the "sulfur formation reaction" of the battery can be expressed by formula (17):
$$Pb{O}_{2}+Pb+{H}_{2}S{O}_{4}+2e\rightleftarrows 2PbS{O}_{4}+2{H}_{2}O$$
17
Figure 5 is a schematic diagram of charging and discharging a lead-acid battery.
During the charging process, a corresponding reaction will occur inside the lead-acid battery, and the solicitation reaction can be expressed by Formula (18) and Formula (19):
$$Pb-2e+2{H}_{2}O⟶Pb{O}_{2}+{H}_{2}S{O}_{4}+2{H}^{+}$$
18
$${H}_{2}O-2e⟶2{H}^{+}+\frac{1}{2}{O}_{2}$$
19
At present, the commonly used methods of battery power-off are timing control shutdown, shutdown based on battery temperature changes, and shutdown based on voltage changes.
For battery charging, the judgment basis for timely disconnecting the charging power supply is to ensure that the battery has sufficient power, which is a key issue to be considered in both traditional charging mode and rapid charging mode.
(1) Timing control
Timing control requires setting the charging time in the system in advance based on the usual battery charging time and current level, so that the system will automatically cut off the charging power supply after meeting the set time. The timing control method is relatively simple and convenient to operate, but it is also necessary to set an appropriate time based on the type and size of the battery, otherwise it may lead to excessive charging time.
(2) Negative increment control of battery terminal voltage
In addition, if the ambient temperature of the storage battery is relatively high, the voltage change of the storage battery is not significant, so it is prone to errors.
(3) Battery temperature control
During the charging process of the battery, if the battery power is insufficient, the battery will continue to absorb the charge from the charging power source. Continuing to transmit the charge after the battery is fully charged can cause the battery temperature to continuously increase. Therefore, monitoring the temperature change of the battery can also understand the charging situation of the battery. Once the temperature continues to rise, the charging power source should be immediately disconnected.
Currently, charging with rechargeable lithium batteries generally involves the same stages. The time spent in the trickle and float charging stages is relatively short, but in the constant voltage stage, the voltage tends to decline rapidly. In the constant current stage, the charging time is relatively short, and the current starts to rise slowly without significant changes. This stage of charging takes a relatively long time, Therefore, this stage can be used to analyze the impact of charging time on the charging efficiency of electric vehicles. The ratio of the remaining battery power to its original power can be calculated using formula (20):
$$SOC=\frac{{Q}_{c}}{Q}\times 100\text{\%}$$
20
For users, it is considerable to ensure demand while saving charging costs. During the charging process, almost all users hope that the charging pile can complete charging for their car within the time they expect. If the charging cost can be minimized, users will moderately accept the measures of orderly control strategy to guide charging. The setting of charging standards determines the charging cost that users need to spend. The charging cost can be calculated using formula (21):
$${f}_{l\left(i\right)}={\text{c}}_{p}\bullet {t}_{p\left(i\right)}+{c}_{f}\bullet {t}_{f\left(i\right)}+{c}_{g}\bullet {t}_{\left(i\right)}$$
21
The load situation of the power grid can be calculated using formula (22):
$${P}_{The total(i,t)}={P}_{c(i,t)}+{P}_{The total(i-1,t-1)}$$
22
Use a multi-objective function to select operators to calculate the fitness value of each individual in the population. The calculation process is shown in Formula (23)
The roulette algorithm is used to select paternal individuals that can be inherited. At this time, the probability of each individual being selected in the population can be calculated using formula (24):
$${P}_{t}={f}_{t}/\sum _{t=1}^{20}{f}_{t}$$
24
This study compares and analyzes ordered charging and disordered charging, and the calculation results of relevant values are shown in Table 3.
Table 3
Comparison of Ordered Charging and Disordered Charging Results
Type | Minimum load /kW | Maximum load /kW | Peak valley difference ratio | Charging fee/yuan |
Original load | 1250.98 | 4804.385 | 73.98 | - |
1000EV disorderly charging load | 1252.68 | 5203.5 | 75.92 | 16712 |
1000EV disorderly charging load | 1584.35 | 4811.3 | 67.01 | 12816 |
2000EV disorderly charging load | 1394.65 | 5623.34 | 75.12 | 33041 |
2000EV disorderly charging load | 1677.85 | 4874.2 | 65.56 | 26202 |
3000EV disorderly charging load | 1250.65 | 6001.25 | 79.15 | 49678 |
3000EV disorderly charging load | 1798.65 | 49.9.3 | 63.35 | 38648 |
4000EV disorderly charging load | 1264.35 | 6470.2 | 80.46 | 66408 |
4000EV disorderly charging load | 1997.25 | 5027.66 | 60.24 | 51652 |
5000EV disorderly charging load | 1264.65 | 6862.3 | 81.56 | 82935 |
5000EV disorderly charging load | 2238.25 | 5140.02 | 56.42 | 63766 |
The data in the table confirms the reliability of the orderly charging strategy using genetic algorithms. This strategy has significant benefits in saving users' charging costs and maintaining the stability of the power grid. Moreover, the more vehicles there are, the better the effect of this strategy.