Abstract: With the rapid development of highway construction and the increase of automobile ownership in China, the traditional collection system is difficult to meet the development needs. Thus the intelligent transportation electronic toll collection system (ETC) is one of the effective means to solve the problem, and the on-board equipment OBU is an important part of ETC system. This paper gives a brief description the working characteristics of OBU and power consumption mode in the process of transaction, and develops a dual power supply solution of ER14250 (1400mAh) +HPC1520+ photovoltaic panels with high reliability and long lifespan as well as high power output based on the practical application characteristics of OBU and the advantages and disadvantages of power demand. Through the test of the market, the solution can meet the work demand of OBU well, which is better than other options.
Keywords: ETC System; On-board Unit OBU; ER14250 + HPC1520 + Capacitive Power Supply for Photovoltaic Composite Battery; Dual Power Supply; Long-term Reliability
I. Overview of ETC
The electronic toll collection system is the most advanced road and bridge toll collection system in the world. Through the microwave special short-range communication between the OBU electronic tag installed on the vehicle windshield and the microwave antenna in the toll station ETC lane, it uses the computer networking technology and bank settlement process in the background, so as to achieve the vehicle through the road and bridge toll station without parking and can achieve the purpose of road and bridge fees. High sensitivity OBU system modules can be divided into three working states: dormant, pre-trade and on-trade. The change in communication state is a reflection of the OBU transaction status as it passes through the ETC system. And the largest power consumption in the whole trading process is the card reading process in the trading state, and the instantaneous current can reach more than 150mA.
II. Analysis of power consumption and lifespan of OBU in practical application environment
According to the characteristics of OBU power supply in ETC, it needs to meet not only the discharge requirements of extreme outdoor weather such as winter (-40℃) in Northeast China, summer (+85℃) in Southwest China and high humidity (85%RH) in Southeast coastal areas, but also the requirements of long lifespan, high reliability, safety and environmental protection at the same time. In addition, it also requires instantaneous large current pulse for power supply. On average, the vast majority of OBU in the current Chinese market are traded less than 10 times or even less than 1 time per day. Calculated from the power consumption data measured in the laboratory, as long as the power supply has a capacity of more than 1000mAh, it can theoretically achieve a total of 20 thousand transactions and a working life of more than 5 years.
III. Comparison of OBU power solutions
3.1 Rechargeable batteries + photovoltaic batteries
They have features of low material cost, high discharge current, high self-discharge rate, narrow range of applicable temperature, low capacity retention, complex power management circuits, solar batteries + voltage conversion chip, high conversion efficiency, good voltage stabilization performance, high price of voltage conversion chip, interference with OBU electrical lines.
3.2 ER14250+HPC1520+photovoltaic batteries (as shown in figure) :
They have features of high reliability, wide operating temperature range, low self-discharge rate, limited output capacity of large current pulse. In order to solve the problem of OBU power supply safety and reliability and environmental adaptability of intelligent transportation ETC system, Shenzhen Langsheng New Energy Technology Co., Ltd. has designed and developed a composite power supply which combines the lithium-thionyl chloride batteries ER14250 (capacity 1400mAh) of high energy density as the backup power supply and the battery capacitor HPC1520 with high power characteristics according to the outdoor temperature collector dual power supply solution which has been applied for five years. The composite power supply has the features of long lifespan, high power output, safety and reliability, wide temperature operation, etc., especially suitable for outdoor low and high temperature environment, and it also has the features of high pulse power output of HPC battery capacitor and high reliability and long lifespan of ER lithium-thionyl chloride battery. Generally speaking, HPC1520 is charged by photovoltaic battery. When large current pulse output is needed, power is supplied directly from the HPC battery capacitor; while when the photovoltaic does not work, the lithium-thionyl chloride battery charges the HPC with a small current, so that the capacity of the lithium-thionyl chloride battery can be fully released under a small current, and the maximum utilization of the battery capacity can be obtained. And this working principle is also in line with the requirements of OBU power intermittent operation.
Ⅳ. Tracking evaluation of long-term reliability
4.1 Test of performance reliability
Whether the composite power supply can provide stable output capacity during the entire lifespan cycle of the OBU is an issue that needs to be paid attention to in practical applications. The HPC1520, which has been used in on-board unit OBU of ETC system on the market for five years, was collected to conduct discharge performance test with new battery: -10℃ 800mA, 10 ms/1min, compare to the performance changes of HPC1520 as follows: Through testing and comparison, it is found that HPC1520 has been working for five years at low temperature -10℃, and the load voltage of 800mA pulse discharge is slightly lower than the new load voltage of 0.1V, but it still has the ability to provide pulse discharge of large current in practical application.
4.2 Test of long-term reliability
The on-board unit OBU installed in the windshield of the vehicle is in an outdoor enclosed environment for a long time. In order to better evaluate the reliability of the performance of the composite power supply under such use conditions, it is necessary to put the battery pack in the glass sun room to conduct insolation experiment to simulate the actual working conditions, and to keep track of changes in the performance of battery pack under these storage conditions. Through long-term tracking in the insolation experiment, both the open circuit voltage and load voltage of the composite power supply remain stable. Besides, there is no failed battery, which can maintain a high reliability according to the current actual storage data of 450 days. The Arrhenius formula shows that the reaction rate constant is exponentially related to temperature, which means the rate of chemical reaction increases by 2 to 4 times for every 10 degree increase in temperature, and 200 days stored at 60℃ is equivalent to 10 years stored at room temperature. Through the long-term tracking of the storage experiment at 60℃, both the open circuit voltage and load voltage of the composite power supply remain stable. And from the current data of 300 days (equivalent to 15 years of room temperature storage), there is no failure of the battery, which can provide a very stable current output capability.
Ⅴ. Summary of application
ER14250+HPC1520+photovoltaic batteries, the dual power supply composite power supply solution has the features of full sealing, wide temperature range, low self-discharge rate, long lifespan, high reliability, light weight, safe design, etc, which has been successfully applied in various products of intelligent transportation system. Through the long-term test of the market, its stability has been widely recognized, and it is preferred by comparing performance with others.