With the increasing battery capacity of electric vehicles aimed at delivering more driving range and reducing range anxiety, there is an obvious global demand emerging for DC fast-charging stations. Designs of ESVEs (Electric Vehicle Supply Equipment) are complicated by the industries' inability to standardize which is resulting in different charging interfaces and power levels depending on car type and location of charging stations. Current DC charging standards include CCS or NACS in Europe and North America, CHAdeMO in Japan, and GB/T in China. Each of these use different charging connectors and utilize different communication protocols.
The internal components of DC EVSEs include AC-DC power modules, charging controllers, communication modules, user interfaces, DC electric meters, circuit breakers, leakage current monitors, safety protection circuits, and more. The reliability of charging stations is a crucial topic, prompting EVSE manufacturers to consider improving product quality on the production side while also ensuring testing efficiency.
Regarding the application of an Automatic Test System (ATS)
for DC EVSE in End of Line (EOL) testing on the production line.
Based on years of industry experience, Chroma has summarized several technical issues that production may encounter and provided recommended solutions. This article addresses the following 5 challenges of EOL testing of EVSE's:
Challenge #1: Differing Interfaces and Communication Protocols
Since the charging connector connects to the electric vehicle's inlet, the dual-sided contact detection and communication protocol activate. During various states such as charging preparation, initiation, and termination, if there are any abnormalities in the interaction between SECC (Supply Equipment Communication Controller) and EVCC (Electric Vehicle Communication Controller), including timeouts, data format recognition, anomaly handling, timing, etc., the charging process will be forcibly interrupted. Therefore, an ATS must possess the capability to simulate electric vehicle communication, and it can even inducing abnormalities to test DC EVSE interruption protection.
Furthermore, the ATS must simulate the EV's charging inlet, and meeting electrical testing requirements. It should include features such as electronic locks, temperature monitoring, high-power circuit breakers, and heat dissipation functions to ensure the safety of the testing process. The testing system is recommended to adopt modular and interchangeable EV simulators and charging inlets. This allows for quick replacements to avoid prolonged testing interruptions caused by abnormal issues on the production line. Another benefit is offering flexibility for varying standards, power levels, and future upgrades.
▲ Figure01. EV Charging Test Scenario
In addition, it is recommended to integrate the EV simulator with a suitable battery simulator and testing software that allows for loading charging power profiles. This functionality can be used to simulate real-time changes in charging current requirements for DC EVSE, enabling the verification of its response time and accuracy.
Challenge #2: Enhancing the Testing Time and Capability for Multi-Coupler DC EVSE
To accommodate the varying DC charging standards used by different electric vehicles (EVs), EVSE manufacturers have introduced products with multiple DC charging coupler interfaces. These connectors include standards such as CCS and CHAdeMO, and even contain an AC charging coupler. The configuration of testing systems for such DC EVSE requires multiple EV emulators for different charging standards, making the testing process more complex. Furthermore, If testing multi-coupler products sequentially, it extends testing time, unlike real scenarios with simultaneous coupler use.
Therefore, the Automated Test System (ATS) on the production line must simulate various interfaces concurrently and support multiple EVs charging simultaneously. This allows testing communication protocols between EVSE and EVs with different protocols, also detecting their output statuses. This is particularly important to verify the EVSE’s internal intelligent output power distribution functionality when multiple EVs simultaneously demand charging current, ensuring that no malfunctions or protective actions occur. The output of each charging coupler can also be synchronized to assess the accuracy specifications of output voltage, current, and watt-hour measurements. This approach can effectively reduce testing time on the production line.
Challenge #3: Safety Standards and Protection Mechanisms
Various protection mechanisms are essential functions of DC EVSE. These include safeguards against output over-voltage/over-current, ground and insulation anomalies, emergency stops, over-temperature conditions, and abnormal power input. The testing system needs to encompass various scenario simulations to thoroughly test the proper functioning of the protection mechanisms in each DC EVSE.
For instance, during insulation monitoring tests for EVSE, the testing system can simulate impedance variations between the battery and the vehicle body by setting up an insulation resistance box. It can also be connected at different communication handshake stages, overlaid onto the output DC+ or DC- to ground circuit, in order to verify the insulation detection function of the EVSE. During system integration, appropriate relay and resistor specifications for voltage and current tolerance must be selected to prevent easy damage.
Additionally, if the internal Y-capacitor of the battery simulator is too large, it might affect the functionality test of insulation monitoring and even lead to false reporting, resulting in test interruptions. Special attention must be paid to this aspect.
▲ Figure03. Insulation Impedance Testing Circuit
Challenge # 4: Battery Simulation
The DC EVSE receives power demand from the connected electric vehicle then uses an internal power supply module to convert alternating current (AC) from the grid to charge the battery. The testing system needs to integrate a battery simulator that can be set in constant voltage or constant current mode to verify the accuracy of the charging output parameters of the EVSE. Given its output power range from 20 kW to 400 kW, it is advisable to use equipment with energy recovery capabilities. This allows charging energy to be fed back into the grid for reuse, saving energy consumption during charging tests and helping control the factory’s room temperature.
Furthermore, selecting a bidirectional battery simulator that can both charge and discharge to prepare for the future functionality requirements of Vehicle-to-Grid (V2G). The DC EVSE's input is powered by the grid simulator power supply, and it needs to be verified that it can operate properly within the specified voltage and frequency range. Additionally, it is crucial to simulate abnormal variations in the grid, as required by standards like SAE-J1772 and UL2231-2. Test such as Voltage Dip, Voltage Interruption, Voltage Variation, and Harmonic Distortion Immunity are suggested to ensure that the EVSE remains undamaged and can smoothly transition into protection mode.
▲ Figure04. Various Waveforms Generated by a Programmable Grid Simulator
DC EVSE also requires testing for standby power consumption, power factor, and charging efficiency specifications. The power meter of the Automated Test System (ATS) should not only meet high measurement accuracy(<0.2%), but also offer milliwatt-level power resolution. Additionally, it is recommended to have different measurement modes such as Power Integration mode and Smart Range functionality to provide greater flexibility in applications.
As the power of DC EVSE continues to increase, it is advisable for the testing system to have expandability in terms of power capabilities. Furthermore, future communication protocol upgrades should also be taken into consideration to reduce the cost of production line expansions.
▲ Figure05. The Scalability of DC EVSE ATS
Challenge #5: Integration Across Different Fields of Production Line
The End-of-Line (EOL) testing station for DC EVSE also needs to interface with the automated line's Programmable Logic Controller (PLC) to verify the proper positioning of the test subject and to confirm the factory's operational safety conditions and fire safety level through PLC communication. It also requires communication with the Manufacturing Execution System (MES) to verify the testing status and results from the previous station, determining whether to proceed with testing. To enhance production efficiency and consolidate test reports, it will be better to control the part number by the MES, it could help for generating the necessary test results and data analysis for production records. Additionally, automatic downloading of corresponding Test Procedures (TP) can facilitate flexible test control for different models during production, minimizing potential human errors. Finally, careful coordination of procedures between PLC, MES, and ATS is extremely important.
Developing EOL testing system involves integrating diverse technologies from various fields and requires accumulated experience. It is hoped that the five technologies points in this article can serve as a reference for EVSE manufacturers when planning their production line testing. This can contribute to improved test coverage, product safety, and quality assurance, ultimately enhancing user satisfaction. Multi-tasking expedites automated testing could boosting factory output. Chroma provides testing solutions for AC & DC EVES, Onboard Chargers, Motor Drivers, and Battery Packs. For more information, please refer to the Chroma website.