With the increasing adoption of electric vehicles (EVs) in the US, charging infrastructure is continuing to evolve, bringing new use cases along with it. A notable example is Vehicle-to-Everything (V2X), where unused energy stored in the EV can be supplied to home appliances, other vehicles, the electrical grid, or even something as niche as an electric barbecue powered by a camper van battery.
These bidirectional energy flows require bidirectional onboard chargers (OBCs). These systems must integrate residual current monitoring (RCM) technology to maintain safety and efficiency.
Ac charging trends
According to the consultancy firm PwC, the number of EVs on American roads is expected to reach 27 million by 2030, driving a tenfold increase in charging demand. The number of charge points in the US is projected to grow from fewer than four million in 2024 to over 35 million by 2030.
Research and Markets reports that the US market for EV ac chargers was valued at $2 billion in 2024 and is expected to grow rapidly. The global ac charger market, valued at $7.5 billion in 2024, is forecast to reach $36 billion by 2030, with a compound annual growth rate of nearly 30%.
While ac charging may be viewed as a mature segment with established specifications, innovation continues to evolve. Developments include significantly faster chargers, wireless charging, and EV roaming capabilities.
Similar to mobile phone roaming, EV roaming allows drivers to charge across networks using a single RFID card or app, simplifying access across charging providers. Ac charging is now more efficient and customizable than ever, offering EV owners greater flexibility in managing their charging routines and enabling advanced features such as remote monitoring. However, this is only the beginning — further innovation is expected as the EV ecosystem expands.
How cars are charged
Electric vehicles are charged by connecting them to the electrical grid using ac chargers, which can take the form of:
- Wallboxes: Compact ac charging units mounted on walls, typically used in residential or private parking settings for convenient EV charging at home or work.
- In-cable control and protection devices (IC-CPDs): Portable charging cables with built-in control and safety features, allowing users to charge EVs from standard ac outlets while ensuring protection from faults or overloads (Figure 1).
- Pedestal-mounted units: Free-standing ac charging stations commonly found in public or commercial spaces, offering more durable and accessible charging for multiple vehicle types.
Figure 1. The positioning of current sensing devices in a wallbox and IC-CPD units.
The vehicle’s OBC converts the ac power from the grid into dc power required by the battery. Since high voltages are involved, residual current monitoring (RCM) sensors protect the system and the user from electric shock. These sensors send a tripping signal instructing the system to switch to a safety mode when needed.
The sensor must be reliable to avoid false alerts that could interrupt charging and be resistant to external electromagnetic interference.
A Type-A residual current device (RCD) is commonly used to detect leakage currents during charging and shut down the circuit if necessary. However, this type of RCD can experience a “blinding effect,” where it misinterprets dc leakage as a typical ac. A dc fault greater than 6 mA can saturate the device’s core, preventing it from tripping and increasing the risk of electric shock.
Replacing a Type-A RCD with a Type-B RCD is a more effective solution. Type-B RCDs accurately detect ac and dc residual currents, avoiding the limitations associated with Type-A devices (Table 1).
Table 1. The types of devices that tackle residual and leakage currents.
A dc leakage current sensor can be integrated into the cable or wallbox to support residential and commercial charging stations. These sensors are designed to operate in single or three-phase system topologies and help ensure safety in modern EV charging infrastructure.
A dc leakage current sensor based on fluxgate technology is well-suited for a wide range of ac charging systems, typically from 3.3 to 22 kW.
Fluxgate technology
Many dc leakage current sensors used in EV charging applications rely on fluxgate technology (Figure 2). This magnetic sensing technique uses a ferromagnetic core and alternating excitation to detect small changes in magnetic fields. It offers high resolution and stability over a wide temperature range, enabling accurate detection of extremely low-level leakage currents.
Figure 2. Ac charging configurations with wallboxes, in-cable devices, and onboard chargers using RCM sensors based on fluxgate technology for precise detection of ac and dc leakage, enabling safe, bidirectional power flow in V2X systems.
Fluxgate-based sensors are well-suited for single and three-phase ac systems and can support built-in diagnostics, dynamic tripping thresholds, and communication interfaces such as SPI. These features help ensure compliance with functional safety requirements and simplify integration into vehicle-to-load (V2L) and vehicle-to-grid (V2G) architectures.
This type of sensor meets key international safety and performance standards for EV charging infrastructure, including IEC 62752, UL 2231, IEC 62955, and ISO 5474 for bidirectional OBCs. It enables precise monitoring of ac and dc leakage currents, detecting values as low as 5 mA — approximately 10,000 times smaller than the typical current in the main conductor.
Its vertical form supports easy integration onto compact printed circuit boards (PCBs), making it ideal for space-constrained applications such as residential wall-mounted chargers.
Residual current monitoring for charging cables
On the charging cable side, automotive-grade type-B residual current monitoring (RCM) devices support single-phase and three-phase ac systems.
These devices can deliver high accuracy using fluxgate sensing technology, detecting leakage currents down to ±0.5 mA at the 5 mA threshold. Many RCMs are designed with embedded diagnostics, including communication interfaces such as SPI, allowing engineers to set tripping thresholds to match application-specific or regional safety standards.
Additional features may include support for safety monitoring protocols, detection of dc and ac faults, real-time leakage current measurement, thermal and supply diagnostics, power-saving modes, and firmware update capabilities. These built-in functionalities enable efficient system integration and faster product development.
Current sensors in onboard chargers
A growing trend in EVs is V2X, which enables stored battery energy to be repurposed for external loads such as homes, appliances, and other vehicles or even fed back to the grid. This bidirectional energy flow requires OBCs capable of ac-to-dc conversion for charging and dc-to-ac inversion for energy export.
In these bidirectional OBCs (Figure 3), current sensors play a critical role. They not only measure the current flowing in both directions but also detect and monitor any ac or dc leakage current that may pose a safety risk. Accurate leakage current detection is essential to prevent electrical hazards when vehicles are connected to the ac network.
Figure 3. Example of bidirectional ac charging enabled by RCM type B devices with integrated fluxgate-based current sensors. These systems support energy flow to and from the vehicle, powering household appliances or feeding back to the grid in V2X scenarios.
Advanced automotive-grade current sensors can serve dual functions by supporting both current monitoring and RCD in a single compact unit. These sensors detect differences between conductor currents and quickly identify and isolate fault conditions. In the context of EV charging, they ensure compliance with strict requirements for detecting ac and dc leakage currents.
To meet automotive functional safety requirements, such sensors may be designed in accordance with ISO 26262 and be ready for integration into systems targeting standards like ISO 5474 for bidirectional OBCs. Their integration helps accelerate development timelines while ensuring compliance and safety.
Conclusion
In the rapidly evolving world of EV ac charging, a robust residual current monitoring (RCM) solution is essential. The increasing complexity of bidirectional and V2X applications, alongside the need to comply with the latest safety standards, demands sensors that are accurate, reliable, and easy to integrate.
Advanced current sensing technologies, such as fluxgate-based designs, can meet these requirements, helping engineers simplify design, improve safety, and ensure the long-term performance of modern charging systems.
Filed Under: Charging, FAQs, Vehicle-to-Grid (V2G)