EV Manufacturers-Understanding On-board Charger - Solterra Technologies Pvt. Ltd.

The on-board charger (OBC) is an integral component of a vehicle’s power architecture, as the capabilities and connectivity of EVs increase. An OBC is more than just a device that changes AC electricity from a charging point to usable DC electricity for the battery. It has an impact on charging, battery protection, thermal characteristics, communications, vehicle packaging and system reliability.

Selecting the right OBC is therefore an integration decision, not just one of picking a charger that has an appropriate power rating, for EV manufacturers.

What Is an On-Board Charger and How Does It Work?

An on-board charger is the power conversion system mounted on the electric vehicle that transforms AC power from the charging source into DC power that is regulated by the on-board charger to power the traction battery.

In AC charging, the current flows from the grid via the charging interface towards the OBC. The OBC usually includes a power factor correction (PFC), an AC/DC and DC/DC conversion and regulation as well as a power delivery to the battery system.

The OBC also works closely with the Battery Management System (BMS). The BMS supplies data on battery state, temperature and charging limits to the OBC that regulates the charging behaviour based on these parameters. This communication aids in the safe charging of the battery within the safe range.

In AC charging, the OBC does the main AC to DC conversion and provides the DC to the battery system, whereas in DC fast charging, the external charger does the main AC to DC conversion and the charger provides the DC to the battery system.

OBC Selection: 2W, 3W or 4W?

Unlike gas-powered vehicles, there is no one-size-fits-all on-board charger for EVs. It’s important to note that the right OBC can vary depending on the vehicle’s battery architecture, the charging infrastructure in place, packaging requirements, and the use case.

Electric 2-Wheelers

Compactness, weight, efficiency and cost are very significant for two wheelers powered by electricity. Most platforms have low voltage battery systems, which allows for the use of a low voltage OBC that is suitable for the size of the battery.

To design an OBC for an electric 2-wheeler, manufacturers should take into account:

The nominal and maximum voltage of a battery.
Required charging time
Available AC input
The size and weight of chargers.The size and weight of the chargers.
The interior temperature of the vehicle.
Communication needs to BMS / vehicle controller.

This can be an important application for a small charger that delivers the right voltage and current but does not become cumbersome or heavy.

Electric 3-Wheelers

The electric 3-wheelers can be used longer and have a higher utilisation rate than private 2-wheelers. Thus, in addition to charging speed, the continuous duty operation, thermal performance and environmental durability are also important aspects to take into account for OBC selection.

The charger needs to be compatible with the battery voltage range, charging current limits and be able to communicate with the vehicle system architecture.

Electric 4-Wheelers

Four-wheelers will generally require greater charging power and more complex integration. OBCs could be based on a higher voltage battery platform or power levels like 3.3 kW, 6.6 kW and more.

Efficiency, power density, electromagnetic compatibility and thermal management are becoming ever more important design choices for these platforms.

Voltage Compatibility and Power Ratings

One of the key issues in battery integration with the OBC is ensuring the output voltage range of the charger matches the battery’s operating range.

The OBC is not to be chosen based solely on the nominal voltage of the battery. Engineering team should take into account the minimum and maximum battery voltage, charging current, cell chemistry, BMS limits and charging profile.

The power rating is also crucial.

The basic relationship is:

Charging Power = Battery Voltage × Charging Current

But the higher the OBC rating, the faster the charging does not automatically result. The actual charging rate is limited based on battery, BMS, vehicle wiring, thermal system and charging source.

The key factors with respect to OBCs are therefore a match between the charging requirements and OBC efficiency, packaging, thermal performance and system cost.

BMS Integration and CAN Communication

The ability of the different parts of the system to communicate with one another is an essential aspect of successful integration of the EV OBC.

In automotive systems, CANbus is a protocol extensively employed for communicating between electronic control units. In an OBC architecture, communication with the BMS can enable the charger to receive the battery information and charging limits and fault conditions. The OBC can thus adjust its output or cease charging as needed.

The following should be covered in a clear communication plan:

Start/stop charging commands
Current limits and voltage limits
Battery temperature conditions
State-of-charge information
The fault/diagnostic messages are shown.Fault and diagnostic messages are displayed.
Derating and protection conditions.

For Manufacturers this means, compatibility with CAN is not sufficient. Communication protocol, message structure, diagnostics and behaviour of the vehicle’s firmware of the OBC should be synchronised with the BMS and VCU architecture of the vehicle.

Thermal Management Matters

Power conversion creates heat, which becomes more critical the higher the charging power and the power density.

Semiconductor losses, magnetic components, switching frequency, enclosure design, ambient temperature and installation location are factors that should be taken into consideration for the thermal design of the OBC. Cooling can be done by forced air, conduction through enclosure or liquid-cooled interfaces, depending on application.

Automotive environmental standards (ISO 16750) cover environmental stresses on electrical and electronic components such as electrical, mechanical and climatic loads.

Thermal validation should therefore be more than just ensuring that the charger runs at room temperature for EV manufacturers. It should consider performance during the actual operating conditions and temperature of the vehicle.

Certification and Automotive Compliance

Consideration of electromagnetic compatibility, electrical safety and automotive environmental requirements for OBC integration is also required.

The limits and measurement techniques of CISPR 25 are relevant to radio disturbances that are likely to affect on-board receivers, especially in vehicle components. This is the latest CISPR 25:2021 edition which specifies conducted and radiated radio disturbances in a wide frequency range for electrical and electronic components for use in vehicles.

Depending on the vehicle architecture, market and OEM requirement, other standards might become relevant as well. These can include electrical transients requirements, environmental durability, functional safety and power-electronic system safety. For instance, ISO 7637-2 defines conducted electrical transients in vehicle supply lines and IEC 62477-1 gives general safety requirements for power electronic converter systems, if applicable.

The compliance package always depends on the vehicle category, target markets, OEM package and relevant regulations.

Choosing an OBC as a System, Not a Component

On-board charger selection should not be the starting point for EV manufacturers, it should be part of the complete vehicle architecture.

All of these parameters need to be compatible, meet the required power specifications, integrate with BMS, communicate via CAN, manage temperatures, perform EMC tests, and be environmentally durable and certified.

An integrated OBC can help ensure reliable charging, enhanced battery management, and a more robust vehicle platform. The OBC will play an increasingly vital role between the charging network and the electrical system of the vehicle as the EV architectures expand to all three and four-wheelers.

Our strategy for power electronics at Solterra Technologies is to design the charging systems to meet the real vehicle requirements, which includes power conversion, communication, protection and ruggedisation to suit the changing needs of the electric car industry.