Electric car on-board charger: the essential guide

When people talk about charging an electric vehicle, attention usually turns to the EV charger, the power available from the building’s electrical system or the number of kilowatts shown on the charging station. Yet one less visible component plays a crucial role in the process: the electric car on-board charger.

Here is a common example. A 22 kW EV charger has been installed, the electrical system has been correctly designed and a three-phase supply is available. Everything appears ready for high-power charging. Once the car is connected, however, the display shows only 11 kW. In other cases, it may show 7.4 kW or even less.

What is happening?

In most cases, there is no fault in the vehicle or the charging station. The car is simply drawing the maximum power permitted by its internal charger, known as the On-Board Charger, or OBC.

This component converts the alternating current supplied by the grid into the direct current required by the battery. Its capacity therefore determines how much power the vehicle can actually accept during an AC charging session.

Understanding how the on-board charger works makes it easier to interpret the information shown by the vehicle or charging station, select the right wallbox power and distinguish a normal technical limitation from a potential problem.

In other words, knowing the OBC helps avoid unrealistic expectations and makes it possible to design a charging system that genuinely matches the vehicle, the building and the driver’s everyday needs.

What is an electric car on-board charger?

On-Board Charger and OBC: meaning and purpose

The on-board charger is an electronic device installed inside an electric vehicle. Its name is often shortened to OBC, from the English term On-Board Charger.

Its main purpose is to convert alternating current, or AC, into direct current, or DC. This conversion is necessary because public and residential electricity grids supply alternating current, while the traction battery of an electric vehicle stores energy as direct current.

The OBC can therefore be thought of as a bridge between the charging infrastructure and the vehicle’s battery.

During an AC charging session, the on-board charger:

• receives electricity from the EV charger or AC charging station;

• converts alternating current into direct current;

• adjusts voltage and current;

• communicates with the vehicle’s electronic systems;

• helps protect the battery and high-voltage components;

• ensures that power consumption remains within the vehicle’s permitted limits.

It is not simply a transformer. It is a sophisticated power-electronics system integrated with the vehicle’s software, battery management system and safety architecture.

Its design influences charging performance, efficiency, heat generation and the maximum AC power that the vehicle can accept.

Where is the on-board charger located?

The on-board charger is installed inside the vehicle, although its exact position varies according to the manufacturer, model and platform.

It may be located:

• beneath the bonnet or front compartment;

• near the electric drive unit;

• under the vehicle floor;

• close to the traction battery;

• inside an integrated power-electronics module.

In newer vehicles, the OBC may share a housing with other components, such as the DC/DC converter or parts of the drive inverter. This integration can reduce weight, size, cabling and production costs while improving overall system efficiency.

For the driver, however, the physical position of the charger is rarely important. What matters most is its maximum power rating, whether it supports single-phase or three-phase charging and how it behaves under real charging conditions.

Difference between the on-board charger, EV charger and traction battery

The on-board charger, the wall-mounted EV charger and the traction battery are sometimes confused, but they perform very different functions.

The EV charger, often called a wallbox in Europe, is installed in a home, workplace, apartment building, hotel or public car park. It manages the connection to the electrical system, checks that charging can begin safely and communicates how much current is available to the vehicle.

The on-board charger is part of the vehicle. It receives AC electricity from the charging point and converts it into DC electricity.

The traction battery stores that energy and later supplies it to the electric motor.

Put simply:

• the charging point makes electricity safely available;

• the on-board charger converts it;

• the battery stores it.

An AC charging point does not normally perform the main conversion from AC to DC. That conversion takes place inside the vehicle through the OBC.

This is one of the most important distinctions to understand when comparing charging-station power with the actual charging rate shown by the car.

Why does an EV battery need direct current?

Lithium-ion batteries store and release energy as direct current. Electrical grids, on the other hand, distribute alternating current because it can be generated, transformed and transmitted efficiently across different voltage levels and distances.

When an electric car is connected to a household outlet or an AC charging point, the incoming electricity cannot be sent directly into the traction battery. It must first be converted and regulated.

That is where the electric car on-board charger comes in.

The OBC receives alternating current, rectifies it and adapts the voltage and current to the battery’s requirements. This process must be carefully controlled because the battery cannot accept the same amount of power under every condition.

Its ability to receive energy may change according to:

• battery temperature;

• state of charge;

• cell voltage;

• battery condition;

• thermal-management requirements;

• charging limits set by the manufacturer.

The conversion process is therefore not merely an electrical formality. It is part of a coordinated system designed to charge the battery safely and efficiently.

How does the On-Board Charger work during AC charging?

The path of electricity from the grid to the battery

During AC charging, electricity passes through several components before it reaches the traction battery.

The process begins with the building’s electrical system. Power reaches the EV charger, which verifies the connection with the vehicle and communicates with the car through the charging cable.

If the required safety conditions are met, the charging station informs the vehicle of the maximum current it can provide. The vehicle then decides how much current to draw, considering the limits of its on-board charger and the condition of its battery.

The energy reaches the OBC, which converts it into direct current. After further monitoring and regulation, the electricity is sent to the traction battery.

The path can be summarised as follows:

electrical grid → EV charger → charging cable → vehicle inlet → on-board charger → battery management system → traction battery

Each part of this chain can affect the final charging rate. This is why charging speed cannot be predicted from a single value printed on the charging station.

The charging cable and connector must also be compatible with the vehicle, power level and regional charging standard.

Converting alternating current into direct current

The conversion performed by the OBC is a complex electronic process.

First, the charger rectifies the alternating current, turning it into direct current. It then adjusts the voltage and current to match the needs of the battery and the high-voltage electrical architecture of the vehicle.

The system must also:

• improve the power factor;

• minimise electrical interference;

• regulate the output accurately;

• manage heat generated during conversion;

• maintain electrical isolation;

• respond to commands from the vehicle;

• stop charging if an unsafe condition is detected.

The design and quality of the on-board charger therefore influence several aspects of the charging experience:

• energy efficiency;

• charging stability;

• thermal performance;

• compatibility with different grids;

• system reliability;

• vehicle safety.

A more efficient OBC loses less energy as heat and transfers a larger proportion of the electricity drawn from the grid into the battery.

Communication between the charging point and the electric vehicle

One of the most common misunderstandings is that a charging station automatically sends its full rated power into the car.

That is not how AC charging works.

The charging point communicates the maximum current it can safely provide. The vehicle then decides how much current to draw according to:

• the capacity of its on-board charger;

• the number of phases it can use;

• the battery’s current condition;

• charging limits configured in the vehicle;

• the power made available by the infrastructure.

For example, a 22 kW three-phase EV charger may communicate that it can provide up to 32 A on each phase. If the connected vehicle has an 11 kW OBC, it will normally draw around 16 A per phase and charge at a maximum of approximately 11 kW.

The charging point does not force the vehicle to accept more power. It provides an upper limit, and the car remains in control of its own energy intake.

This communication allows vehicles with different power ratings to use the same charging infrastructure safely.

The role of the Battery Management System

The on-board charger does not work alone. It constantly exchanges information with the Battery Management System, commonly known as the BMS.

The BMS monitors the traction battery and evaluates parameters such as:

• cell temperature;

• pack voltage;

• state of charge;

• voltage balance between cells;

• maximum acceptable current;

• battery health;

• possible abnormal conditions.

If the battery is too cold, too hot or close to full charge, the BMS may request a reduction in charging power.

This means that even a car equipped with an 11 kW on-board charger will not necessarily draw 11 kW at all times. The OBC rating is the highest power that the charger is designed to manage under suitable conditions. It is not a guarantee of constant power throughout every session.

The BMS may also stop charging completely if it detects a condition that could affect safety or battery durability.

Why does the on-board charger limit AC charging power?

What does the rated power of an OBC mean?

The rated power of an on-board charger indicates the maximum AC power that the component can convert under its specified operating conditions.

Common ratings include:

• 3.7 kW;

• 7.4 kW;

• 11 kW;

• 22 kW.

Other ratings also exist, particularly in markets with different grid voltages, phase arrangements or vehicle designs.

An 11 kW on-board charger can manage up to approximately 11 kW of AC power when connected to a suitable three-phase supply. Even if the vehicle is plugged into a 22 kW charging point, the OBC will continue to operate within its own 11 kW limit.

The rating is a structural feature of the vehicle. It depends on:

• power-electronics design;

• size and cost of components;

• cooling capacity;

• electrical architecture;

• software controls;

• packaging constraints;

• manufacturer strategy.

In most cases, the maximum power cannot be increased by changing a setting or installing a more powerful wallbox.

Why charging-station power does not always match the power drawn by the car

The rating shown on an EV charger represents the maximum power it can make available under suitable conditions. It does not indicate what every connected vehicle will draw.

A 22 kW charger could supply:

• close to 22 kW to a vehicle equipped with a compatible 22 kW OBC;

• around 11 kW to a vehicle with an 11 kW three-phase OBC;

• around 7.4 kW to a vehicle with a 7.4 kW single-phase charger;

• around 3.7 kW to a plug-in hybrid limited to 3.7 kW.

The same charging point can therefore produce very different results depending on the vehicle.

A useful comparison is a water supply capable of delivering a high flow rate. The actual flow also depends on the diameter and capacity of the pipe connected to it. The charging station makes power available, but the on-board charger determines how much of that power the vehicle can process.

The most restrictive component sets the limit

The real charging power is determined by the most restrictive element in the entire system.

Possible limitations include:

• the capacity of the local grid connection;

• the electrical service available at the site;

• single-phase or three-phase supply;

• the current rating of the circuit;

• the EV charger’s configured limit;

• the charging cable;

• the on-board charger;

• vehicle charging settings;

• dynamic load management;

• battery temperature;

• battery state of charge.

If one element allows less power than the others, the entire session will operate at that lower level.

Consider a site with a 22 kW charging point, a suitable three-phase electrical system and a vehicle with an 11 kW OBC. The car will be the limiting element, so the charging rate will be approximately 11 kW.

Connect the same vehicle to a charging point limited to 7 kW, and the infrastructure becomes the bottleneck instead.

The real charging rate is therefore the result of the whole system, not just the rating of the car or the charger.

What happens when the EV charger is more powerful than the OBC?

When the EV charger has a higher rating than the vehicle’s on-board charger, the vehicle draws only the power it can use.

A 22 kW charging point connected to an 11 kW car does not continuously deliver 22 kW. The vehicle limits its intake to approximately 11 kW.

This is normal and is part of the charging system’s design.

The unused capacity does not automatically become wasted electricity, nor does it create an overload inside the vehicle. The charging station supplies only the current requested within the agreed safety limit.

What happens when the EV charger supplies less power than the vehicle can accept?

The opposite situation occurs when the charging infrastructure provides less power than the OBC can handle.

An electric vehicle with an 11 kW on-board charger connected to a 7.4 kW charging point will charge at no more than approximately 7.4 kW.

The vehicle could technically process more power, but the charging point or electrical system cannot provide it.

This is not necessarily a disadvantage. In residential settings, a lower charging rate may be perfectly adequate when the vehicle remains parked for several hours or overnight.

The right power is not always the highest possible power. It is the level that meets the driver’s mobility needs without unnecessarily increasing installation complexity or electricity demand.

Single-phase and three-phase on-board chargers: what is the difference?

How single-phase EV charging works

A single-phase on-board charger draws electricity from one phase of the grid.

In regions using supply voltages of around 220–240 V, common charging levels include approximately:

• 2.3 kW at 10 A;

• 3.7 kW at 16 A;

• 7.4 kW at 32 A.

Actual values may differ according to local voltage, regulations, charger settings and electrical-system conditions.

Single-phase EV charging is common in residential installations and is supported by many electric vehicles and plug-in hybrids.

In other markets, particularly where residential voltage is lower or split-phase supplies are used, the relationship between current and charging power is different. The main principle remains the same: the OBC can use only the electrical configuration for which it has been designed.

A single-phase vehicle may be connected to a three-phase charger, but it will normally draw power from only one phase.

How three-phase charging works

A three-phase OBC draws power from three phases at the same time.

In electrical systems commonly found across Europe and many other regions, typical configurations are:

• approximately 11 kW at 16 A per phase;

• approximately 22 kW at 32 A per phase.

Three-phase charging distributes the electrical load and allows higher AC power without placing the full current on a single phase.

Many modern electric vehicles support 11 kW three-phase charging. Compatibility with 22 kW is less common because a 22 kW OBC is larger, more expensive and more demanding in terms of cooling.

A three-phase charging point does not guarantee three-phase charging. The vehicle must also contain a compatible three-phase on-board charger.

What happens when a single-phase car is connected to a three-phase charger?

A single-phase vehicle can usually charge from a three-phase charging station, provided that the connector and communication standard are compatible.

However, the vehicle will normally use only one phase.

If the car accepts up to 7.4 kW in single-phase, it will reach that power only if the charging point and site can provide the required current on the phase being used.

In installations with per-phase limits or strict phase-balancing requirements, the available power may be lower.

For this reason, a single-phase car connected to a 22 kW three-phase charging point may still charge at 7.4 kW, 3.7 kW or another lower value. The charging station is not necessarily malfunctioning. The result reflects the configuration of the vehicle and electrical system.

What happens when a three-phase vehicle is connected to a single-phase supply?

Many vehicles with three-phase OBCs can also charge from a single-phase source, although the maximum single-phase power varies between models.

An 11 kW three-phase vehicle might charge at approximately 3.7 kW or 7.4 kW when connected to a suitable single-phase charger. Some models may accept different current levels, while others impose a lower single-phase limit.

The maximum three-phase rating does not automatically reveal the vehicle’s maximum single-phase charging power.

This is another reason to check the technical specification of the exact model rather than relying only on the headline OBC rating.

How to find out whether an EV supports one or three phases

The most reliable approach is to consult the vehicle’s official specifications or owner’s manual.

Look for terms such as:

• maximum AC charging power;

• on-board charger;

• OBC;

• single-phase charging;

• three-phase charging;

• maximum current per phase.

The information may also be available in the manufacturer’s configuration tool or technical data sheet.

It is important to verify the exact version of the vehicle. The same model may be sold with different OBCs depending on:

• country or market;

• model year;

• battery version;

• trim level;

• optional equipment.

A more powerful charger may be standard in one market and optional or unavailable in another.

What are the most common On-Board Charger power ratings?

3.7 kW on-board charger

A 3.7 kW OBC commonly operates in single-phase at around 16 A on a 230 V supply.

This rating is found on some older electric vehicles and many plug-in hybrids.

For a smaller battery, 3.7 kW may be sufficient, especially when the car is regularly parked overnight. For a battery-electric vehicle with a larger pack, however, charging times can become considerably longer.

A 3.7 kW car will not benefit from the full power of a 7.4, 11 or 22 kW charging station. The higher-rated charger can still be used safely, but the vehicle will remain limited by its OBC.

7.4 kW on-board charger

A 7.4 kW charger is typically designed for single-phase charging at up to approximately 32 A on a 230 V supply.

This solution is common in markets where residential buildings mainly use single-phase electrical services.

Compared with a 3.7 kW charger, it can theoretically halve charging time, provided that the electrical system and charging point can supply the required current.

Not every property can dedicate 7.4 kW to a vehicle without an electrical upgrade or intelligent power management. Household appliances, heating systems and other loads must also be considered.

11 kW on-board charger

An 11 kW OBC usually operates on three phases at approximately 16 A per phase.

It has become one of the most widespread solutions among modern electric vehicles because it provides a good balance between:

• charging speed;

• component size;

• cost;

• heat management;

• residential and commercial compatibility.

At 11 kW, many medium and large EV batteries can recover a substantial amount of energy during an overnight stay.

For that reason, 11 kW charging is widely used in homes with suitable three-phase supplies, workplaces, hotels, apartment buildings and long-stay car parks.

22 kW on-board charger

A 22 kW OBC generally uses three phases at approximately 32 A per phase in compatible electrical systems.

It can significantly reduce AC charging times, but both the charging infrastructure and the local electrical connection must support that level of power.

A 22 kW on-board charger is less common than an 11 kW unit. It requires larger or more advanced power-electronics components and may need more substantial cooling.

For many drivers, 11 kW is already enough to restore the energy used during the day while the vehicle is parked overnight. Manufacturers must therefore weigh the benefit of 22 kW charging against cost, weight and packaging requirements.

Why is the 11 kW OBC so common?

The popularity of the 11 kW on-board charger is largely due to the compromise it offers.

It is much faster than low-power single-phase charging while requiring less current per phase than a 22 kW system.

For manufacturers, this can mean:

• smaller components;

• lower cost;

• reduced cooling requirements;

• easier vehicle integration;

• lower weight.

For drivers, it often means that a medium or large battery can be replenished within a normal overnight parking period.

Of course, the suitability of 11 kW depends on the availability of three-phase electricity. In regions where residential three-phase services are uncommon, manufacturers may prioritise different OBC configurations.

Why can AC charging power vary between versions of the same vehicle?

The same electric-car model may be available with several AC charging specifications.

Differences can depend on:

• production year;

• destination market;

• trim level;

• battery capacity;

• platform update;

• optional charging package.

One version may include a 7.4 kW OBC as standard and offer 11 kW as an option. A later model-year update may introduce a more powerful charger or change the maximum single-phase current.

Before choosing an EV charger, it is always worth checking the exact vehicle configuration rather than assuming that every version of the model has the same charging capability.

22 kW EV charger and 11 kW car: how much power is actually used?

Why the vehicle cannot exceed the OBC limit

When a vehicle with an 11 kW on-board charger is connected to a 22 kW AC charging point, its maximum charging rate will normally remain around 11 kW.

The charging point announces that it can provide up to 22 kW, but the vehicle requests only what it can process.

The OBC limit cannot be overcome by installing a more powerful AC charger.

The 22 kW rating describes the maximum capacity of the infrastructure. The 11 kW rating describes the maximum AC conversion capacity of the vehicle.

Can a more powerful charging station damage the car?

No. A correctly installed and compliant EV charger does not damage a vehicle simply because its maximum power rating is higher than the car’s OBC rating.

The communication between the vehicle and charging point determines the maximum permitted current. The vehicle then controls its own consumption.

The situation is similar to connecting an electrical device to a supply capable of delivering more current than the device needs. The device draws the current it requires; it is not forced to consume the full capacity of the supply.

The installation must, of course, be properly designed, protected and commissioned according to applicable local standards.

Is unused charging power consumed or wasted?

The unused capacity is not automatically consumed.

A 22 kW EV charger does not continuously draw 22 kW from the grid. It draws only what the vehicle requests, plus the relatively small operating losses associated with charging.

If the car is charging at 11 kW, the electricity demand will be close to that amount rather than 22 kW.

The remaining capacity is simply available but unused.

When can a 22 kW charger still be useful for an 11 kW car?

A higher-rated charging point may still be a sensible choice when future flexibility is important.

For example, it may be useful when:

• the driver expects to buy a vehicle with a 22 kW OBC later;

• several different vehicles will use the charging point;

• the site may eventually include more chargers;

• the installation serves a workplace or shared car park;

• the electrical infrastructure is already suitable for 22 kW;

• upgrading the charger later would be more difficult or expensive.

That does not mean a 22 kW unit is always necessary. Selecting an EV charger compatible with the vehicle and electrical system requires considering current needs, future plans, installation cost and available power.

How can you find out how many AC kilowatts your car accepts?

Where to find maximum AC charging power in the technical specifications

The OBC rating is usually shown in the vehicle’s official specification sheet.

Look for wording such as:

• maximum AC charging power;

• AC charging capacity;

• On-Board Charger;

• OBC;

• maximum AC charging rate.

Do not confuse this figure with the maximum DC fast-charging power, which may be significantly higher.

A vehicle may accept 11 kW in AC and 150 kW in DC. These numbers refer to different charging systems and cannot be compared directly.

What information should you look for in the owner’s manual?

The owner’s manual may specify:

• supported number of phases;

• maximum current per phase;

• nominal AC power;

• charging-cable requirements;

• adjustable current limits;

• charging behaviour on different grid configurations.

Some manufacturers provide separate values for single-phase and three-phase charging.

These details can explain why the car does not reach the maximum headline power under a particular set of conditions.

How to distinguish AC charging power from DC fast-charging power

AC power refers to charging that uses the vehicle’s on-board charger.

DC charging uses an external converter inside the fast-charging station. Direct current is sent toward the traction battery without passing through the normal AC conversion stage of the OBC.

A value of 150 kW in the vehicle specification does not mean that the car can charge at 150 kW from a home wallbox.

Likewise, an 11 kW OBC does not limit DC fast charging to 11 kW.

Why should you check the model year, version and trim?

Charging specifications can vary within the same vehicle range.

To identify the correct information, it may be helpful to check:

• the vehicle identification number;

• the original order configuration;

• the manual for the specific model year;

• official manufacturer documentation;

• information supplied by the retailer.

Relying only on the model name can lead to the wrong conclusion, especially where several battery or charging packages are available.

How should you interpret charging data shown by the car or EV charger?

Many electric vehicles and smart charging stations display real-time charging power.

This information is useful, but it must be interpreted carefully.

If an 11 kW vehicle is charging at 8 kW, that does not necessarily mean the OBC is limited to 8 kW. Power may be temporarily reduced because of:

• dynamic load management;

• vehicle settings;

• battery temperature;

• state of charge;

• grid voltage;

• charger configuration.

A single session is not enough to determine the OBC’s nominal rating. Official vehicle documentation remains the most reliable source.

Why can an EV charge more slowly than its OBC rating?

The electrical system cannot provide the full power

The fact that a car can accept 11 kW does not mean that the building can supply 11 kW.

Available charging power may be limited by:

• the site’s electrical connection;

• contracted or permitted capacity;

• cable sizing;

• protective devices;

• distribution-board capacity;

• EV charger configuration;

• other building loads.

In residential and commercial settings, the charging point is often configured below its maximum rating to suit the real capacity of the installation.

The vehicle is using one phase instead of three

An 11 kW vehicle normally reaches that power by drawing electricity from three phases.

If it is connected to a single-phase installation, the available power will be lower.

A wiring issue, configuration error or missing phase may also reduce performance. If a three-phase vehicle charges at roughly one-third of the expected rate, it may be worth checking whether all phases are present and being used correctly.

A charging limit has been configured in the vehicle or station

Some vehicles allow drivers to reduce the maximum charging current through the infotainment system or mobile app.

The charging station may also have been configured with a lower current limit.

These settings are useful for managing electricity demand, avoiding peak loads or adapting charging to a specific installation. However, they can cause confusion if they are changed and later forgotten.

Before assuming that the OBC is faulty, check the current limit in both the vehicle and the charger.

Dynamic load management is reducing power

An intelligent EV charger can adjust charging power when electricity consumption elsewhere in the building increases.

If high-demand appliances, heating, cooling, industrial equipment or other chargers are operating, the system may temporarily reduce the power assigned to the vehicle.

When the building’s consumption falls, charging power can rise again.

This behaviour is normal. Smart energy management is designed to prevent overloads and make better use of the available connection.

A smart EV charger does not increase the maximum power of the OBC, but it can distribute the site’s available energy more effectively.

Battery temperature and state of charge are affecting the session

The battery cannot accept the same power under every condition.

Very low or high temperatures may cause the system to limit charging. Power can also decrease when the battery approaches a high state of charge, particularly if the vehicle is balancing cells or protecting battery longevity.

The effect is often more visible during DC fast charging, but it can also influence AC charging.

Instantaneous power is not the same as rated power

The OBC rating represents a maximum capability, not a constant value.

During a session, charging power may vary because of:

• supply-voltage fluctuations;

• battery temperature;

• state of charge;

• dynamic load control;

• auxiliary consumption;

• vehicle software strategies.

A momentary reading below the maximum does not necessarily indicate a problem.

For a broader overview of the factors involved, see the Daze guide to what slows down EV charging.

On-board chargers and DC charging

Why is the OBC used for AC charging?

During AC charging, alternating current reaches the vehicle from the grid. The on-board charger must convert it into direct current before the energy can be stored in the battery.

The OBC’s power rating therefore sets the maximum AC conversion rate.

This is why a vehicle with an 11 kW OBC cannot normally exceed 11 kW when connected to an AC charging station.

How is energy conversion different at a DC charging station?

At a DC fast charger, the AC-to-DC conversion takes place inside the charging station rather than in the vehicle.

Fast-charging stations contain large power-conversion modules that can process substantially more power than a typical OBC.

The station supplies regulated direct current to the vehicle, where it is managed by the battery and high-voltage control systems.

The car’s normal AC on-board charger is therefore not responsible for the main conversion process.

Why can a car with an 11 kW OBC charge at much higher power in DC?

The 11 kW rating describes the vehicle’s AC charging limit, not the maximum power that the battery can accept from a DC charger.

During DC charging, the same vehicle may be able to receive 50, 100, 150 kW or more, depending on its design.

The maximum DC charging power is influenced by:

• battery chemistry;

• battery voltage;

• thermal-management system;

• cell design;

• high-voltage architecture;

• state of charge;

• charging software.

AC and DC power ratings must therefore be evaluated separately.

Maximum AC power and the DC charging curve are not the same thing

The maximum AC charging rate is largely determined by the OBC and may remain relatively stable during much of the session.

DC fast-charging power is generally more variable. It follows a charging curve that depends strongly on battery temperature and state of charge.

A vehicle may reach its advertised DC peak only for a limited period. As the battery fills, power is normally reduced to protect the cells.

For a complete introduction to electric car charging times, it is important to consider both the maximum power and the way that power changes during the session.

How does the on-board charger affect charging time and efficiency?

Relationship between battery capacity and AC charging power

With the same battery capacity, a more powerful OBC can reduce AC charging time, provided that the infrastructure can supply the corresponding power.

A 60 kWh battery will take less time to charge through an 11 kW OBC than through a 3.7 kW unit.

A basic estimate can be calculated by dividing the energy to be added by the effective charging power:

estimated charging time = energy required ÷ effective charging power

However, this is only a simplified calculation.

Real charging time also depends on:

• starting and target state of charge;

• conversion losses;

• variations in charging power;

• temperature;

• auxiliary systems;

• vehicle energy-management strategy.

Difference between theoretical and real charging time

Suppose that 55 kWh must be added to a battery and the car charges at 11 kW.

The simple theoretical calculation gives approximately five hours.

In practice, the session will usually take longer.

Some energy is lost during conversion, while the vehicle’s electronic and thermal-management systems consume electricity during charging.

The amount of energy measured by the charging station may therefore be slightly higher than the amount ultimately stored in the battery.

Energy losses during AC-to-DC conversion

No power-conversion system is 100% efficient.

During charging, some energy is lost as heat or consumed by:

• control electronics;

• cooling pumps or fans;

• battery heating;

• battery cooling;

• communication systems;

• vehicle computers.

The level of loss depends on:

• OBC design;

• charging power;

• ambient and component temperature;

• supply voltage;

• vehicle condition;

• active auxiliary systems.

An efficient on-board charger reduces these losses, but cannot eliminate them entirely.

Why very low charging power is not always the most efficient

It may seem reasonable to assume that slower charging is always more efficient. That is not necessarily the case.

Some systems in the vehicle remain active throughout the charging session and consume a relatively constant amount of power.

If charging takes many additional hours, these fixed loads may represent a larger proportion of the total energy used.

This does not mean that charging at the highest possible power is always best. The most efficient level depends on the vehicle, OBC efficiency curve, temperature, infrastructure and charging strategy.

How to choose an EV charger that matches the on-board charger

Must the EV charger have the same power rating as the OBC?

No. The charging point and on-board charger do not need to have identical ratings.

An EV charger can be:

• less powerful than the OBC;

• equal in power to the OBC;

• more powerful than the OBC.

If it is less powerful, the charging station will set the limit.

If it matches the OBC, the vehicle may be able to use its full AC capability when the electrical supply is suitable.

If the charging station is more powerful, the vehicle will still draw only what its OBC allows.

When does it make sense to choose a charger that is more powerful than the current vehicle?

A higher-rated charger may be useful when planning for future needs.

It may make sense when:

• the vehicle will probably be replaced;

• several cars will use the same charging point;

• another charging point may be added later;

• the building already has a suitable three-phase connection;

• upgrading the charging hardware later would be expensive;

• the site is used by employees, visitors or customers.

However, unnecessary oversizing should be avoided. A 22 kW charging point brings no immediate charging-speed benefit if the vehicle, cable, site connection or electrical system cannot support that power.

Why should a future vehicle change be considered?

EV charging equipment may remain installed for longer than the current car is owned.

Someone driving a plug-in hybrid with a 3.7 kW OBC today may switch to a battery-electric vehicle with an 11 kW OBC in the future.

Installing a configurable charging point and preparing the cable route for a higher power level can make the system more flexible.

This does not necessarily mean that the charger must operate at maximum power from day one. Many products can be configured to a lower current and adjusted later if the electrical system is upgraded.

Why are the building supply and number of phases still decisive?

Knowing the vehicle’s OBC rating is only one part of the process.

To use 11 kW three-phase charging, the site generally needs:

• a compatible three-phase supply;

• sufficient available capacity;

• a correctly sized dedicated circuit;

• suitable protection devices;

• a compatible EV charger;

• professional installation and commissioning.

If any of these elements is missing, charging power will be lower.

The installation should be assessed by a qualified EV-charger installer familiar with the standards and requirements applicable in the country where the charger will be used.

The role of intelligent energy management

A smart EV charger can adapt the vehicle’s charging power to the real-time consumption of the building.

This does not increase the OBC limit. Instead, it enables the available electricity to be used more effectively.

When the building is using little power, more can be allocated to the vehicle. When other loads increase, charging can be reduced temporarily.

In a multi-charger installation, intelligent systems may also distribute power between several connected vehicles. The principle is similar to dynamic power sharing: the available capacity is allocated according to real demand rather than divided through a rigid fixed limit.

This reduces the risk of overload and can avoid an unnecessary increase in the site’s maximum electrical capacity.

How can you recognise a possible on-board charger problem?

AC charging does not start or stops repeatedly

A faulty OBC may prevent AC charging from starting or cause repeated interruptions.

However, the same symptoms may be caused by:

• a damaged charging cable;

• dirty or damaged connectors;

• an incorrectly configured EV charger;

• an electrical-installation problem;

• a vehicle charging schedule;

• a communication error;

• an issue with protective devices.

It is therefore important not to assume immediately that the on-board charger has failed.

DC charging works, but AC charging does not

If the vehicle can charge successfully from a DC fast charger but cannot charge from any compatible AC station, the on-board charger may be one of the components that requires investigation.

During DC charging, the main conversion takes place in the external charging station. The usual AC conversion path through the OBC is bypassed.

This behaviour is not a definitive diagnosis, but it is useful information for the vehicle service centre.

Charging power remains unusually low at different stations

If the vehicle charges at an unexpectedly low rate on several known-compatible charging points, possible causes include:

• a software current limit;

• failure of one charging phase;

• an OBC fault;

• a thermal problem;

• a vehicle-side communication issue.

A useful check involves testing the vehicle:

• at more than one charging point;

• with another cable, where applicable;

• at different battery levels;

• under different temperature conditions.

One isolated session is rarely enough to identify the cause.

How to distinguish an OBC problem from a cable or charging-point fault

The most practical method is to compare the vehicle’s behaviour in different situations.

For example:

• Does another vehicle charge correctly from the same EV charger?

• Does the affected vehicle charge correctly from another AC station?

• Does changing the detachable cable solve the problem?

• Does the vehicle display a specific fault message?

• Does DC charging continue to work normally?

These checks can help narrow the possibilities, but they do not replace professional diagnostics.

When should the vehicle be inspected?

Vehicle support should be contacted when:

• AC charging fails at several different stations;

• charging power remains abnormally low without an obvious external cause;

• the vehicle reports recurring charging errors;

• charging stops repeatedly;

• one or more phases are not used;

• DC charging works but AC charging does not.

The OBC operates within the vehicle’s high-voltage system and should never be opened, modified or repaired through a do-it-yourself intervention.

Electric car on-board charger: the key points to remember

The on-board charger is one of the most important components involved in AC charging.

It converts the alternating current supplied by the grid into direct current that can be stored in the traction battery.

Its power rating determines how much AC power the vehicle can accept, regardless of the maximum rating shown on the charging point.

A 22 kW charger will not make a car with an 11 kW OBC charge at 22 kW. At the same time, it will not damage the vehicle or waste the unused capacity.

Effective charging depends on the interaction between:

• the on-board charger;

• the EV charging point;

• the electrical installation;

• the available phases;

• the site’s power capacity;

• energy-management settings;

• battery conditions.

Knowing the vehicle’s maximum AC charging power makes it easier to select the right infrastructure and understand what is happening during each charging session.

The goal is not always to achieve the highest possible number of kilowatts. Often, the best charging solution is the one that fits the driver’s daily mileage, parking time, electrical system and future mobility plans.

A correctly configured smart charger can coordinate the needs of the car and the building, making everyday charging simpler, safer and more efficient.

Frequently asked questions about electric car on-board chargers

What is an electric vehicle On-Board Charger?

The On-Board Charger is an electronic unit installed inside the vehicle. It converts alternating current supplied by a household outlet, EV charger or AC charging station into the direct current required by the traction battery.

What is the difference between an on-board charger and an EV charger?

The EV charger manages the safe connection to the electrical supply and communicates the available current to the vehicle. The on-board charger is installed inside the car and performs the AC-to-DC conversion.

How can I find out how many AC kilowatts my car accepts?

Check the vehicle specification sheet, owner’s manual or official manufacturer documentation. Look for “maximum AC charging power”, “On-Board Charger”, “OBC” or “AC charging capacity”.

Can an 11 kW car be connected to a 22 kW EV charger?

Yes. The car will normally charge at no more than 11 kW, or at a lower power if another part of the system imposes a stricter limit. The charging point will not force the vehicle to draw 22 kW.

Why does my car charge at 7.4 kW instead of 11 kW?

It may be connected to a single-phase supply, limited by the site’s electrical capacity, subject to dynamic load management or configured with a lower current limit in the vehicle or charging point.

Can a single-phase car use a three-phase EV charger?

Yes, provided that the charging standard and connector are compatible. However, the vehicle will normally use only one phase and will not benefit from the charger’s full three-phase capacity.

Is the on-board charger used during DC fast charging?

Not for the main AC-to-DC conversion. At a DC station, conversion takes place inside the external charger and regulated direct current is supplied toward the vehicle’s battery system.

Is a 22 kW OBC always better than an 11 kW OBC?

A 22 kW OBC can reduce AC charging time where compatible infrastructure is available. However, it may offer little practical benefit to drivers who can already replenish their daily energy needs overnight at 11 kW or less.

Does the on-board charger affect charging efficiency?

Yes. Its conversion efficiency affects how much of the electricity drawn from the grid reaches the battery. Temperature, charging power and auxiliary vehicle systems can also influence overall losses.

Can an on-board charger be upgraded after buying the vehicle?

Usually not. The OBC is integrated with the vehicle’s electrical architecture, cooling system, software and safety controls. Increasing its power generally requires hardware changes that are not supported as a normal vehicle upgrade.

How can I tell whether the on-board charger is faulty?

Possible signs include AC charging that fails to start, repeated interruptions, unusually low power at several different charging points or vehicle error messages. A professional diagnosis is required because similar symptoms can also be caused by the cable, charging point or electrical installation.