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Why do electric vehicles have onboard chargers?

Oct 25,2022 | TCcharger

Onboard chargers and electric cars

Every EV has an onboard charger, the main purpose of which is to convert Alternating Current from the grid into Direct Current for the battery.

When you plug your electric car into an AC outlet, power from the outlet is converted from AC to DC by your car’s onboard charger.

Power from the grid is Alternating Current (AC), but electric vehicle batteries store Direct Current (DC). EVs have an onboard charger to convert AC to DC, while some public charging stations have an in-built converter that works even faster (these are known as off-board chargers).

OBCs also perform other functions, like charge rate monitoring, current protection and handling the conversion for V2L.

In this guide, we’ll discuss onboard chargers in more detail, covering the basics so you can properly understand them.

In this guide
  • What is an onboard charger?
  • What is a two-way on-board charger (V2G)?
  • What is an off-board charger?
  • Design challenges with onboard chargers
  • Onboard charger power ratings
  • Why are PHEV onboard chargers slower?
  • What is an onboard charger used for?
  • Why is an onboard charger required in an electric car?
  • DC charging stations also have an onboard charger
  • How does an onboard charger work?
  • How efficient is the onboard charger conversion process?
  • How different is an onboard charger and an off-board charger for EV?
  • EV onboard charger manufacturers
  • Onboard charger physical limitations
  • Summary

What is an onboard charger?

An Onboard Charger (OBC) is a device in an electric vehicle that converts Alternating Current (AC) to Direct Current (DC).

Here’s a GIF animation showing how the process works:

Source: Tesla

Onboard chargers are necessary for electric vehicles because EV batteries are charged with DC power, but all home chargers (and some public chargers) are AC.

When AC power is fed into the onboard charger, the power passes through one or more diodes which only allow current to pass in one direction.

Here’s what an onboard charger looks like:

Onboard charger for an EV
Source: AVID

Onboard chargers vary in size, with thicker designs for more powerful devices. The image above shows a 22kW onboard charger.

Innovations in onboard chargers mean they are getting smaller and lighter, although they need to be relatively large to handle the conversion process.

What is a two-way on-board charger (V2G)?

A two-way onboard charger is a vehicle-to-grid (V2G) onboard charger that converts AC to DC to charge an electric vehicle, and DC to AC to send power back to the grid. In other words, it converts current both ways to power the grid and your car.

Vehicle-to-grid technology is proven to save drivers money by feeding the grid with electricity stored in the vehicle’s battery, which the grid pays for. However, V2G chargers are expensive, so the technology hasn’t caught on with drivers yet.

A simpler technology that has caught on is V2L (vehicle-to-load), which allows an electric car to share power with connected devices.

Another variant of bi-directional charging is V2H (vehicle-to-home), which requires the onboard charger in an electric car to convert DC current to AC.

In the future, we expect V2G and other bi-directional charging technology to catch on because electric vehicles have enormous batteries that can feed the grid.

What is an off-board charger?

An off-board charger is an AC-DC converter built into a public charging station. All DC public charging stations have an off-board charger that converts AC power from the grid to DC power for your electric car. Off-board chargers are larger and more powerful than the onboard chargers found in cars, enabling faster-charging speeds.

Your electric car’s onboard charger still handles power transfer to the battery at a DC charging station, but no conversion is required in the vehicle.

Design challenges with onboard chargers

An onboard charger is packed into a tight space in an electric vehicle, which creates thermal management and conversion efficiency challenges.

In a confined space, the onboard charger produces more heat and power is lost during the conversion process when the onboard charger exceeds temperature limits.

Onboard charger

Onboard chargers require efficient, simple designs that pack as much power into as small a package as possible. The less waste heat that is generated, the more efficient the onboard charger is.

Heat = inefficiency in an onboard charger in the same way it does with a 12v battery.

Sometimes, heat sinks are necessary to improve cooling for the onboard charger. A heat sink improves thermal efficiency but adds weight and complexity to the design. Weight is the killer of range in electric vehicles, and heat sinks can add 50kg.

Size is another challenge. Larger onboard chargers have greater power conversion ratings. For example, an 11kW onboard charger is typically around 25% bigger than a 7kW onboard charger.

Onboard charger power ratings

Onboard chargers are rated to convert a certain level of power in kW. The rating defines the speed at which the charger will charge your EV.

These are the most common OBC ratings:

  • 3.3kW onboard charger (typically used in PHEVs)
  • 7kW onboard charger (most common)
  • 11kW onboard charger (becoming more common, but requires three-phase for the max charge speed)
  • 22kW onboard charger (uncommon and only useful with 22kW AC chargers. Most 22kW chargers are DC)

The kW rating is the maximum speed the onboard charger accepts. For example, the KIA EV6 has an 11kW onboard charger.

Why are PHEV onboard chargers slower?

PHEV’s have a slower onboard charger because the battery is small and would overheat with faster charge speeds. 3.3kW to 3.6kW is the typical charging speed for PHEV onboard chargers because anything higher would create too much heat.

What is an onboard charger used for?

An onboard charger converts AC power from the grid to the DC power your battery needs. It is used whenever you plug your electric vehicle into an AC charger. It converts the AC input to DC input to charge the battery.

Why is an onboard charger required in an electric car?

An electric car needs an onboard charger because power from the grid is AC (Alternative Current) and batteries are charged with DC (Direct Current). The onboard charger converts AC to DC, allowing your electric car to charge.

DC charging stations also have an onboard charger

DC public charging stations take power from the grid (AC) and convert it to DC in a single charging unit, bypassing your EV’s onboard charger. The benefit to this is significantly faster charge speeds, while the trade-offs are a significantly larger and more expensive charging unit.

How does an onboard charger work?

When you plug your EV into a charger, the current passes from the cable to the onboard charger, which converts AC current to DC and sends it to the battery. The Battery Management System (BMS) is responsible for accepting the power.

Here’s a vehicle schematic showing the components of an electric car:

oyota Motor Europe
Source: Toyota Motor Europe

Onboard chargers secure accurate voltage matching for the EV battery. While the grid voltage is fixed, the battery voltage varies from EV to EV. 

Here’s a diagram of how the onboard charger works:


As you can see, the process workflow is relatively simple:

AC power flows through the cable > AC power is converted to DC by the onboard charger > the battery management system manages flow to the battery.

All modern fully electric vehicles have an onboard charger rated for 7KW to 7.4kW, with some supporting 11kW three-phase. Plug-in hybrid electric vehicles tend to have slower onboard chargers up to 3.6kW.

How efficient is the onboard charger conversion process?

The conversion from alternating to direct current takes place onboard your EV, in a process that is automated and imperceptible.

The conversion efficiency of AC-DC converters today is around 90%, although advancements are being made – this wide-band-gap based bidirectional onboard charger is over 96% efficient, according to its researchers.

How different is an onboard charger and an off-board charger for EV?

Onboard chargers in electric cars are smaller than off-board chargers in public charging stations, so they convert current at slower speeds. This is the biggest reason why public chargers are a lot faster than home chargers – they convert the current with a bigger, more powerful off-board charger.

EV onboard charger manufacturers

The biggest EV onboard charger manufacturers include:

  • AVID Technology Limited
  • BRUSA Elektronik AG
  • Bel Power solution
  • Current Ways Inc
  • Toyota Industries Corporation
  • Innoelectric GmbH
  • Eaton
  • Stercom Power Solutions GmbH
  • Xepics Italia SRL
  • Delphi Technologies
  • Infineon Technologies AG

Most of these manufacturers make EV onboard chargers in addition to other EV components like wiring looms, battery cells and semiconductors.

Onboard charger physical limitations

Onboard chargers have limited conversion capability based on physical size. A bigger onboard charger takes up more space and adds more weight, reducing vehicle range.

Also, the bigger the onboard charger, the more expensive it is. There is a reason why some electric cars have 6.6kW or even 5kW onboard chargers – budget constraints. Faster onboard chargers are more expensive, creating a barrier to purchase.


Onboard chargers in an electric car convert AC power from the grid to DC power for the battery.

Onboard chargers are essential electrical devices in electric vehicles, and they will be for as long as AC power needs to be converted into DC power in the vehicle.

The need for more efficient, higher range electric vehicles means engineers are tasked with making smaller and lighter onboard chargers without sacrificing longevity. An onboard charger should last as long as the vehicle battery!