Electric Vehicle On Board Charger

In my last post on Type of EV Charging Connectors we looked at different types of Connectors for Charging Electric Vehicles. These connectors are either AC or DC. Depending whether they are AC or DC they connect to either the On Board Charger (OBC) AC-DC or directly to the Battery through an DC to DC converter.

Electric Vehicle Chargers

There are two types of Chargers:

  1. OBC: On Board Chargers that are built into the Vehicle
  2. EVSE: Electric Vehicle Supply Equipment or Charging Station (Can be AC or DC). These are outside the Vehicle either Type 1 or 2 AC in our Home, Office or Public or Type 3 DC Fast Charging Public Stations.

In this article, I will go through the On Board Chargers (OBC) which are primarily AC to DC.

How does the On Board Chargers (OBC) work?

Current from the Grid (AC) goes through EVSE (Charging Station Either Level 1 or 2) through a connector to the On Board Charger. The Onboard Charger rectifies or converts the current into DC and sends it to the Battery Pack. Whether the battery pack closes the contactors and what power it accepts is dependent on the Battery Management System (BMS). The On Board Charger will be communicating with the BMS as this will determine if the Battery Pack is OK to charge and what charge power it will accept. The Alternating Current AC can be either Single or 3 phase depending on the Chargers.

Communication is required between the Electric Vehicle (EV) and the EVSE. This is done through Pilot Wire. Pilot wire helps in the communication and to identify the Type of EVSE and requirement of Current for the OBC. Then the EVSE supplies the required input current.

If DC Charging is used then the Onboard Charger is Bypassed and the current is directly sent to the Battery Pack through BMS and a DC to DC converter.

Lets have a look at Vehicle schematics as below:


What Happens inside the On Board Charger?

There are 4 Stages in a OBC:

  1. EMI Filter
  2. Power Factor Correction (PFC)
  3. DC to DC Converter
  4. Filter

1. EMI Filter

First Stage of OBC is EMI Filter. This stage removes and filters unwanted Noise from the AC Power. Unwanted Noise which may be cause by Cellular Network Transformers, Electric Motor, Other Grid Variables. It also protects the other stages of OBC by limiting the voltage and Current spikes.

2. Power Factor Correction (PFC)

The Power Factor Correction stage is the AC to DC conversion stage.

Power Factor is ratio of True Power (W) to Apparent Power (VA). If the Ratio is less than 1 then Circuit wiring has to carry more current than necessary. Power Factor Correction is a series of methods using electronic devices to improve the power factor to near to 1. Power Factor losses can be either by displacement or distortion of the signal. Capacitors, inductors are used to to resolve displacement and bring the current wave in phase with the Voltage phase. Systems Distortion which are usually in Non linear circuits is bit more complicated than displacement which is in Linear Circuits. System Distortion can be corrected either by passive or active correction.

3. DC to DC Converter

The Input DC Power (Approx 700V DC) coming from Power Factor Correction is either Stepped up/down in the Transformer in the middle of the circuit. For most Electric Vehicles the voltage is between 400 and 1000V DC.

The isolated DC/DC Converter isolates the Power Grid from the High Voltage DC Bus. Note that a DC/DC Converter also sits between the HV vehicle bus and the 12V vehicle bus. In both cases these convert between bus voltages and isolate the buses for safety.

4. Filter

In the final Stage the output Filter is placed at the end to remove any remaining Harmonics. This is a fine tuning Stage. The Fine Tuned Voltage is then provided to the HV battery of the Electric Vehicle.

What is the Role of OBC?

OBC controls the Current and the Voltage the Battery needs to be charged either by the Constant Current Mode or Constant Voltage mode. Both charging modes have their own advantages and disadvantages. Constant current Charging is highly efficient and good charging speed but battery may be overcharged and reduce its lifetime later on. In Constant Voltage mode there may be too much current going into the battery at the start which may overheat the battery and reduce its lifetime.

Thus the charger goes for bit of both called the Charging Strategy, at the start it charges at constant current mode until the Voltage reaches a given amplitude and then it changes to constant voltage charging.

Source: https://www.hindawi.com/journals/jat/2019/2613893/

What Power Output does an OBC have?

Many Electric Vehicle Manufacturers have their own On Board Chargers but also there many independent Tier 1 Automotive Electronic Suppliers with their own On Board Chargers which they supply to the EV OEM.

Depending if its 1 phase or 3 phase the Power Output of On Board Charger usually varies between 3.7kW to 22 kW.

Voltages can be 120 to 240 V AC for one Phase or 240 to 480 V for Three Phase.

Some EV OEM Onboard Charger examples below:

Tesla On Board Charger: https://www.tesla.com/support/home-charging-installation/onboard-charger

Volkswagen ID 4: 7.2 kW https://www.myevreview.com/tech-specs/volkswagen/id-4/pure

Volkswagen ID Buzz: 11 kW https://www.myevreview.com/tech-specs/volkswagen/id-buzz/82-kwh

BMW: All BMW for example BMW iX3 currently have 11 kW https://www.bmw.com/en/innovation/how-to-charge-an-electric-car.html

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