The world is entering a period of rapid buildout of EV charging infrastructure. Average total cost of EV ownership is falling below that of comparable internal combustion vehicles while EV adoption rates continue to trend upwards.  Expect to see more and more electric vehicle chargers in parking lots and gas stations.

Porticos has broad experience in the engineering design of electric vehicle chargers.  In this series of posts we’ll explore the intricacies of electric vehicle chargers from the perspective of new product development.

Read Previous Post: EV Charging Basics

Welcome back to Electric Vehicle Chargers, A Development Perspective. Part 1 of this series defined EVSEs and discussed the international standards defining their performance. But product development and product engineering require deep understanding.  Today, we crack open the case and discuss the inner workings of EVSEs.

AC Level 2

Residential and commercial AC Level 1 and 2 charger design is governed by the UL Standard “UL2594: Standard Testing for Electric Vehicle Charging Stations”.  As noted in our first post, these AC EVSEs are simpler than DC chargers, therefore they provide a good starting point when discussing the inner workings of EVSEs. Below is a circuit diagram of the OpenEVSE Project charger followed by a description of each component and its function.  OpenEVSE is a publicly available Arduino-based charging station which uses open source hardware and software.  OpenEVSE is the basis for many commercially available EVSEs on the market today.

Diagram of the OpenEVSE project.
Diagram of the OpenEVSE project. Corner icons added by Porticos. Source:

Mains Power

Let’s start by first discussing the EVSE connection to grid power. This can be as simple as plugging in an AC Level 1 charger directly into a standard household 120V outlet (sometimes referred to as “mains” or “line” power).

In the United States, higher-power (Level 2) AC chargers generally use 240V (Split Phase) power. 240V power can be supplied either through a clothes dryer-type outlet or it can be hard-wired directly into the charger for a more permanent and robust installation.

EVSEs that can be plugged into a wall outlet are most applicable for individual residential ownership, since they can easily move with the owner. The hard-wired dual AC Level 2 charger shown above is marketed towards the commercial market. Despite being much larger and more commercial-looking, it still charges vehicles at the same rate as residential AC Level 2 chargers.

Commercial hard-wired dual AC Level 2 EVSE
Commercial hard-wired dual AC Level 2 EVSE. Chargepoint media repository.
Charging Relay detail view.
Charging Relay detail view. Source:

Due to the low power consumption of AC Level 2 chargers (~7W while charging), they do not produce much waste heat. As a result, they do not require constant cooling airflow over their components and are generally housed in rugged weatherproof boxes. More on that in later sections.

Grid power is run to two places within the EVSE. First, large gauge (high-power) wires direct power to the charging relay.  The current rating of this relay determines the maximum charge rate of the EVSE. Grid power also runs through small wires to an AC-DC Power Module.  This provides DC power to the onboard electronics and also provides the DC power necessary to activate the relay.


The controller is responsible for safely managing the charging cycle. Its performance is governed by UL991: Tests for Safety-Related Controls Employing Solid-State Devices. The controller relies on several channels of information to determine the state of the charging system:

  1. A GFCI sensor compares how much current is entering and leaving the vehicle. Any difference between the two means power is flowing to ground, and the system has a ground fault. This sensor features a third wire that allows the controller to test the GFCI before initializing a charge. GFCI requirements are covered under UL2231-1&2.
  2. A current sensor (black oval with black wires) monitors the charging current, ensuring that the vehicle doesn’t pull more current that the infrastructure and EVSE can support.
  3. Using proper isolation methods, voltage is monitored on the EV side of the charging relay in order to ensure that the charging relay is operating as intended and that the grid voltage is within an acceptable range.


The controller communicates with EVs by sending a 1kHz ±12V square wave signal to the EV along the control pin (purple wire). This is called the Control Pilot signal. The EVSE broadcasts the maximum allowed charging current by manipulating how long the square wave remains at +12V. This is called Pulse Width Modulation (PWM). The EV can communicate its state to the EVSE by adding resistors between this signal and ground, which changes the voltage of the square wave. These level shifts are detected and decoded by the EVSE controller.

Controller detail view
Controller detail view. Source:
GFCI sensor detail
GFCI sensor detail. Source: CR Magnetics.

“Keep in mind that since the EV controls AC charging with its on-board charger, the EV will not always charge at the maximum rate allowed by the EVSE. Charge rate can be limited by the maximum charge rate of the OBC and the state of charge and temperature of the battery.

Sam Mayes, Porticos Electrical Engineer Tweet

More detail on the Control Pilot signal can be found by visiting the Signaling section of the SAE J1772 Wikipedia page. The OpenEVSE project also has an excellent video showing a live oscilloscope view of the Control Pilot signal that is highly recommended to interested readers.

The controller also handles input and output (I/O) of the EVSE . In the example of OpenEVSE, this includes a display and a WiFi module.  Other I/O features common to many EVSEs include:
  • Indicator LEDs
  • Buttons
  • Speaker
  • Touchscreen
  • NFC card reader
  • Numeric keypad

The Vehicle

To initiate charging, the controller provides 12V power to the charging relay. The charging relay then closes, allowing high-current AC to pass to the EV. The EV then reads the maximum allowed current from the Control Pilot signal and charges at or below this rate.  While charging, the controller monitors the charging current and the GFCI signal (to protect against a ground fault failure) and will turn off the charging relay if a problem is detected.

More to Learn

This high-level breakdown of AC charging station systems and components is intended to provide a basic understanding of how the various components of AC Level 2 EVSEs work together to provide a safe and reliable charging solution for electric vehicles.  While the inner workings are relatively straightforward, there is still plenty of flexibility for product differentiation when it comes to form factor, special requirements, and User Interface/User Experience.

Porticos continues to help clients with their varied and specialized EVSE design and development needs.  If this blog has inspired any general questions related to EVSEs, or any specific product development questions or needs, please let us know – we are here to help.  Our next blog in this series will explore the more complicated DC charging hardware and thermal design.

XL200P Exploded View

About Porticos


About Porticos — Porticos, Inc. is a mechanical engineering and product design development company located in Research Triangle Park, NC.

Established in 2003, Porticos continues to produce innovative and effective solutions for its clients and the markets they serve. Porticos, Inc. provides mechanical design, analysis, research, and development services to clients including Dell, Motorola, Raytheon, and many others.