This DIY Level 2 Electric Vehicle Charger Costs About $200
James Fotherby managed to build a reliable and powerful electric vehicle charger inexpensively.
The motivation
Over the past decade, electric vehicles have gone from nearly unheard of to being quite common in nearly any urban/suburban area, and with this recent proliferation has come some challenges regarding the electrical infrastructure required to charge them. In response, James Fotherby wanted to create his own charger that could compete with off-the-shelf solutions while also being safe and simple to operate.
Introducing some safety
As with any projects that involve dangerous levels of voltage and current, Fotherby wanted to be extremely careful in order to minimize the chance of serious injury from a malfunction. Typically, this involves using a simple "pilot" signal that tells the charger to turn on whenever the car is plugged in, and ensuring no current flows if there is no car present. Another important aspect to consider was ground current detection which, similar to GFCI outlets, detect if there is a short between a live wire and electrical ground, such as in the case where a live wire contacts the car's chassis.
The power supply design
To address the issue of ground fault detection, Fotherby employed a current transformer in his power supply schematic that measures the amount of current flowing through both the live and neutral wires. If the values read are not equal, then the relay controlling power delivery should immediately shut off. For power delivery to the more sensitive electronics, an RAC10-15DK/277 AC to DC power module was included that changes the incoming 220V AC power into 15V DC, after which an additional linear voltage regulator decreases the voltage further so that the 5V Arduino Pro Mini can function.
Constructing the charger
Once the schematic was completed and parts had been sourced, Fotherby designed a simple two-layer PCB and had it fabricated. The board allows for the two high-power lines to enter the relays and exit, all while being controlled by the Arduino Pro Mini that continuously monitors the circuit for any issues. The now-assembled board was then placed into an enclosure sourced from a "showroom" EV charger, meaning that it lacks any electronics inside but still has a functional connector.
Software and testing
In terms of software, the code running on the Pro Mini keeps track of which charging state it is currently in and indicates it via an LED to the user. The addition of a self-test coil also allows the microcontroller to simulate a ground fault and catch it, with a fault detection time of less than 100ms being considered a passed test. To see more information about how Fotherby built this project, you can read his write-up here on Instructables.