This project is intended to manage the charging of an electric car so that the charging current is varied in accordance with available solar power from a PV array.

Electric cars require an EVSE (Electric Vehicle Supply Equipment) to connect to the domestic electricity supply. It is the responsibility of the EVSE to signal to the car the maximum current it is allowed to draw based on factors like the rating of the domestic wiring, and the cable connecting the vehicle to the EVSE. A smart controller inside the EVSE called an EPC (Electronic Protocol Controller) performs the function by setting the duty cycle on a 12V pilot signal sent to the car via the charging cable. One of the leading models of EPC is manufactured by Viridian (sometimes labelled as MainPine) What makes the Viridian EPC interesting to a DIYer is the fact that the available current signalled to the car can be controlled by applying a voltage on a control pin on the Viridian EPC. This project is for an Arduino based device to set that control voltage based on a calculation of available solar power made by an OpenEnergyMonitor system, and transmitted to the Arduino via a radio link. The Arduino takes no part in the decision making process. It merely applies the voltage dictated by OpenEnergyMonitor. The Viridian EPC has its own fail safes to prevent an unsafe current being signalled to the vehicle.

or clone, or stripboard with Atmel328P No crystal required. I used a Xino which is a minimalist Arduino clone with no usb interface, and I modfied it to run off 3.3V power which has the advantage of allowing finer control of the Digital to Analogue Converter, and means no step down resistors are required for the RFM69CW radio module which requires a 3.3V supply. By burning a new bootloader into the Atmega328P chip I was able to run it using the 8MHz internal clock and so saved on the cost of a crystal, and reduced power consumption. Power was drawn from the 5V rail of the Viridian EPC itself by wiring in an extra screw connector into the housing of the EPC. Using a Xino at 8MHz the current requirement is 20mA.

• MCP4725 Digital to Analogue converter module. It is very cheap to buy MCP4725 chips on ebay, ready mounted on a pcb which saves the trouble of trying to solder surface mount components.
• RFM69CW 433MHz or 868MHz radio module. This is the radio module used in the OpenEnergyMonitor project. They are cheap on ebay.
• Viridian Tethered EPC
  Viridian make their own EVSEs, but I was unable to buy one at a sensible price so instead I bought a Rolec EVSE and replaced the Rolec EPC with a Viridian. Since both the Rolec and Viridian EPCs are DIN rail mounted modules this was a simple exchange.
• EmonPi or Emonbase from OpenEnergyMonitor.org

• Jeelib
• Wire
• Adafruit_MCP4725

• Emonhub Part of EmonPi or Emonbase. Used to transmit commands over radio link.
  A patch is required for emonhub to allow it to transmit control messages to Solar_EVSE. See GitHub for required patch to emonhub.
• Node-Red Included as part of EmonPi. Used to calculate Solar PV power PV available. See Node-Red folder for an example flow.

This sketch is very simple. The process loops continually waiting for a packet ready flag from the RFM69CW radio unit. When the packet arrives it is validated for CRC and correct address and a single integer variable is read. If the variable has msb bit set, then it is a command to set a new default Power on Voltage for the DAC to use. If msb bit is zero it is a new command to set a voltage on the DAC. The voltage set on the DAC is passed to the Viridian EPC which uses it to vary the duty cycle of the Pilot signal given to an attached electric car, and so regulates the current drawn while charging.

Installation notes are here