OpenEVSE II starts by separating the HV and logic systems. The display board, which was optional for OpenEVSE, is mandatory for OpenEVSE II. It includes the display, controller, a real-time clock chip and a temperature sensor (to allow thermal shutdown). It is designed - as the OpenEVSE display board is - to be a "backpack" on an RGB backlit 2x16 LCD module. It has just about all of the low voltage logic, which means that troubleshooting can be done on the bench with nothing more than a +12 VDC power supply. It draws less than 100 mA of 12 volts in operation.
The logic/display board also includes the pilot generator and state transition monitoring systems, a GFI (with a test circuit), and an ammeter (based on a current transformer).
The logic/display board connects with a 10 pin FFC cable to the HV board. There are three variants of the HV board available. One is for a line-powered contactor, one is for an external 12 VDC relay, and the third has two board-mounted SPST relays, which simplifies chassis wiring (but is limited to 24A of charging current).
The current rating of the design for the external relay or contactor boards is limited only by the ampacity of the relay or contactor and the wiring of the high current path (which includes the power and J1772 cabling).
The HV board is responsible for supplying 12 VDC power, switching the relay or contactor on and off, and performing the stuck relay and ground continuity test. It also has a crude opto-isolated voltmeter, which allows OpenEVSE II to monitor power consumption rather than just current.