MAILBOX: EV Charging Off-Grid

Intermediate

I have been following the discussion about off-grid electric vehicle (EV) charging with interest. I thought I would pass on details about my system for charging a Chevrolet Bolt.

To run our home’s PV system, I learned computer coding, and then built a diversion control using an Arduino microcontroller that interfaces with our MidNite Classic charge controller’s RS-232 interface. This control diverts all available energy that’s not needed by the batteries into our domestic water heater tank. This hot water is then used for domestic purposes and also heats the house using our radiant floor heating system. The solution to our EV charging was to find an electric vehicle supply equipment (EVSE) device that could be interfaced with the diversion control and also was capable of being user-friendly to the needs of an off-grid PV system.

I found the OpenEVSE. It is easy to communicate with the device at short distances using transistor-to-transistor logic (TTL) serial communications. Using TTL allows us to turn the device on and off remotely and increase or decrease the charge rate. Because it is suitable for use in an automatic control system and is capable of being controlled to vary its charge rate in proportion to the energy available for it, it’s very solar-friendly!

EVSE on, off, load-shed, increase charge rate, and decrease charge rate algorithms are coded into the diversion control. When the array output is above a certain wattage (2,000 W); the hot water tank is above a certain temperature (140°F); and the battery SOC is above a certain point (90%), the EVSE turns on at the minimum charge rate—6 A at 240 VAC. The diversion control then monitors certain system parameters. If more power becomes available as the batteries increase their SOC and hot water tank heats up, the EVSE charge rate is increased. Care is taken not to exceed the inverter’s maximum load. Provisions are made to shed loads and for a rapid shutdown if various system parameters are exceeded. As insolation diminishes at the end of the solar day, the EVSE charge rate is reduced to a minimum. The EVSE turns itself off at the end of the day when surplus electricity is no longer available.

The system functions automatically and manages the available energy for the house to ensure that, as the first priority, the batteries get charged; that, secondly, some hot water is always produced; and that, thirdly, the car charges. So long as there is enough sun on the modules, the batteries always remain within 0.3 V of the MidNite Classic charge settings regardless of system conditions. There is also a manual on/off button coded into the control, enabling the car to be charged manually, regardless of system parameters.

So far, we’ve been able to put the energy equivalent of 87 miles into the car in one day, while automatically charging our batteries and heating our 120-gallon water tank to 158°F. This code is open-source; it can be found on MidNite Solar’s forum (see bit.ly/MidniteEVSEcode) and also on the OpenEVSE forum.

Will Eert • Rossendale, Manitoba, Canada

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