Lessons in automatically solar charging an electric car

There now appears to be broad agreement that within a few years the electric car (EV) is going to be a fundamental component of our energy systems.
This raises the question of how the EVs will be fuelled. Are we going to have a car fleet fuelled, at least in part, by coal up until the time we have fully decarbonised our electricity grid?  Do we have to accept this? How easy is it to achieve fossil fuel free motoring with an EV today?
At the start of this article I must stress that this is not intended to be the promotion of particular products or ways of doing things. It is simply an account of what my family has done in an effort to have a fossil fuel free EV. I am sure there will be other ways to achieve similar outcomes.
When we bought our EV, a Nissan Leaf, in early 2014 I was totally focussed on achieving carbon neutrality. After considering a few options, we decided to install a 2 kW solar PV system to offset the electricity we were using in our EV. This easily generated more electricity per year than we were using in the EV travelling about 15,000 km per year. However, we were charging the car each night with grid (ie mainly coal based) electricity – we were carbon neutral but not carbon free.
After about two years I came to the point where I didn’t feel too comfortable with just achieving a carbon neutral EV, I wanted to see if we could become fossil fuel free. So I began to ‘manually’ solar charge the car directly from our solar PV panels (opportunistically turning on the charger when we had plenty of solar production) but I really wanted to find a way to automatically direct solar charge our EV.
At this time, we were in the middle of an energy make-over for our household and the direct solar charging of our EV was a key step in this project. Quite coincidentally, as part of this energy transition project we had installed an Immersun unit to manage the heating of our house hot water using solar PV electricity.  It was only after I had installed the Immersun that I realised that it could also be used as a very effective device for automating the solar PV charging of our EV. While the Immersun is primarily marketed as a device for diverting solar PV electricity into resistive loads, particularly water heaters, the unit also contains what the User Manual terms a ‘multifunction relay’. This relay, which in effect is a programmable switch, can be programmed to switch external devices on/off under conditions specified by the user.
It was a very simple job for my electrician to wire up the relay in the Immersun to the Level 1 charger (EVSE) that came with the Leaf when we bought it.

The Immersun unit on the wall in my study
The Immersun unit on the wall in my study

Operation Overview
Whenever my EV is in my garage I plug in the Level 1 charger. I have set up the relay so that when the electricity export from our house exceeds the power draw of the charger (2.5kW) it turns on and stays on for 15 minutes. At the end of the 15-minute period the system stops and checks whether the export still exceeds 2.5kW, if it does charging recommences. If clouds have come across and solar production has dropped, the system does not start charging again until sufficient solar production has returned. This process continues throughout the day without any intervention from me until the battery is fully charged, or I disconnect the charger in order to use the car.
My only intervention in the charging process happens each night when the car is docked and the sun has gone down. I have to decide whether I need to put any electricity into the battery overnight (using off-peak electricity) so that I can use the EV the next day. This is a relatively straightforward decision which looks at things like how full I need the battery to be and whether there are likely to be a few hours of solar charging time the next day before I need to use the car.
The sign showing my EV charger is controlled by the Immersun
The sign showing my EV charger is controlled by the Immersun

How well does it work?
For the first 20 months I owned the EV I ran it almost totally on off-peak grid electricity – direct input from our solar PV system was negligible. Over the summer of 2015-16, when I was ‘manually’ charging the car, my monitoring indicated that about 60% of the electricity used by the EV was generated by our solar PV system. I was expecting that if I continued with the same routine, this level of self-consumption would drop dramatically over winter as the level of solar PV production fell.
Screen Shot 2016-06-14 at 9.50.50 AM
I had the Immersun solar charging set up installed at the end of March. Over April and May about 50% of the electricity used by my EV came from our solar PV. The graph above, which is based on continuous monitoring of the charger circuit, shows the time distribution of electricity input into the car over the two months. About 85% of the electricity input during the hours between 8am and 4pm came from our solar PV system. Given that the last two months have been at a time of the year with significantly reduced solar production, I estimate that over a whole year around 70% of the electricity used by the EV will be provided by our solar PV system.
To date, our Immersun solar charging system is working beautifully. While this works well for us it is important to point out that this will not be a solution for everyone. Clearly, your EV needs to be at home and available for charging for at least a few hours a day. The amount of available solar power will also determine how well this works – we have a 6.5kW PV system which is available for charging our car.
Further Details
I have extracted this article from a longer document which I have recently compiled using Microsoft Sway.  This Sway contains specific details about the settings I am using in the Immersun unit; it also contains a range of general observations about solar charging.  You can access this more detailed document from here.

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