One occasional issue with my solar powered charger has been an occasional glitch on the immerSUN which once in a while fails to turn off its relay output. The effect of this is that, as the output is held on, the charger output rises to 16 Amps and holds that level for an extended period. The illustration shows this happening late on August bank holiday Monday.

Manual intervention has shown that turning the charger off causes the the immerSUN to reset after a few minutes and normal charging can continue.

I’ve recently revised the charger software to identify this issue and to reproduce the manual intervention automatically i.e. turning off the charger until the immerSUN output resets. The issue is identified if, during daytime operation, the immerSUN output is held on for 15 minutes. If working correctly after 15 minutes the output will have ramped up to so close to the maximum 4kWp output of the panels that the immerSUN should have turned the output off. Today is the first day that I’ve seen this at work.

Although today’s generation at mid November is very different to late August, there was still sufficient power to run the charger occasionally but at 13:22 the intermittent issue recurred and the output latched on until 13:51 – just shy of 30 minutes – as captured by the ‘Threshold On’ and ‘Threshold Off’ messages in the daily log file.

After 15 minutes the charger shutdown. During the shutdown period the PLC holds on its ‘off’ output forcing the protocol controller to disable charging, while flashing its 16 Amp output at 1Hz to indicate its error state to any observer. It remained shutdown until the immerSUN relay output turned off after which normal operation resumed.

Today I read a number of comments elsewhere from those who didn’t think it was possible to run an electric vehicle in the UK with a ‘significant’ degree of solar charging. I’m not sure how they come to that conclusion when some of us are doing it.

Now clearly the vehicle needs to be home, sometimes, and there’s a limit to how much can be generated; but it certainly works for me. I average around 20 miles per day, which is about 6 kWh of electricity; but my average daily generation is 11 kWh. That’s clearly seasonal, so I doubt that I can solar charge through the depths of winter – November to January I don’t generate 6 kWh on average let alone have 6 kWh available for EV charging.

It needs a certain mindset. I think most EV users try to get a full charge when they charge but many go for several days between charges. With solar charging I try to ensure that I have enough charge on board for the day ahead (potentially charging a little overnight to achieve this when necessary), but on a sunny day with the car at home I can get a full charge during the day. I only try to get a full charge by the start of a day on imported electricity when I know I’m going on a relatively lengthy trip, otherwise I deliberately don’t aim for a full charge to leave space for solar input if available. Whenever the car is at home it’s plugged in ready for when the sun shines. Typically I’ll leave the weekend with the vehicle almost fully charged (1 full day of sunshine can get a full charge), that will drift a bit during the week, possibly benefitting from a day at home during the week if I have meetings in London, then by the weekend it’s almost empty.

Top-ups during the week (at least during the summer) are often before I leave for work in the overlap where I have some solar (but not enough to run the charger) but am also in the Economy 7 window.

Overall I think that I have a good claim that most of the electricity for charging comes from my own solar during the course of a year.

Today is my first day with the car plugged in all day since I completed the variable charging which directs the car to charge at 0, 6, 10, or 16 Amps depending on availability from my solar panels.

The green area shows the electricity generated by the solar panels. The purple line shows the demand for electricity – the larger changes are the car charger switching on and up/down. I myself witnessed it switching between 6 and 10 Amps while working in the garage. The blue line shows the second priority – electricity being used for water heating. The water heating control is more dynamic and adjustable in finer increments so it mops up what the car charger cannot.

In total:

Well, with a bit of tinkering, my solar car charger is working with the charger turning on automatically when the output from the solar panels is sufficient, but I already have in mind a few improvements.

My issues include:

• As it stands the system turns off as expected when household demand rises, such as when boiling a kettle, but it takes some time for charging to restart after the signal is sent, presumably down to some logic in the charging equipment. I’d prefer something a bit less dynamic in responding to short term demand changes, but that restarted sooner.
• The signal from the ImmerSUN tends to turn off too quickly to my mind as, having turn on a relatively big load, it decides there isn’t enough capacity to run a big load. The logic effectively determines that there isn’t enough spare capacity to turn on the load a second time, turns off the load, and then decides that there is enough capacity. To my mind this just cycles the power unnecessarily and, with the delays in restarting already reported, also reduces total energy transfer.

Consequently I’m already working on improvements:

1. Firstly I’m changing the protocol controller than coordinates the handshake with the vehicle. The new one should enable a more rapid restart after interruption without the uncertainties of the delays in the OEM controller. It will also provide an alternative mechanism to stop charging. Instead of suggesting to the vehicle that the cable is about to be disconnected, the new one allows more direct control of the current by resistor selection – or turning on/off via a contact across the resistor.
2. Secondly I’m upgrading to variable current rather than a simple on/off at low current. The new protocol controller documents maximum current choice by resistor selection, but I think that I’ve worked out how to provide infinitely variable current via an analogue voltage. The car will respond to the infinitely variable signal by drawing the highest compatible current from 0, 6, 10 and 14 Amps.
3. Thirdly I’m reconfiguring the output of the ImmerSUN to be more dynamic, but then..
4. Finally I’m building a module to sit between the transmitter receiver and the protocol controller. The new module will set upper and lower current limits and set the ramp rate / time constant for the system response.

My intention is that the result of this is that in response to a jump in available power the controller will slowly ramp up the output to meet the availability and then slightly cycle the analogue output as the ImmerSUN output cycles on and off. The vehicle will respond by cycling between the currents immediately above and immediately below the available current. As an alternative strategy it can be configured to cycle just below the available power, and thus theoretically not import any power from the grid, at the expense of not fully using the available solar power.

Which ever strategy is used the more highly dynamic water heating will continue to mop up any available power between what the vehicle takes and what’s available from PV.

This weekend I finished my radio-controlled charger.

You may recall that I wanted to link the charging equipment for my electric car to my solar panels so that the car automatically charged when there was enough power from the panels. I’d previously settled on a simple on/off system charging at 6 Amps (the minimum setting) since analysis had suggested that this would maximise energy transfer as it seems to be more important to charge for the most hours than, for example, charge at more current for fewer hours.

It seemed that I would be easily able to configure my ImmerSUN to turn the charger on and off using its Relay: Export Threshold feature but I wanted to extend control to my car in the garage some way from the ImmerSUN without running a cable so far.

My solution was to make use of a HomeEasy remote control light switch and remotely-controlled switched socket (available on Amazon for example). The switch is designed to send a radio signal up to 30 meters to operate the socket – a range of transmitters and receivers are available separately which can be paired by the user.

When I dismantled the switch I found that in normal use the operation of the rocker operated two pushbuttons – one for on and one for off. I removed the pushbuttons from the circuit board and replaced them by cables to the relay outputs of the ImmerSUN. At the same time I reasoned that the button cell in the transmitter might not last very long in service, so instead I repurposed an old mobile phone charger as a power supply. The mobile phone charger was notionally 3.6 Volts, rather than the 3.0 Volts of the button cell, so I also added a simple voltage regulator chip and smoothing capacitor to ensure that I didn’t damage the transmitter from any over-voltage.

If you simply wanted to switch some mains powered device then you’d need to do little more than plug it into the remote-controlled socket to be able to switch it on and off automatically, but for my car charging equipment I wanted to do something a little more sophisticated. I had previously decided to interrupt a low voltage control signal so that the charging system turned on and off gracefully as if the user was engaged in the normal process of stopping and starting charger rather than forcibly turning the power off as if there was a power cut. To this end I fitted a small mains relay inside an empty case intended for a small power supply like those used for a mobile phone, so that when power is turned on or off volts-free relay contacts open and close. These contacts are wired in series with a push button that normally starts or stops charging so that, as far as the car and charger are concerned, the button release that indicates ready-to-charge doesn’t happen until there’s enough solar power available (by default).

I also have the option to manually enable charging or set up a schedule from the ImmerSUN for times when there either hasn’t been enough solar generation or the car isn’t home long enough to benefit from it.

All I need now is a reasonably sunny day when the car is at home to try it.

As part of my electric car charger control project I thought I’d try to estimate the benefit of such a system. I’d already decided on resource grounds to do a simple on/off system. The car offers the choice of charging at 6 or 10 Amps. 10 Amps would of course charge more quickly, but would be available for fewer hours each day, so I thought I’d look at what might be possible with 6 Amp charging. My chosen date for analysis was March 16th a day on which we generated 19 kWh. The Immersun provided a hourly profile of consumption and generation through the day which included diversion of 5.4 kWh to generate hot water via the Immersun.

For my charging analysis I decided to prioritise EV charging over making hot water (an option within the Immersun) which would result in hot water being generated until 1.5 kW was available, once 1.5 kW was available then the car charger would be turned on, and then as the power increases beyond 1.5 kW then the water heater turns back on to absorb the excess.

The result of this strategy was that 7.5 kWh was diverted to charge the car, while water heating received 4.8 kWh (a slight reduction on the 5.4 kWh delivered for water heating without EV charging). Exported electricity dropped from from 7.5 kWh to 0.2 kWh – so effectively all the available power was used. 7.5 kWh is more than 50% of a full charge for the car and probably exceeds my average daily charge.

For the alternative 10 Amp charging strategy only a single hour could provide 2.3 kW for charging (a total of 2.3 kWh) so the 6 Amp charging strategy provided around three times the energy transfer to the vehicle.

If the project saved me the same 7.5 kWh from my night time electricity (my current pattern of car charging) that would save me 60 pence per day. Of course I wouldn’t save that every day – some days (particularly in winter) we don’t generate enough power and other days I’m at work (although it should work quite well at the weekend when I spend more of the day at home). Normally I start each day with the car fully charged, but on most days I return with a significant amount of unused charged (typically 50%), but I could choose a charging strategy where I don’t attempt to fully charge each day allowing some charging to be displaced from weekdays to the weekend. It thus gets quite hard to estimate an annual benefit.