Category Archives: Charger control project

Best use of generated power

Over the last few days I’ve been rethinking the best use of generated power.
The prioritisation of battery charging over water heating is clear due to the significant cost difference between day time electricity and any time gas, but the situation on car charging is more complex. It occurred to me that there could be times when prioritising battery charging and water might not always be the lowest cost solution since car charging avoiding mid-price nighttime electricity might be a bigger saving than a lesser amount of high value battery charging combined with low value gas-replacement.

For example, if we look at the lowest level of EV charging that amounts to about 1.4 kW. With our night-time rate of 7.87 p/kWh, 1.4kWh of solar power used for car charging saves 11.0 p of night time electricity.  If the battery is maxed out at 800 VAh that saves 7.34 p of later day time electricity. The water heating using the balance of 0.6 kW saves a further 1.76 pence of any time gas. Thus the total save from 1.4 kWh used for a combination of battery charging plus water heating is 9.1 p, compared to 11.0 p from car charging – so it would appear to be better value to do 100% car charging when a 1.4 kW surplus exists.

A bit of further analysis aimed to establish the point at which it became better value to charge the car, rather than combine battery charging and water heating, even if that involved a small level of mains import. The answer is that, with my energy costs, it makes sense to enable 1.4 kW of charger when 1.3 kW of export would have existed thereby potentially importing 0.1 kW. In practice this 0.1 kW may be supplied by the battery.

Given that the battery has priority by the way it’s wired, and takes up to 800 VA, then I intend to try a 500 W export threshold to start the car charger since 800 VA + 500W ~ 1.3 kW.

Charger software upgrade

img_0609One 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.

img_0610Although 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.

Solar PV Installation – 1 year on


We’ve now had our solar panels for 1 year, so it would seem time for an update on how things are going. Our 4kW SSE-facing system has an expected annual generation of 3,668 kWh. However after a year in service I’m pleased to see that we’ve generated 4,056 kWh – 10% more than expected.

The financial returns on a solar PV system are a combination of 2 things: (i) payments from your chosen electricity company for energy generated and exported to the grid and (ii) savings from not having to buy so much power as you use that which you’ve generated instead.

When it comes to payments from the electricity company, my electricity company (like many) chooses not to go to the expense of installing an export meter and instead assumes that half of the power which I generate is exported to the grid. The total annual revenue for the first 12 months of operation, including both generation and export (assumed to be 50% of generation) is £629.62.

Then there’s the question of how much electricity I save. I’ve only had monitoring of usage since March (approximately 6 months) but in that time I’ve used 41% of the generated electricity to replace bought electricity. 41% of 4,056 kWh is 1,663 kWh. What is less clear, is what the unit saving for this energy is. Some of this is daytime usage like standby loads, the fridge, cooker and other daytime loads at 11.7 p/kWh; but some would otherwise be night time loads like dishwasher, washing machine, or car charging at 7.57 p/kWh. I don’t measure the split so I’m simply going to assume an average unit rate = (11.7 + 7.57)/2 p/kWh = 9.6 p/kWh. 1,663 kWh @ 9.6 p/kWh = £159.65.

Finally, there’s the question of how much gas I save by making hot water from solar PV electricity rather than gas. Since March I’ve used 813 kWh or 27% of the generated power for water heating, so for a whole year 27% of 4,056 kWh is 1,095 kWh. The immerSUN itself records 995 kWh used for water heating since December. I’m also going to assume that not all the heat from the gas boiler would have ended up in the tank as hot water since some is lost via the boiler flue to the outside world, and some is lost via the pipes to the inside of the house – so let’s say 80% efficient on gas. 1,095 kWh @ 3.01 p/kWh @ 80% efficient = £41.20.

The combined total of my revenue and savings for a whole year would have been £830.46 – a 13.6% return on the investment or payback in 7.3 years.

The tariff scheme will have provided me with about 20 years of income by the time it closes, so the investment in the panels, as well as helping me reduce my carbon footprint, will have generated 12+ years of profit having paid for itself during the 8th year.

Charging ahead

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.

Results with variable charging

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.

imageThe 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:

Electricity Sources Electricity Uses
4.1 kWh of electricity was bought/imported. 13.8 kWh of electricity was used by the house (including car charging)
15.9 kWh of electricity was generated. 5.1 kWh of electricity was diverted for water heating.
1.1 kWh of electricity was exported to the grid.
20.0 kWh of electricity total input. 20.0 kWh of electricity total output.
93% of electricity generated used productively. 79% of electricity used self-generated.

For some time now my car charger has been linked to my solar panels so that it automatically turned on when enough surplus solar power was available enabling me to charge my electric car for free. This has helped me get up to an average of 70% of my generated electricity being used my me rather than export to the grid.

The latest refinement is intended to help me increase that beyond 70% while charging the car more quickly and reducing my use of bought electricity.

Variable rate charger

The new refinement allows the charger to switch between 0, 6, 10 and 16 Amps rather than simply off/on between 0 and 6 Amps. I needed a new case to get all the bits in, but the main technical difference is the use of a Programmable Logic Controller (PLC) to determine the desired charge rate.

The PLC is programmed from a laptop via a USB lead to create a programme in ladder logic which combines inputs (currently export threshold and contactor closed) with timers to switch on one of four different outputs corresponding to the four different charge currents (0, 6, 10 and 16 Amps).

Solar car charging results

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.

My radio-controlled car charger

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.

HomeEasyMy 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.

My remote controlWhen 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.

Electric Vehicle Charging Opportunity from PV

16-03-2016 no EVAs 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.

16-03-2016 6A EVFor 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.

Making best use of energy

If I want to make best use of my solar PV (i.e. use as much of it as possible) then it seems logical to prioritise replacing my most expensive energy purchases. Examination of my bills suggests that my different sources of energy have the following costs:

  1. Day-time electricity: 12.31 p/kWh
  2. Night-time electricity: 8.02 p/kWh
  3. Any-time gas: 2.87 p/kWh
  4. Solar PV (when available): 0.00 p/kWh

In winter when little PV electric is available our normal practice is to shift as much electric load as possible to night time use saving 4.29 p/kWh by use of timers. Washing machine, dishwasher, and electric car charging all get shifted to Economy 7 by use of timers. The washing machine is the oldest and uses a external timer plug while both dishwasher and car have their own inbuilt timers allowing consumption to be delayed.

In summer the same approach can be used to shift these loads to daytime to make use of solar PV although if the weather is dull and cloudy there’s no guarantee that sufficient PV will be available risking consumption of bought daytime electricity.

My Immersun already minimises exported/surplus PV by running the immersion heater to mop up surplus electricity, although as gas is cheap the savings are smaller than might be achieved by avoiding a similar amount of night-time electricity consumption. In winter we minimise gas use for water heating, while still providing a gas safety net by displacing in both time and temperature. The Immersun naturally heats water in daylight hours only and generates hot water at 60C (not adjustable) as defined by the immersion heater thermostat. The gas is set to complement this being set by timer not to overlap Immersun hours and is set to 50C by the cylinder thermostat. The water temperature difference is not noticeable in practice especially as the showers are thermostatically regulated.

Car charging 1The device that consumes most electric power is undoubtedly my electric car. Its usable battery capacity is about 10.5 kWh and, with charging efficiencies, it can take up to 12 kW of mains electricity to charge (less in summer). The appeal of charging this on solar PV is high, although the cost of doing this if it inadvertently draws day-time electricity is also high (but still much cheaper than petrol or Diesel). It occurred to me that it would be good to use spare channels on the Immersun to control car charging as my largest consumer of my most expensive fuel. It has three channels – two variable power and one on/off – although it can be configured to produce three variable outputs using the on/off to switch between devices.

It seems to me that there are three alternatives for control:

  1. Proportional control.
  2. Stepped control.
  3. On/Off control.

Proportional control would be the most capable. The Immersun has the capability to provide proportional control to multiple resistive loads, and the Mode 3 communications standard to which virtually all electric cars adhere also provides for the charging equipment sending a proportional signal to the car via PWM, so there’s scope for a proportional system. However it seemed that the time to develop this was beyond what I have available, and in practice although the car would receive a proportional control signal it would normally respond in a stepped manner so there’s little to be gained by investing my time to develop a fully proportionate system..

Stepped control is easier to achieve. As it happens my car model is more steppy than some as I believe that it switches between 6, 10 or 15 Amps consumption only. 15 Amps is very close to the full 4 kW output of my panels and, given that there are always other loads in the house, then I don’t think it would ever use this setting, which leaves me with switching between 0, 6 and 10 Amps. Unfortunately of course you can’t switch between 3 states with an on/off signal that only has 2 states, so I’d still need to decode an analogue Immersun output or invest in a second Immersun to get another on/off output but I deemed the latter cost-prohibitive. Generating a variable current signal to the vehicle would also require some logic to reduce the pulse width on the PWM signal on demand – not impossible but still requiring some time to create.

That leaves me with on/off control at a fixed current. This is readily achieved from the Immersun which has a relay output that can be configured to operate when a certain solar power is available (such as 1.5 kW to drive a 6 Amp charger). I decided not simply to turn the whole charger on or off by switching the power as this would cause the mains contactors in the charger and the HV DC contactors in the vehicle to drop out under load which is bad for longevity. Instead I’ll interrupt a low voltage control signal within the charger to trigger stop and restart in a more controlled manner – effectively car and charging equipment will see the signal as the user wishing to disconnect the cable as you might if you wished to drive before charging was complete.

Finally I considered how to connect the Immersun to the charging equipment. Unfortunately the Immersun is in the airing cupboard upstairs (where it drivers the immersion heater) and the garage where the car charges is a separate building alongside the opposite side of house. Neither a second Immersun or a physical cable looked like an attractive prospect on cost grounds so I decided on radio control using around £20 of home automation parts and a total investment of around £40. More on this solution over the next few weeks.