Category Archives: Charger control project

Zappi?

It’s now over a year since I built my solar-powered car charger which is enabled automatically by surplus electricity output from my solar panels, but can also be run on a timer when required to use cheap night-time power too.

However since then a new product has come to market from myenergi (a new company led by some of the team responsible for developing the ImmerSUN) which provides the same sorts of capabilities although in a professionally-produced solution. I don’t own one, I’ve never used one, but I think that this is a solution which I’d be seriously considering if I hadn’t already made something similar.

The only downside that I can see is that it’s available in either Type 1 or Type 2 forms with tethered leads only, so if you have a mixture of EVs then a single Zappi may not be compatible with all your vehicles; whereas a charger with a Type 2 socket outlet would be.

You can learn more about Zappi here

Generations of my solar charger

I just came across some of the pictures from last year of different iterations as I was developing my solar powered car charger.


The left picture shows my first attempt using a Mode 2 charger (i.e. one that plugs into a standard socket outlet). The design attempted to turn the car on and off by the equivalent of pushing the latch button on the vehicle connector. That approach stopped charging effectively, but starting charging was subject to long delays so that wasn’t a practical solution.

The middle picture show the second attempt using a commercial Mode 3 charger (i.e. one that’s hardwired into the fixed wiring). In this iteration the commercial charger was gutted so that, although it retained the original external appearance, inside was all different content including a protocol controller and a radio receiver. This was an effective on/off solution.

The right picture shows the third iteration which addd a programmable logic controller to generate a variable charge rate for the electric car i.e. more than just simple on/off. The hardware to achieve this is too bulky for the case of the commercial charger, and so it was repackaged in consumer unit case. A consumer unit case is cost-effective solution for a bigger box to house the DIN rail mounting components, but is of course only suitable for indoor use as it’s not waterproof to the required standard for outdoor use.

Contents of my solar car charger

My electric car charger is built into a case more normally used for household consumer units.  From left to right its contents are:

  1. 2 slots – Double pole on/off switch to isolate the incoming mains supply entering from below.
  2. 4 slots – Programmable Logic Controller (PLC) which takes 2 inputs (remote on/off via radio link and contactor status – see item #4) and generates 1 of 4 outputs (corresponding to off, 6 Amps, 10 Amps or 16 Amps).  Beneath the PLC (and not visible inside the case) sits a circuit board with an array of resistors corresponding to the required current settings.
  3. 2 slots – Protocol controller which handles the Mode 3 handshake with the car and switches between current settings based on the selected resistor.
  4. 1 slot – Contactor which turns the power to the vehicle on/off based on the output from the protocol controller.  Cable to car exits below.
  5. 1 slot – unused.

The dedicated charger circuit is fed from a RCBO in a small consumer unit on the other side of the garage which combines overcurrent protection (20 Amps) and Type A residual current detection (30 mA).

 

 

 

A mixed August day of charging


Today I was at home working on a DIY project while the car was on charge for much of the day, a day which was fairly mixed in weather terms.  I thought it would be appropriate for an update on the car charger which has been in operation for around a year.

You may recall that the car generally remains plugged into the charger whenever it is at home, but doesn’t generally charge until there’s sufficient surplus on the solar panels, unless timed charging has been enabled for when the weather isn’t so sunny.

The picture shows the charger itself built into a case intended for a consumer unit.  Alongside the charger sits the receiver for the Mainslink system which provides for a radio signal from the house turning on the charger in the garage.  The smaller black unit is the holster for the vehicle connector so that it doesn’t lie on the floor when not in use.

The screenshot to the left shows the electricity consumption of the house including the car charger (the purple line) tracking the output of the solar panels (the green area).  Any failure to fully use all the electricity available causes the remaining electrical surplus to be diverted via a proportional control to the immersion heater to make hot water (the blue line).

Over the course of the day although we’ve used 17.1 kWh of electricity directly, and another 2.8 kWh of electricity for water heating (making 19.9 kWh used in total); but we’ve bought only 3.1 kWh of electricity.

Latest software update

Part of the ladder logic program in the PLC

This afternoon I’ve spent a little time working on the charger. The charger stopped working on Monday after the power was interrupted for an hour or so while other electrical work was completed. A quick check at the time concluded that the radio link between the immerSUN and the charger had stopped working, and that the problem wasn’t a fuse, but other time commitments prevented further investigation until now.

The radio link carries the control signal from the immerSUN in the airing cupboard to the car charger in the detached garage.  The control signal is generated either when the immerSUN has detected that a suitable surplus of PV electricity is available to want the car charger enabled or its output current increased; or when a timed boost is programmed in the immerSUN typically to do overnight charging on cheap rate Economy 7 power; or indeed when a manual boost is demand via the immerSUN front panel, app or web portal.

Fortunately lack of the control signal doesn’t prevent car charging as I can disable the integral PLC (which leaves the charger on continuously) and allow the car’s internal timer to control when the car charges.

This afternoon I’ve successfully re-paired the receiver to the transmitter to restore normal charger operation and uploaded my latest software to the PLC.

The capabilities of the charger are now as follows:

Operating currents: 6, 10 and 16 Amps
Maximum continuous current (continuous grid load current):10 Amps
Peak current (from PV + battery only):16 Amps
Range switch interval:6 minutes
Minimum on time:10 minutes
Control signal watchdog (not during Economy 7 hours):15 minutes
Control source:ImmerSUN

Best use of generated power (cont.)

In order to illustrate how the combination of battery and immerSUN distributes generated electric power at different levels of generation I created this chart.

For different levels of power generation across the bottom, the chart shows how the power is divided between battery charging (and occasionally discharging), electric car charging, and water heating; which are generally prioritised in that order. My prior post explained the rationale for the 500 Watt switching threshold for the vehicle charger – based on 1.4 kW of mid-value car charging being better value than a mix of 800 Watts of high value battery charging and 600 Watts of low value water heating.

Alternatively you might like to consider that the horizontal axis represents passing time after daylight comes and that the chart shows how diversion changes as the sun reaches its zenith.  You might then view the end of the day as a mirror image of this as the output of the panels ramps down in late afternoon, although at the end of the day there’s the greatest possibility that one or more of the storage devices is/are full and thus the greatest chance of electricity being exported.

Of course all of this assumes that the storage devices aren’t already full, and indeed that the electric car is present at all. As storage devices fill, or indeed if the car is absent, the system automatically switches to the next best value alternative:

  • Battery full – more car charging and/or water heating.
  • Car full or absent – more battery charging and/or water heating depending on power output.
  • Hot water at maximum temperature – this is the lowest priority electricity use so when this is full we don’t currently have another use to divert power to. However there is an unused output on the immerSUN so it would be possible to drive another load. The underfloor heating in the kitchen would be a possibility, although there’s unlikely to be much overlap between days when there’s enough surplus to reach this point and days when kitchen heating is required so it may never repay the cost of fitting the cables.

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.

img_0612

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

solar-panel-ikea

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

image
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.