It costs about £900 in the UK to have an EV charger installed by an electrician. This motivated me to question whether I could build my own. On researching the topic further I found an open source charger (the EVSE) with good documentation. This gave me the confidence to build my own.
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A word about safety. I’m not an electrician. I’m not allowed to make alterations to my consumer unit. I will have to remove the charger when I leave my house. I’m sure there are countless people that would condemn what I have done here and I can understand why. Building your own outdoor charger which switches 240V 32A could be dangerous if not done right. I have learnt about earthing systems, PEN faults, RCDs, cable current capacities etc. I think I have educated myself enough to have built an adequately safe system. Nevertheless, I welcome constructive criticism and discussion.
EV chargers use a simple “pilot” signal to detect when they are plugged into a car and to tell the car how much current it is allowed to draw from the charger. They don’t modify the mains at all, they just switch it on/off to the car via some relays. In addition to this they also incorporate the functionality of an RCD. But to be honest, that’s about it!
I managed to buy a second hand “showroom” Zappi charger. It came without any of the electronics inside but it gave me an enclosure, cable and plug to work with. I paid £120 inclusive of postage.
I bought 5 metres of 6mm² SWA cable to run from my consumer unit to my desired charger location. I added a 50A MCB into my consumer unit on the non-RCD side and routed the SWA cable using cleats and stainless steel screws.
The SWA cable enters the Zappi enclosure through an outdoor water resistant gland. The live and neutral pass together through a current transformer before attaching to the PCB.
One of the most important safety mechanisms to include is a ground current detection system. The chassis of the car is grounded via the earth wire through the charging plug. The earth supply comes from the consumer unit (we have TN-C-S supply).
There is quite a bit of theory behind grounding systems. John Ward has some instructional YouTube videos on the topic which I have watched. He discusses the problem of PEN faults etc. It’s worth spending the time educating yourself about earthing if you are doing any electrical work.
Although unlikely, it is possible for a fault to occur such that a live wire contacts the car chassis. Perhaps it's pulled loose inside the car somewhere and is touching the chassis or perhaps a wet connector is bridging a path to the chassis.
Either way the live will source current into the chassis which will drain straight to ground (In a TN-C-S supply the ground and neutral conductors are bonded at the consumer unit). The amount of current will depend on the resistance of the faulty bridge. (water is unlikely to allow many amps to flow). Given the chassis is well earthed, it shouldn’t rise up in voltage enough to present a shock hazard to someone that touches it.
Nevertheless, this is a fault situation that should be detected and dealt with. If some water is bridging the live to the chassis perhaps a couple of amps will flow (not enough to trip the 50A MCB for the charger) but enough to cause localised heating and further damage.
So we need to measure current flowing to ground (should be zero in normal operation). If this is more than 20 mA we want to isolate the car by opening the relays. The reason RCDs typically trip at 5-30mA is because this amount of current for a couple hundred milliseconds doesn’t cause permanent injury to humans. I like the wikipedia article on electrical injury.
AC-1: imperceptible, AC-2: perceptible but no muscle reaction, AC-3: muscle contraction with reversible effects, AC-4: possible irreversible effects, AC-4.1: up to 5% probability of ventricular fibrillation, AC-4.2: 5–50% probability of fibrillation, AC-4.3: over 50% probability of fibrillation
The way to measure current to earth is simple. We employ a current transformer measuring the common mode current of the live and neutral wires. All current should be differential (all current flowing out of the live wire should go through the load and return back through the neutral). If there is a fault and some current does not return then it must be going to earth. This is a common mode current and we want to be measuring this!
There's a bit more to deciding what speed home EV charger you need than is apparent at first. We'll work through how to calculate charging times as well as some factors to consider. Finally, we'll give you our recommendations based on some common use cases.
For petrol cars we think about fuel use in terms of Litres per 100 kms. For EVs we mostly use Watt hours per Kilometre.
Driving 10kms per day in a Model 3 will use around 180 x 10 = 1,800 Watt hours or 1.8 kilowatt hours (kWh's) per day.
Let's work out daily energy used based on how far you typically drive in a year. Each day will be different but it gives you a reference point.
Annual kms / 365 = kms/day.
To find your daily EV charging energy usage, multiply your kms/day x Wh/km for the car. A Tesla model 3 doing 41 kms/day = 41 * 180 / 1000 = 7.38 kWh/day
Medium EV - Tesla Model 3
Large EV - Tesla Model S
SUV EV - Tesla Model X
While you've probably never thought about it, the "charging speed" for a petrol car is the speed the fuel comes out the bowser, we could measure this in litres per second.
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For EV charging we measure this in kWs. There are three common charging speeds we see for home chargers:
A 7kW wall charger will give you 7kWh of energy for each 1 hour of charging.
We can calculate how long you need to charge by multiplying how much energy you need by how fast you put that energy in your EV.
A Tesla Model 3 doing 41 kms/day needs about 7 kWh per day. On a 2.3 kW charger you'll charge in 3 hours, on a 7 kW charger you'll charge in 1 hour, at 11 kW you'll charge in 40 minutes, assuming you charge every day.
Medium EV - Tesla Model 3 with a 2.3 kW charger
Medium EV - Tesla Model 3 with a 7 kW charger
Medium EV - Tesla Model 3 with a 11 kW charger
Say 8-12 hours, which, unless you're doing lots of kms each day, is much more than what most users will need as we calculated above. Many EV owners could probably make do with a slow charger, or even charge from a regular wall outlet.
Let's consider some other reasons you might want to be able to charge faster.
In order to charge from your rooftop solar, you need the car plugged in whilst the sun is shinning. Allowing for work, family, outings, errands there might be days when the EV is only plugged in for some of the time your solar is generating power.
When you are plugged in, to use all the available solar energy, you need to be able to charge at the same rate as that you're generating solar energy.
New solar PV installations are typically around 8 kWp, for much of the day they'll be producing energy at a rate of between 2 kW and 8 kW. On average, such a system will create 32 kWh energy per day. If you're charging from a regular wall outlet at 2.3 kW, you can only make use of less than a third of that solar energy. On the other hand, if you had a 7kW fast charger then most of the time you could make use of all of the excess solar, and if you're only plugged in for part of the day it might still be enough to meet your EV charging needs.
From another angle: it's not sunny all day every day, there are plenty of days where you get a few hours of sunshine and the rest is overcast or raining. To charge from your solar on these days, ideally you need to be able to capture all the available energy when it's there.
So to charge from your solar you often won't have that much time, it could be as little as 1-2 hours per day, the faster you can charge the better.
But, keep in mind if you move from a single phase (7 kW) charger up to a 3 phase (11 kW +), the minimum increments for charging speed adjustment become larger which may impact the accuracy of solar tracking.
In 2022, most Australian states (other than South Australia and parts of Queensland) provide a 9 hour overnight window when power is cheap via off-peak network tariffs. This gives you a lot of time for charging. Per above, the average user could get away with a slow charger on a regular wall outlet.
South Australia has some of the highest penetrations of renewables in the world and offers a window into the future. Their off-peak window has already been shifted to the middle of the day and is only 5 hours long, the next cheapest period is overnight but is also only 5 hours long.
As the energy system transitions to one primarily made up of renewable generation, energy will be increasingly cheap when there's lots of sun or wind and more expensive when there is not. Equally, moving loads which can be moved (such as EV charging) to times when renewable energy is abundant will play an important role in reducing fossil fuel use for generating electricity, because the supply market will adjust to meet demand.
Whilst you might be able to plug your EV into a charger for 10+ hours per day, the overlap of times when you're plugged in and energy is cheap or renewables are abundant could be much shorter in future.
It might not happen often, but you may from time to time you find yourself low on charge. You forgot to plug the car in overnight? You're doing a weekend outing and you forgot to increase your charge limit from 70% to 100% the night before?
When this happens you might only have 1-2 hours to add as much charge as you can to the vehicle to avoid having to use a fast charger on the highway. Faster charging in these times helps.
Taking all this into consideration, here's our recommendation:
NOTE: if you do install a 3 phase charger, the circuit should be 32 amps if possible, not 16 amps, because if you purchase a vehicle that only supports single-phase charging (which is most of them) you will be limited to 3.6 kW on 16 amps.
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