Have you ever wondered what’s inside one of these electric car chargers? Well then, this is the blog for you! The short answer is, not a lot really. The longer answer has to do with how electric vehicles charge themselves because this isn’t actually a charger. The industry name for this device is EVSE, which stands for electric vehicle supply equipment. Kinda weird that we’re calling it “an equipment” but let’s not get any more pedantic than this blog is already guaranteed to be. Fundamentally, this device is merely a controlled access point to the power grid. I’ve been wanting to make this blog for a long time but finally got motivated to do it thanks to a series of interactions on Twitter. You know who you are. First, qualifications; this blog is discussing Level 2 AC charging in North America. Those of you with your fancy three phases need not comment on your Type 2 connectors because yes, I know they exist in Europe, Australia, and elsewhere but they do not exist over here so mentioning them is irrelevant. DC Fast Charging, the most famous system of which being the Tesla Superchargers, is not where we’re headed either.

Not today, anyway. Instead, this blog is about quote unquote “slow” at-home or at-the-office charging and the devices which make that work. Unless you plan on exceeding your vehicle’s range in a day (in other words are doing long distance travel) this is how the vast majority of your charging will (and in my opinion, should) occur. I very much hope we focus on getting more level 2 chargers in more places to help accommodate those who need a car but don’t live in a single family home rather than make that huge percentage of the population reliant on DC fast charging, thus replicating the fueling infrastructure we have today but with high-speed chargers instead of gas pumps. Plugging in at home or at work is not only much more convenient, but requires less radical infrastructure and from my perspective seems a lot easier to manage on a connected smart grid. But the main reason why I hope we go with the more Level 2 less DC charging route is because these things are stupidly simple devices.

I said earlier that these aren’t chargers; in fact, these are basically just fancy light switches. Let’s explain what I mean. Most things we call chargers are really DC power supplies. Yes even those aren’t often really chargers. They contain electronic circuitry designed to turn AC mains voltage into a defined and stable DC voltage; in the case of standard USB that would be 5 volts. The charging is done by the device you give the 5 volts to because it knows how to treat its battery best. Electric cars are pretty much the same. In the case of Level 1 and 2 charging, they handle the charging themselves. But they go one step further than your phone or laptop. Because electric vehicles come in all sorts of shapes, sizes, battery chemistries, and battery pack voltages, it’s not really feasible to put the power supply outside of the car. So it isn’t. This device has no voltage conversion circuitry in it of any kind, except for the wee bit it needs for its own electronics. A reminder to the keyboard warriors out there; we’re not talking about DC fast charging in this blog. This device has one job and one job only. To safely deliver AC power to a car.

The car itself contains the battery charger. Want to see it? Well, in the Chevy Bolt EV it’s here. These four boxes represent all of the electrical bits of a Bolt. The Bolt’s Bits. Up on top here to the left is a junction box which splits out traction battery voltage to the inverter module and high-power accessories like the air conditioning compressor and cabin heater. Below it is the inverter module which actually drives the traction motor (and thus wheels) and recovers charge under regenerative braking. On the right up top you have a DC-DC converter which takes high traction battery voltage and steps it down to 13.8 volts or so to power accessories and charge the 12V battery (in other words this is the equivalent of an alternator in a conventional car). And below it is the onboard charger. Side-note, a lot of you may be asking why an electric car needs a standard 12V lead-acid battery. Technically, it doesn’t *need* it but the design of the car becomes much safer and also easier if you can run conventional things like lighting, infotainment, power windows, the computers, etc off of low voltage. It’s easier because it means the things that are the same from a gas car to an EV don’t need to change – imagine a 400 volt turn signal bulb. And it’s safer because, well for one you don’t have high voltage wiring running all over the place, but more importantly when the car is off and not charging, contactors inside the battery open removing traction battery voltage from everything.

What are contactors? Well, you’ll find out shortly. You can actually hear those contactors close when you turn on the car. [two dinstinct thunks] [Classic General Motors Seatbelt Bong] Or plug it in. [A single clunk, followed by a beep] And obviously when the high voltage system is shut down, you need something else to close those contactors and turn it back on, and that’s why the bulk of the car’s control systems run at the conventional 12 volts and why there’s a standard battery, too. Anyway, again, this is the onboard charger. It’s kind of hard to see in situ, if you’d like a closer look at these components I can’t recommend this blog from the Weber Automotive YouTube channel highly enough, it’s fantastic. Also everything is a lot cleaner. But this module here is the actual charger. It takes AC power coming from the car’s charge port, rectifies it to DC, boosts that up to the battery pack’s required charge voltage, and sends that into the battery pack. Inside the pack itself you’ve also got various modules which monitor each cell and help balance everything out. Now, here’s where things get a little complicated and where the critical role of the EVSE comes in.

The car’s onboard charger may be capable of pulling more power from the grid than a given circuit can safely provide. It’s the EVSE’s job to tell the car how much current it can pull and then to supply it with voltage when requested. Let’s now take a closer look at the device itself. This is a Siemens VersiCharge unit, a fairly basic EVSE. It has a NEMA 6-50 plug on one end, and the industry-standard SAE J1772 connector on the other. Since 2010, every single battery electric vehicle and plug-in hybrid for sale in the US that doesn’t begin with T and end in esla has this very connector on it. From the Chevy Volt, the Ford Focus Electric, the Nissan Leaf, the Volkswagen E-Golf, the Toyota Prius Prime, the Kia Soul EV, the Hyundai Kona Electric, and yes, even the Wheego LiFe, they all have this charge port. This connector is part of the universal nationwide (except for Tesla) Levels 1 and 2 charging standard capable of delivering up to just shy of 20 kW, though typically most units deliver between 6 and 7.2. This unit, like many out there, is rated for 30A, so depending on the voltage it receives it can deliver between 6.2 and 7.2 kW. We find a user interface on the front of this device, though not all EVSEs are going to have one. The most useful thing this does is allow you to delay charging for up to 8 hours in two hour increments, a fairly easy way to take advantage of time-of-use rates if your utility offers them. Though it should be noted most cars can do this themselves, in fact on the Bolt EV you can even program that based on GPS location so it charges immediately when on a public charger, but only between certain times at home.

A lot of the symbology on here is, I believe, shared between multiple models because I don’t think this unit has any sort of WiFi connectivity (though frankly, I don’t really care either way). Oh, and of course it has fancy lights on it because what good is driving an electric vehicle if you don’t get to be smug about? Let’s now open it up. You might be surprised to learn that most of what’s in here is simply empty space. The unit is way larger than it actually needs to be, partly so it can have those fancy lights to help your smugness, and partly because it helps manage the cord when not in use. The power leads from the plug (which interestingly are not colored properly, the white wire should be red since there is no neutral but whatever) go to a terminal block, allowing you to replace the plug or hardwire the unit if so desired. One reason to do that is that this particular model is weatherproofed and so can be outside if installed and wired properly. From there a few small feeders go to the circuit board, and two large conductors go to one side of a contactor. Out the other side of the contactor you’ll see that it goes to the main conductors of the charge cable. This contactor is the single control device of this unit.

A contactor is simply an electromagnetic switch. When energized it closes, connecting the charge cord directly to incoming power.When it’s open, it doesn’t. That’s it. That’s all these devices do. They’re a fancy light switch. [loud CLACK of contactor] But it is a critical safety device and what keeps your car from overloading a circuit. Let’s talk about that part first. Keep in mind that the car is the load. The charger is inside the car. This device is simply a gateway to the grid. It is labeled as though it’s a 30A device which makes things easier for regulators and electricians, but in reality the device itself consumes maybe 5 watts. The car is where the load actually comes from. So first and foremost, it needs to tell the car how much power it can safely pull. The device can only supply 30A because it’s on a 40A circuit. Technically it’s allowed provide up to 32 continuously, and truthfully I don’t know why they capped it at 30, but anyway the device needs to tell the car “Hey! Don’t pull more than 30A. You’ll make trouble if you do.” And it does that through very rudimentary signalling protocols. So, here’s a close-up of the pins on the car’s side. And now on the plug side. This one helpfully labels which pin is which. The three largest pins are Line 1, Line 2/Neutral, and a safety ground or protective earth. If you’re confused on why pin two is only sometimes neutral, you might want to check out this blog I recently made on the US electrical system. In short, we use split phase power, and on a 240 or 208V circuit both pins will be hot, but on a 120V circuit only one of them is. The other smaller pins are the control pilot and the proximity pilot. Let’s start with proximity. The main purpose of these pins is to tell the car and the EVSE that they’re connected to each other. On the EVSE side, the proximity pilot and ground pin are connected via a resistor which allows the car to know it’s plugged into an EVSE even in the event the EVSE is dead.

This completely passive method ensures that when the car is plugged in, even if the EVSE is faulty or there’s no power to it, it knows it’s connected to an EVSE and won’t let you shift from park. A well-thought-out design preventing the careless from driving away with the charge point. The control pilot is a little more complicated, but still not all that much. If it’s awake and ready to charge, the EVSE puts a 1 kHz square wave signal out on the control pilot pin. Without a car plugged in, that circuit is open so nothing happens at all, but when a car is plugged in, just as there’s a resistor on the EVSE side for the car, there’s a resistor on the car for the EVSE side. In fact, the car can manipulate the resistance in order to signal different things to the EVSE. Per the spec, when the car is connected it should have a 2740 ohm resistance across the protective earth and the control pilot pin. This signals presence to the EVSE. To request power the car lowers that resistance value down to 882 ohms.

That will cause the EVSE to close the contactor, and then the vehicle can charge. There is also a very special and rare case where the car will lower the pilot circuit resistance further to 246 ohms to signal that it requires ventilation when charging. This is used to prevent such a hypothetical vehicle from being charged indoors. I’m not aware of any consumer-facing applications where this is in use, but if you were ever wondering why your charger says “ventilation not required” on there – that’s why. It’s to prevent such a car (perhaps one with lead acid batteries which could produce a lot of hydrogen when charging? Really not sure what would require ventilation) But anyway, to prevent such a car from being charged with this supply. Most importantly, though, that 1kHz square wave being sent on the control pin is pulse-width modulated to signal the maximum charge current the car is permitted to take. That is arguably the single most important thing the EVSE does. See, a Chevy Bolt has a 7.2 kW onboard charger. That happens to match the rating of this EVSE so we’re all hunky dory. But what if I wanted to charge it on a smaller circuit? Say I had installed a 3.6 kW charger on a 20A circuit. The car needs to know it’s only allowed to pull 3.6 kW or else it would overload the circuit and trip the breaker.

Which is of course inconvenient, but also you’re then relying on the breaker to actually trip, and if it’s faulty you could very well have a fire on your hands. Better to not tempt fate. The EVSE is also monitoring the circuit for any ground-faults and will open the contactor should one occur. They’re usually designed to self-reset at least a few times so that you aren’t left without a charge in the morning. And they’ll also self-test things like the integrity of the protective earth and provide other various protections. But at a core level, all this does is announce its presence and capacity, and wait for a signal to initiate charging. Then it goes *CLACK* and the car does the rest. So if these are merely fancy light switches, why are they so gosh darn expensive? It’s not exactly hyperbole to say the functions of this EVSE could be replicated with an arduino, a contactor from an air conditioning unit, a power supply, and a willing middle schooler. The answer? [incredibly annoying rising "ehhhh" sound] OK well in fairness, there are plenty of options these days which can provide all that this car can take for around $300. The greatest cost of installing a charging station will always be simply running a new circuit to wherever you need it to be. Which is why it would be *great* if we could require at least one 40A 240V circuit for garages in new building codes. But anyway, the charger itself need not be expensive because it’s a pretty dumb device. I believe I’ve said this a few times now, but it is basically a fancy light switch.

The greatest cost in these units is almost certainly the charge cable itself. It’s a fairly specialized and quite beefy multi-conductor cable, built to withstand a fair bit of abuse. This entire unit weighs something like 20 pounds or roughly 9 kilograms, but the vast majority of that is just this cable. The actual unit is a plastic box. The connector itself is designed for 10,000 insertion / removal events, which means it should last a couple of decades in a private setting, and a good few years at least in a public one. The connector’s handle is also a little more specialized than you might realize. Can you hear the little microswitch in here when I depress the latch? [clicky clicky noise] This actually adds another resistor across the proximity pin and the protective ground, which signals to the car that it’s about to be unplugged. This is actually among the most elegant parts of this design spec. More or less the instant the car sees that resistance change, it stops pulling power from the grid. That means that when you interrupt a charge by unplugging the connector, current flow has stopped before the pins are actually separated (and indeed before the contactor opens) which prevents arcing and prolongs the life of both connectors and the contactor inside the EVSE. Pretty neat. Now, just because these devices are really quite simple doesn’t mean there isn’t room for innovation here. The biggest limitation with this standard is that the car can’t really communicate with the charger other than to say “please provide power” and (rarely) “I need ventilation.” There really isn’t anything like negotiation going on, and the charger is completely unaware of the characteristics of the car such as its state of charge, battery capacity, or indeed maximum charging rate. That could be quite useful for things like load sharing and potential back-feeding to the grid should that ever come to fruition.

Let’s talk about load sharing because that is a powerful tool for things like multi-family residential situations. Here, Tesla currently leads the way by quite a margin. Load sharing allows a given circuit to provide multiple vehicles with electricity by managing the current each vehicle can pull when more than one is plugged in. Say you have a 40 amp circuit like this thing is on. Well, you could put all of the allowed 32 amps continuous into one charging station and thus one vehicle, but you could also share that amongst multiple charging locations. See, a 40 amp circuit like what supplies this is capable of providing roughly 600 miles of driving range over a 24 hour period. But if you typically drive just 40 miles in a day, you only need about 7% of that output on a typical day. Sharing it among multiple charge points allows for more people access to one circuit at the same time. So long as the charge points can talk to each other, they can simply command whatever vehicles are plugged into them to pull less current so that it can be spread out between more vehicles. As individual vehicles are unplugged or finish charging, they will allow the remaining vehicles to pull more current.

Now this isn’t unique to Tesla but right now Tesla has the most flexible and also most economical solution available through their quasi-proprietary wall connectors. Up to 16 gen 3 wall connectors can share a single circuit and communicate with each other wirelessly, which greatly simplifies installation. While spreading even 60 amps (the max supported by the wall connector) out between 16 cars leaves a paltry sum for each vehicle, the idea isn’t really to allow 16 cars to charge at once – it’s to allow things like shared parking lots or garages in multi-family dwellings to have more charging points with less capital investment. It is very unlikely that all 16 units will ever be in use at the same time, and Tesla has one other advantage up their sleeve here. Because Tesla is Tesla they’ll have absolutely no reservations about allowing direct communication between the cars and the wall connectors. It’s my understanding that the Tesla wall connectors use the same protocol as SAE J1772 in the cable, they just use Tesla’s proprietary connector rather than the real deal, so I don’t think they talk through the charge cable itself.

I’m absolutely certain I’ll be corrected if I’m wrong so I won’t even bother asking. But the good thing is that this allows non-Tesla vehicles to charge on a Tesla wall connector with an adapter. Thankfully. That’s why they’re not quite proprietary. Barely. But if Tesla vehicles can talk to each other and also the network of wall connectors through some other means like WiFi, they can communicate their states of charge and rather than split the available current equally, it can be prioritized to vehicles with lower charge. From my perusal of the manual of the Gen3 wall connector it doesn’t look like this prioritization feature is currently live, but of course there’s no reason to think it won’t be quite soon via a firmware update. Still though, similar solutions are available from companies like ClipperCreek, and indeed inside this Siemens unit there are some connections which make me think it can be bonded to other units and even limit its charge rate based on a little potentiometer I saw in there but anyway those companies don’t have the advantage of vertical integration and so their products tend to be a little more expensive.

That’s the downside of serving everybody and not perpetuating a walled garden. Plus right now, there’s no codified state-of-charge communication between car and EVSE. However, that could change. A proposed update to the SAE J1772 standard [through gritted teeth] really would have been nice to have given it some sort of name, guys… would enable real vehicle-to-charger integration using power line communication protocols. That would help enable my personal EV charging pipe dream. I would love for power utilities to start installing their own level 2 EVSE equipment all over the place. Help get people in apartment buildings set up, and those who live in areas with on-street parking only. If the car can communicate with the EVSE, and the EVSE is part of a smart grid, an EV driver can register their car, plug it in anywhere on the network and have the electricity it uses automatically added to their own personal electric bill. Public charging today is a mess of competing companies each trying to make a profit in one way or another, and maybe that’s where we should go but personally I kinda wish utilities would just step in. Plus, if the car can tell the grid its state-of-charge, dynamic load-balancing on a huge scale would be easy to implement without stranding drivers. And, should backfeeding of the car’s charge to the grid ever be a thing, well that’s a perfect way to make that happen. If you’re a policy maker and/or someone who works for a utility, I’d just like to say I really think you oughta start looking at this. If you had L2 chargers everywhere, you incentivize people to keep their cars plugged in and if their cars are able to feed the grid in times of excess demand, well now you have access to battery storage which you didn’t need to pay for. And yes I know that idea is unsettling to many people, but the beauty of a smart grid is that you could potentially offer incentives and opt people in, or you could simply codify into vehicle design that every EV has a 10% charge buffer that’s invisible to the driver.

That way there’s some battery capacity that they don’t even know they have and never affects their range. There’s so much we could do, all it takes is some imagination and willingness to regulate some things. Which is obviously a pretty tough sell in the US at present. But for now, your garden variety EVSE is a pretty dumb device. Even Tesla’s wall connectors when installed individually really don’t do anything more than this does. If I can give a piece of advice to those who are in a situation where they can install their own charger in a private garage, it would be this; have an electrician install a NEMA 6-50 receptacle. I installed this one myself, and yes I know that needs to be in conduit– I’m getting to it. Anyway, many EVSEs are available with this plug, and they come in various capacities. It will allow effortless changes in the future, and also offers some peace of mind in case your EVSE happens to develop a fault of some kind and becomes unusable – then you can simply replace it yourself in mere moments. Though as I hope I’ve shown you, there really isn’t much that can go wrong with these. It’s just a fancy light switch.