EV Charging 101

I’m writing this mostly to have something to point to when others are confused about the same technical details that took me a while to figure out myself. I hope to convey a good lay of the land at a technical level between complete layperson and “technical but not an engineer.”

Units

Electricity is extremely analogous to water. The key concepts are voltage, current, power, and energy.

Voltage, measured in volts is electrical potential, analogous to water pressure. In a home where the water comes out fast and strong, that’s like high voltage. In a home where the water pressure is weak and your shower just dribbles on you, that’s like low voltage.

Current, measured in amps, is electrical flow. A water main in your neighborhood has the same pressure as your kitchen faucet, but thousands of gallons a minute flow through it whereas one or two gallons a minute comes out of your tap. The main is higher current than your kitchen faucet. Current is how many molecules (or ounces, or gallons) of water per second flow. In electricity, it’s actually how much charge per second flows, literally how many electrons per second.

Power, measured in watts, is a measure of how quickly work is being done. You can think of it as how quickly a bucket fills from a faucet. High pressure through a small pipe (high voltage, low current), might fill the bucket just as fast as low pressure through a very large pipe (low voltage, high current). If you really want to fill a swimming pool super fast, a fire hose with a pump truck delivering lots of water very fast at high pressure is ideal.

Energy, measured in watt-hours, is how much water you have ready to drink (or shower with or whatever). The unit scientists use is the joule or calorie, which you might be familiar with. But describing a battery pack as “2.7e^8 Joules” is cumbersome at best, so we use watt-hours. A watt hour is the amount of energy you’ve consumed by using a watt of power for an hour of time.

Since they’re very similar, people tend to confuse watts and watt-hours a lot. Luckily, since they’re very similar, it’s also pretty easy to catch when someone’s done it and know what they mean. For instance, my car’s battery pack holds 77.4 thousand watt-hours of energy, or 77.4kWh. If I charged it at 77.4kW of power, it’d take an hour to finish charging. If I said my pack was 77.4kW, technically I’d be saying “it can output 77.4kW of power to a motor” which is very much lower than the truth (it’s good for more like 300kW). But generally people would guess that I meant 77.4kWh is how much energy it can store.

AC Charging

J1172 is the name of the circular port on all EVs in North America. It has 5 pins, 3 of which are for (AC) power and two for communication. (In Europe, they have a similar handle called “Type 2” (J1772 is also known as “Type 1), which has 5 pins for AC power.) Basic at-home charging uses this port/plug. When you park at a movie theater and plug in to the small station in the parking lot, this is that plug. When you use this plug to charge from a normal-looking 120V household outlet (a NEMA 5-15 to be specific), you are L1 (level 1) charging. L1 chargers are limited to 12 amps (1440W) even though the outlet is capable of 15 amps, because the NEC (national electrical code, the building standard to which most wiring in the USA is installed) dictates that constant loads must be limited to 80% of the capacity of the outlet they’re plugged into. If the cord you’re using is connected to 240V, you are L2 (level 2) charging. In short, L1 and L2 are both AC charging, where L1 is less than 1.5kW, and L2 is anywhere from 3kW up to 20kW.

Exact power will depending on the capability/wiring of the EVSE (Electric Vehicle Support Equipment), which is the box of smarts on the charging station (or cable, if you’ve got a portable EVSE like the Tesla UMC) that coordinates the car and circuit. The EVSE’s job is basically to tell the car how much power is available, and disable power until the charge handle is safely locked in the car. An extremely typical EVSE would be something like a JuiceBox with a NEMA 14-50 plug which just makes it easy to swap out the EVSE if the homeowner wants. The 14-50 is a 50A plug, so in this case the JuiceBox would be set to a limit of 40A (50*80%, as car charging is a continuous load). This configuration would supply a maximum of 9.6kW. Another common option would be to directly wire the JuiceBox to a breaker via conduit, to avoid the current bottleneck of the plug. In this scenario, you’d set the JuiceBox to provide 48A, wired via sufficiently thick 6-gauge wire to a 60A breaker. Thus, 48 is 80% of 60 and that rule is satisfied, while the JuiceBox can source 48*240=11.5kW, which is the limit many cars can accept.

Exact power will also depend on the capability of the on-board charger of a given car. Since a battery must be charged with DC, there must be a device to convert AC wall power to DC for charging. Cars come with a device onboard to do this, typically called the “onboard charger” or OBC (this applies specifically to my own EV6 – that’s the acronym the platform uses). Since 14-50 plugs and 60A circuits are common and entirely adequate for most cars, most cars have an onboard charger designed for a maximum of 11.5kW or close to that. SOME cars, especially ones with very large battery packs, have larger OBC modules, including the Tesla Model S which can charge at up to 80A*240V=19.2kW, and the Ford F150 Lightning extended range which has the same capability. 20kW EVSEs are rare, however, since many residential panels are only wired for 100A service and can’t easily handle such a large load without an upgrade or lots of extra legwork.

A note about voltages: Voltage isn’t very tightly regulated on most power grids. That might seem surprising, and you’ll very often measure exactly 120V on your home outlets, but there’s a lot of slop in the system. When someone says 110V or 120V, they really mean the same thing. Some corners of the grid might legitimately have 110V at a plug, and even a 120V plug might sag when many loads are turned on and there’s voltage drop across the wire between your outlet and the pole outside. Similarly, 220 and 240V are the same for the same reasons. An outlier is 208V, which you’ll often see explicitly on appliances like stoves and clothes dryers. 208V isn’t “240V that lost some on the way” – it’s an artifact of how some kinds of buildings are wired, particularly large apartments. All the same kinds of appliances designed for 240V will typically be designed to work on 208V as well, but they might dry your clothes or boil your water or charge your car a bit slower on the lower-voltage wiring.

DC Charging

DC Fast Charging, or DCFC is charging at a station that converts power to direct current BEFORE it gets into your car, rather than relying on the OBC. By doing the conversion offboard, charging isn’t limited by the size and weight of the OBC. The conversion electronics can be much larger, much heavier, and have more fans to manage heat, than if they were stuffed into your vehicle. The plug with which this is done is CCS, or combined charging standard.

There are still all kinds of potential bottlenecks, though: battery size, battery voltage, battery temperature, connector current rating, cable current rating, power converter maximum power, converter current limit, converter voltage limit, and current temperature for every component in the list. I’ll try to cover each as concisely as possible.

Component temperature: Putting power through stuff heats it up, and everything has a temperature limit above which it stops working or gets dangerous. A specific example is the charging handle: the handle heats up a little in use, and if it gets too hot, charging must slow down to let it cool. Of course, the handle could also get hot in the sun. Electrify America specifically (used to deploy) ABB charging units with two cables/handles for redundancy. I’ve absolutely shown up to such a unit where one handle was in the sun and only gave me 60kW, while the other handle on the same charger that had been sitting in the shade gave me more power. CCS handles all have temperature sensors in them so the charging unit can manage safety in this way. This concept also applies to cables. They’re typically water-cooled, but they can still become or start too hot causing the charger to limit power output.

Battery Size: Imagine I have one cell phone. It charges at about 2 amps from a 5V plug. Imagine I had 100 of the same cell phone. If I plugged them all in, they’d be able to charge with 200A at the same 5V. Battery packs generally follow the same trend: larger packs can charge faster. In fact, engineers typically describe battery charging normalized to pack size: Charging at 1kW means very little. Charging a cell phone at 1kW means a vaporized cell phone, while charging a grid storage plant at 1kW is a barely-measurable trickle. But if I say the pack we’re charging is 1kWh in capacity, 1kW/1kWh is “1C” or “1 pack capacity per hour.” This is the C-rate. The C-rate a pack can charge at depends mainly on its cell technology, and most lithium ions we commonly use today like charging at 1C. 2C is fast, and 3C is typically right around the limit a cell can safely tolerate. My EV6 with its 77.4kWh pack that can peak at 240kW charging hits 240/77.4=3.1C. A hummer EV can max-out a 350kW charger, but since its pack is a whopping 212kWh, that’s actually only 1.65C: a 350kW charge on a hummer is much more gentle for the battery than a 240kW charge on my car. A plug-in hybrid with its relatively tiny 15 or 20kWh pack simply can’t pull as much power as a BEV car with a larger battery.

Battery Voltage (architecture): You may be familiar with the concept of “400V cars” and “800V” cars – Teslas are famously all “400V” where the Taycan, EV6, Ioniq 5, and a handful of others are “800V.” Both numbers are EXTREMELY approximate. The EV6 long-range is actually more like 634V (0%) to 797V (100%). The Light model with the 58kWh pack just cuts a couple modules off the top and is about 475 to 598V. The Tesla Model S is a “400V” car, but its pack voltage ranges from 363 to 462V if the internet is to be believed. The hummer EV does this weird thing where it reconfigures the (400V-class) pack on-the-fly to charge faster. It splits the pack down the middle and wires the two halves in series to make it an 800V-class pack while charging. All this to say, “400V” and “800V” aren’t literal. “400V” chargers typically go up to 500V, and “800V” chargers typically go up to 1000V, to cover the wide range of possible pack voltages throughout all states of charge.

P=IV, or Power = Current x Voltage. When charging a battery, the voltage IS the battery’s current operating voltage. Current is whatever you can (safely) make it. Current is also what generates heat, so if you had your choice of a system that charges a 10V battery at 2 amps vs a system that charges a 20V battery at 1A, the latter is likely to be more efficient, get less hot, require less cooling, and so on. In theory, either can get the job done, but in practice, one is likely to be easier for many reasons.

In EVs, this is where we get the dichotomy between “400V” battery systems and “800V” battery systems. Let’s say you want to charge a car at 200kW. If it has a 400V battery, that means pumping 500A into it. If it has an 800V battery, that means it takes 250A to charge it at 200kW. In theory, both are the same outcome. In practice, 500A will generate lots more heat, and lots more voltage drop across the cabling, and so on and so forth, vs the half-as-intense 250A.

You might ask, “well then why does Tesla have some of the fastest charging and also a 400V architecture?” The answer, best I can guess as an engineer who wasn’t there, is that when Tesla made their architecture decisions, there simply weren’t electronic switches viable for a higher voltage design. Semiconductor switches (and even physical metal switches) have limits just like anything. When Tesla made their choices, semiconductors didn’t’ exist to enable higher voltage pack design. The only option for fast charging was to cram tons of current through their cables and deal with the heat by clever design and water-cooling. Now semiconductor tech has better offerings, so we can leverage that technology advancement to make the rest of the design easier.

In summary, higher voltage architecture doesn’t mean higher possible charging power for physical reasons, but it does in practical terms.

Battery Voltage (state of charge): Remember above that Power=Voltage x Current. If your charger is limited as to how many amps it can provide, being required to provide those amps at low voltage means lower delivered power than providing those same amps at higher voltage. Since the pack voltage increases as it charges, that means power increases that it’s possible to deliver within the same amperage limit. This is one reason charging usually starts off at one speed, then gets a bit faster and stays pretty fast until it starts going back down again.

On the flip side, a pack that’s already pretty full can’t charge as fast as a pack that’s pretty empty. This is mainly because charging hits a voltage limit of the pack: the charger can’t set the pack voltage any higher than about 4.2V per cell without risking damage. Therefore, when the pack voltage has hit this point, the charger just has to keep supplying whatever current the pack will take, which will taper down over time as it gets fuller.

This is the reason why charging up to 80% is pretty quick, but charging from 80-100% is pretty slow.

Converter power/current: This is probably one of the most confusing aspects. Chargers themselves have three key limits:

1. The maximum voltage they can provide
2. The maximum current they can provide
3. The maximum power they can provide.

Imagine that the converter hardware looks like a rack of power converters. Each one can convert its mains AC input to any DC voltage up to, say, 1000V. It can do this for, say, up to 50kW of power output, because above that power limit the heat it generates is too much to handle. You might say “great, I want 50kW at 100V!” but then you run into the current limit of the unit: 500A is a lot of amps! 10x as many amps as the converter would have to supply if you asked for 50kW at 1000V. So that 50kW converter will derate in total power depending on what specific voltage and current you’re asking of it.

So let’s apply that to a real-world example. Electrify America has deployed a few different charger configurations, but the most common is an ABB unit that can be configured for either “150kW” or “350kW.” The 350kW unit is a dispenser capable of 1000V and 500A, attached to two power cabinets wired together, each of which can provide 175kW. You can get 350kW out of the unit, but only if you’re asking for 700V or more, since 700V*500A=350kW. any lower than that and you’d be current limited.

The “150kW” stalls are limited to 350A and 175kW, which means you can get 175kW out of them as long as the charging voltage is above 500V. In other words, an 800V-class car can pull the full 175kW from low state of charge up to near 80%, while a 400V-class car will be current-limited. For instance, the Model S plaid above, which CAN take 250kW from a Tesla supercharger, will start out limited to only 126kW and top out at something like 143kW, due to the 350A current limit of the charger.

Battery Temperature. Finally, this one is the most likely to surprise people showing up at a charger. In cold weather, the battery pack also gets cold. One of the key chemical process limitations to charging speed is called “lithium plating,” and unfortunately this process gets worse with decreasing temperatures. The anode (negative terminal – weird, I know) of a typical battery is made of graphite foam. During charging, lithium ions in the electrolyte are combined with electrons on this anode to form metallic lithium. The process of lithium finding its way deep into the foam is called “intercalation” (fun fact!). Taking my EV6 for example: it can charge at the full 3.1C/240kW if every cell is at least 25 degrees celsius, but below that temperature it slows down charging to allow the lithium atoms time to intercalate into the graphite anode. If we charge the battery any faster, there’s risk of the lithium forming a smooth layer of metal over the top of the anode, preventing the rest of the ions from finding a space and limiting both power output and total capacity.

In the EV6, personal experimentation has shown that the throttle-down temperatures are as follows:
Above 25C minimum cell temperature, not-thermally-limited/240kW
20-25C = 180kW limit
15-20C = 130kW limit
10-15C = 60kW limit
<10C = 40kW limit.

10C is 50F, which means that on a modestly chilly day, charging performance drops off to almost nothing.

Tesla came up with a solution to this problem long ago in the early days of superchargers called “battery preconditioning.” The idea is that the driver directs the onboard navigation to a DC fast charging station, and then by knowing how long it’ll take to get there, the car can enable a (very powerful/range-killing) heater to warm the battery up just in time for maximum charging performance. By running a ~6kW heater for ~30min before charging, thereby consuming around 10 or 15 miles of range, the battery will be warm enough to charge, say, to 80% in 20min instead of 90min.

The main takeaway is if it’s less than about 75 degrees outside, your charging performance may be negatively impacted, and if your car supports battery preconditioning, you should familiarize yourself with how to use the feature and do so on every road trip to save yourself some valuable charging time.

What about charging my “800V” car at a “400V” station?

An 800V-class car needs some way to charge even if it’s plugged into a 400V-class charger that can’t provide enough voltage. This is where it gets rough, since every car does something different.

To start off, in the USA, there are VERY few 400V-class CCS DCFC stations. As far as I’m aware, it’s basically ONLY the older 50kW EVGo units in grocery store parking lots, which EVGo is actively replacing with newer and faster. Of course, that changed last week when Tesla started rolling out the Magic Dock CCS adapter at their V3 superchargers, as all Tesla superchargers to date are 400V-class (topping out at something like 600V max output). Still, this is much more relevant in Europe where there are multiple 400V-class networks, and Tesla chargers use the same CCS-2 standard as everyone else and are now open for third party vehicles to charge.

There are a few different strategies, but they all revolve around a boost converter. A boost converter is a power electronics device used to increase a DC voltage at a higher current to a higher voltage at a lower current. It’s actually a specific circuit topology, and there are topologies other than “boost” that also increase output voltage, but it’s pretty common to just refer to all of them as a boost converter.

The Porsche Taycan adopts the strategy of just having a dedicated boost converter box. By default, the car ships with a converter capable of 50kW throughput, meaning you can charge at 50kW from any 400V station capable of at least 50kW (all of them). You can option a larger box capable of 150kW, which you absolutely should do now that V3 superchargers are opening for CCS cars.

The Lucid Air does basically the same thing, with a dedicated box. Not sure about its limits.

The E-GMP platform, including the EV6, Ioniq 5, and GV60, does something extremely clever. A boost converter is basically some switches, a big inductor, and a capacitor. EVs already have two of those things in the form of a motor controller and motor. E-GMP repurposes the rear inverter as the switching converter, the rear motor as a very large value high-power inductor, and presumably just tosses a suitable in there beyond what the motor driver already needed, to complete the design.

This leads to some interesting questions about what kind of performance it supports. The rear motor has a power rating of 168 kW, so that’s probably the highest possible limit. As an arbitrary educated guess, 168 kW is rated at 700V, implying an rms current limit of 240A for the inverter. We know from documentation that the boost converter requests 450 V from the charger, and 450*240 is 108kW.

In the real world, we’ve now seen a GV60 charging at a magic dock location pull 105kW, so that seems to confirm my envelope math.

If you’ve gotten to this point, hopefully you’ve learned something and hopefully uou didn’t need a nap midway through!