Understanding AC Charging Is Critical to Understanding EVs

A vehicle’s AC charging abilities are arguably just as important as its DC fast charging equipment.

byKevin WilliamsMay 23, 2022 3:37 PM
Understanding AC Charging Is Critical to Understanding EVs
Andrew Collins (Mitsubishi, Tesla assets)
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Understanding the intricacies of electricity and how it works in relation to electric vehicles (EVs) can be confusing. Before I bought a Mitsubishi i-MiEV and before I started heavily reporting on EVs, I didn’t know much about currents, amps, or watts, beyond wiring a D battery to a lightbulb back in the fourth grade. Through my journey as an owner and by testing a Tesla Model 3, a Ford Mustang Mach-E, a Volkswagen ID.4, and several other all-new battery-powered vehicles, I’ve learned which metrics are important, which numbers are misleading, and which are simply for marketing. One of the key aspects of EV charging that’s often overlooked is basic alternating current (AC) charging.

I’ve realized that manufacturers often list big charging time promises without explaining exactly how they came up with those numbers. It’s not as simple as universally plugging it in for one amount of time. Even when there are additional details, automakers tend to focus on direct current (DC) fast charging times, but DC fast charging can be expensive, temperamental, inconsistent, restrictive to certain cars, and could have a negative long-term impact on the environment and the economy. AC charging specifications are too often left by the wayside, and that shouldn’t be the case. Heck, I’d say the speed at which a vehicle can AC charge is more important than its DC fast charging abilities.

This cutaway lets you see generally how the Porsche Taycan's plug wiring leads toward the batteries.

Charging Time Requires Detailed Context

In nearly every bit of marketing information for an electric car, there will be some sort of statement that will quantify that the vehicle can charge a certain amount (or fully charge) within a certain time period. On its face, It seems like that statement is a helpful, accurate piece of information that tells potential buyers and journalists alike about the speed at which vehicles can charge. Hogwash.

Barring any weird charging behaviors, such as the Ford Mustang Mach-E’s slowed charging for the last 20 percent of the battery capacity or the i-MiEV’s penchant for stalling while charging, recharging time is a simple math problem. However fast the vehicle can accept charge and however fast the cord can dispense power should determine charging time. No more, no less.

Why Does That Matter?

In addition to the cost and limited availability of DC fast charging, stations that provide the service are also much more expensive to build. They require three-phase power, which is typically only found in high-traffic commercially zoned spaces. By comparison, 240-volt AC chargers can be easily integrated into most residential or business areas without too much effort. That means it’ll be much easier to find a Level 2 charger than it is to find a DC fast charger.

In the past, I was skeptical of Level 2 charging and I initially thought that an EV is useless without DC fast charging. In certain situations, that might have some truth, as my i-MiEV is inarguably a less useful car without the CHAdeMO DC fast charging found on the topmost SE Premium trim. However, after more time with the car, I realized the i-MiEV isn’t really limited by its lack of DC fast charging, it's more limited by its very slow onboard AC charging. I’ll explain.

When it was new, the i-MiEV came with a 16-kWh battery. Mitsubishi claimed an approximately six-hour recharge time from flat to full on a 240-volt power source. By comparison, a brand-new Mach-E with an 88-kWh battery takes about 8.5 hours to recharge from flat to full on a 240-volt outlet. Where did these numbers come from? Why does the i-MiEV fully charge only slightly faster despite its much smaller battery?

It's because the i-MiEV’s onboard charger is tiny. The i-MiEV’s onboard AC charger was only rated for 3.3 kW when new. Add in charging losses and the i-MiEV’s weird battery smoothing, you’ll get Mitsubishi’s six-hour number. Comparatively, the Mach-E in 88-kWh battery trim uses an 11.5-kW charger. Including charging losses, the Mach-E should charge from flat to full in Ford’s claimed 8.5 hours. When you consider how chargers work, things get trickier.

Oh God, More Math

Charging speed is a simple equation: amps (how much volume of electricity) times volts (the speed at which the electricity can flow) equals wattage (amp x volt = watts). Electricity flows in a sine wave. So, 110 volts is basically half of that sine wave. The full sine wave is 240 volts, and three-phase electricity is two additional sine waves that flow in opposite directions of the first sine wave. 

That sounds more complicated than it really is, but I'll illustrate it with Ford’s own charging station. The Ford home charging station, when installed by an electrician, is 48 amps, multiplied by 240 volts equals 11,520 watts, or about 11.5 kilowatts. This is the max speed at which the Mach-E can charge on AC power, meaning it should easily be able to deliver the Mach-E’s 8.5-hour promised time.

However, Ford’s charging station won’t charge my i-MiEV any faster. My i-MiEV only has a 3.3-kilowatt charger, meaning that unless I swap the I-MiEV’s onboard charger for a better one, there’s no way to make the i-MiEV charge faster than six hours.

This 110V outlet is rated for 16 amps, the max my cheap level 1 and 2 home charger is rated for. Here, the car can only charge at a max speed of 1.7KW. Even if I were to plug into 240V power, the cord can only handle 16 amps, meaning the max this cord can handle is 3.8 KW.

The same is true in reverse. I recently bought an Electric Vehicle Supply Equipment (EVSE) cord that doesn’t require any sort of electrician to install, just simple access to a 110-volt or 240-volt cord. This cord is limited to 16 amps; multiplied by the 240 volts expelled from the cord, which means my EVSE can only output 3,840 watts, or about 3.8 kilowatts, per hour. Not much, but perfectly fine for my i-MiEV that can only take 3.3 kilowatts per hour anyway. Unfortunately, the same cord would be woefully small for the Mustang Mach-E. Including charging losses, an 88-kWh battery in a Mach-E premium would take more than 25 hours to replenish from flat to full.

Don’t Underestimate the Importance of AC Charging

There are a couple of reasons why we should talk about AC charging. It’s common for EV owners to warm or cool their vehicles when they’re charging. Because the i-MiEV’s onboard charger is so small, it can’t do that. It can only draw 3.3 kilowatts at a time. Its onboard heating uses 5.5 kilowatts just to start, then settles down to 3 kilowatts when warm. The i-MiEV can’t draw enough power to both charge and heat its cabin. Mitsubishi did include a special remote to allow the car to enter a low-power heating, ventilation, and air conditioning (HVAC) mode while charging, but it will still significantly affect charging speed. Also, mine is missing.

Adding to its importance, AC charging will likely always be far more ubiquitous than DC fast charging. Understanding your vehicle’s AC onboard charging limits will help paint a more realistic picture of how fast an EV will actually charge. There’s no point in spending cash on a home charger that your new EV can’t even utilize entirely. For example, systems like Ford’s 48-amp home chargers or Tesla’s near 100-amp destination charger service is a lot of power. A lot of older homes in the USA are only rated for 100-amp service, but establishing 48- or even 60-amp service per charger or 100-amp service at a business is a lot easier for a lone electrician to do, compared to ripping up the ground for three-phase DC fast charging service.

The i-MiEV isn’t very good at charging, no, but just imagine how much more usable it could be if it had the Mustang’s 11.5-kilowatt charger. Recharging times could come in as little as an hour with my level of battery degradation, without the need for DC fast charging. At the very least, I could charge and use the heat at the same time.

Some pre-facelift Tesla Model S and X cars had a Dual Charger onboard charger option that allowed them to connect from high-speed AC destination chargers and charge at a rate of 22 kilowatts. This means a big-battery tesla could fully charge from AC power in a few hours. Tesla quietly dropped the Dual Charger option when both the Model S and Model X were facelifted in 2016. Sad that so few seemed to mourn its loss. Personally, I think that small low-range EVs, like the Wuling Hongguang Mini EV or Mazda MX-30, could have a stronger use case if they had the ability to “fast charge” nearly anywhere.

To help y’all along, we’ve compiled a nice table that shows the max onboard charging specifications of quite a few EVs and a couple of PHEVs sold in North America.

Capability of More Than 18 kW

  • 2014-2016 Tesla Model S and Tesla Model X when equipped with dual charger (22 kW)
  • Lucid Air (19.2 kW)
  • Porsche Taycan, optional dealer-installed accessory (19.2 kW)
  • Ford F-150 Lightning Long Range (19.2 kW)

11-17.9 kW

  • Tesla Model 3, Model S, Model X, Model Y (11.5 kW)
  • Ford F-150 Lightning Standard Range (11.3 kW)
  • Rivian R1T (11 kW)
  • VW ID.4 (11 kW)
  • Ford Mustang Mach-E (11 kW)
  • 2022+ Chevrolet Bolt EV/EUV (11 kW)
  • Polestar 2 (11 kW)
  • Hyundai Ioniq 5 (11 kW)
  • Kia EV6 (11 kW)
  • Audi E-Tron Q4 (11 kW)
  • BMW iX (11 kW)
  • 2023+ Jaguar I-Pace (11 kW)
  • Volvo XC40 Recharge (11 kW)

6.7-10.9 kW

  • Porsche Taycan (9.6 kW)
  • Audi E-Tron (9.6 kW)
  • Audi E-Tron GT (9.6 kW)
  • Tesla Model 3 RWD (7.7 kW)
  • BMW i3 (7.4 kW)
  • Mini Cooper SE (7.4 kW)
  • Hyundai Kona Electric (7.2 kW)
  • 2018-2021 Chevrolet Bolt EV (7.2 kW)
  • Kia Niro EV (7.2 kW)
  • 2018-2022 Jaguar I-Pace (7 kW)

3.4-6.6 kW

  • Nissan Leaf (6.6 kW)
  • Mazda MX-30 (6.6 kW)
  • Toyota RAV4 Prime PHEV With premium package (6.6 kW)
  • Honda Clarity EV (6.6 kW)

Less than 3.4 kW

  • Toyota RAV4 Prime PHEV (3.3 kW)
  • Mitsubishi i-MiEV (3.3 kW)

EV charging is complicated, but it doesn’t have to. Let’s be proactive and learn a little bit about how things work, before we end up with a car that can’t deliver on its promises because we’ve got the wrong equipment.

Correction: Tuesday, May 24, 2022, 11:09 a.m. ET: Changed some instances of “kilowatt per hour” to “kilowatt” to reflect accurate unit measurements.