Electric Car Rapid Charging: What You Need to Know

When cars like the Nissan LEAF and Mitsubishi i-Miev launched back in 2010, they brought with them the concept of rapid charging: the idea that if you stopped and plugged in to a dedicated, high-power charging station, you could replenish your car’s battery packs in minutes rather than hours. Conceived as a way of rapidly replenishing the charge stored in an electric car battery pack in order to compensate for the limited-range of most electric cars, rapid charging made it theoretically possible for electric cars to be driven longer-distances for the first time with relative ease, provided there was adequate rapid charging infrastructure en-route.

Rapid charging is far more common than it once was.

Rapid charging is far more common than it once was.

While the early days of rapid charging weren’t particularly reliable, today we see more rapid charging than ever before, with multiple different rapid charge standards available around the globe catering to a variety of different makes and models of electric car. While rapid charging used to be a rare bonus for plug-in car drivers, it’s now considered a must-have item, with very few cars on the market today not featuring rapid charging capabilities.

But lately, we’ve seen a growing confusion about how rapid charging works, be it the CHAdeMO quick charge standard found on cars from Nissan, Mitsubishi, Kia, Peugeot and Citroen, the CCS standard favoured by BMW and Volkswagen, the AC quick charging standard found on the Renault Zoe, or Tesla’s own proprietary (but open source) Supercharger standard. These include confusion about how the standards work, what to expect of them, and how quickly they recharge.

So we’ve decided to help out by giving you a few things you really should know about electric car rapid charging — and how you should behave when rapid charging.

How they work

With the exception of the AC quick charging standard favoured by Renault — which uses the car’s on-board motor inverter to turn high-power, three-phase alternating current into high-voltage direct current to feed into the car’s battery pack — all DC quick charge standards, regardless of their connector, all follow a basic mode of operation: they pump high-voltage, high-current electricity directly into the car’s battery pack.

There are a simple set of rules you can follow to ensure rapid charging is fun.

There are a simple set of rules you can follow to ensure rapid charging is hassle free.

Using external power electronics, all DC charging standards bypass the car’s built-in on-board charger and connect directly to the car’s battery pack, dumping electricity at a high rate directly into the battery pack. Capable of handling far more power than the car’s on-board charger can handle, external DC charging stations are designed to bring an electric car’s battery pack from near empty to 80 percent full in as quick a time as possible. How quickly that happens depends on the car you have and the actual power capabilities of the charging station itself, but for most mainstream DC quick-charge enabled cars on the market today, a 0-80 percent charge theoretically takes around 30 minutes.

If you like, you can think of the relationship between a DC fast charger and your car’s charger a little like the difference between receiving an intravenous injection of glucose and eating glucose-rich food. The latter works perfectly fine in most situations, but the former is able to deliver much-needed energy quickly and easily by bypassing your body’s digestive system.

DC charging stations are not only far more expensive than the on-board charger in your car, but also require a much higher power level in order to operate. As a consequence, you’ll usually only find them in places with high-power circuits nearby, like shopping malls, car dealerships and rest stops.

Common sense comes into play when using rapid charging, whatever the type.

Common sense comes into play when using rapid charging, whatever the type.

Why they’re not always as quick as you think

It doesn’t matter if you’re using a CHAdeMO DC quick charger or a fancy Tesla Supercharger: rapid charging doesn’t take the amount of time you think it will, and rarely meets with the headline figures quoted by automakers.

That’s because the speed at which a car recharges on a rapid charging station depends on four different things.

  • The power of the charging station
  • The state of charge of the car’s battery pack
  • The battery pack’s age and state of health
  • The weather

While it’s easy to think that all charging stations are created equal, not every public charging station can deliver the same power level. That’s because not every rapid charging site is capable of providing the theoretical maximum power of each charging standard, either because the hardware itself is limited to a particular power output or the physical power lines feeding the charging station require it to be ‘turned down’.

Thus you may find for example, that some CHAdeMO DC quick charging stations will provide the theoretical 50 kilowatts of maximum power most cars are capable of receiving, while others will be limited to 30 or 40 kilowatts.

While that might sound like a massive reduction in power, it shouldn’t impact charging times more than a five or ten minutes since cars don’t actually charge at full capacity for the entirety of the rapid charging session. Instead, they charge at the maximum available power until the battery pack is about 60 percent full, then slowly ramp down power as the battery approaches full.

Charging beyond 80 percent is something of a fool's game.

Rapid charging beyond 80 percent is something of a fool’s game.

Next we come to the state of charge of the car’s battery pack. If your car has an empty battery pack, you’ll find that the initial rate of charge is pretty swift, reaching 50 percent full in next to no time. But as the battery pack’s state of charge rises, the speed at which power can go into the battery drops. This is to do with the actual chemistry of the battery pack itself and something called internal resistance, but for the purposes of this article you just need to remember that higher state of charge equals slower charge rate.  Try to rapid charge beyond say 80 percent of your car’s state of charge for example, and you’ll find that the rate of charge slows to a crawl, which is why automakers and charging manufacturers alike always quote 0-80 percent full times.

Next,  we come to your the age and state of health of your car’s battery pack. As a general rule, the older the car’s battery pack and the more miles your car has on the clock, the slower its rapid charging will be. That’s because the on-board software which controls how fast the car charges is programmed to do everything it can to keep your car’s battery as healthy as possible. If it detects a higher internal resistance in the battery pack — again, something that is normal with a battery pack as it ages — it will charge the car more slowly than it did when new.

Similarly, if your battery pack isn’t well — there’s a particular battery cell that’s failing or the entire pack is imbalanced — your charge time will increase.

Finally, we come to temperature. While many electric cars on the marketplace today use active liquid cooling to keep the battery pack operating at an ideal temperature, not all do. As a consequence, you’ll notice a differentiation between charging speeds depending on the weather.

In extreme cold for example, charging times may increase, as the battery pack’s internal resistance will be higher than when it is warm. Likewise, in extreme heat, the battery charge rate may be slowed to protect the battery pack from overheating.

Why you should always allow more than the advertised time

Because there are a myriad of different factors that can slow down the rate of charge at a public charging station, you should always allow extra time for your rapid charge, especially if you really do need as full a charge as possible.

For a start, adding an extra ten or twenty minutes for each stop not only helps you cope with any traffic problems but also ensures that you leave your rapid charging location on time and calm. There’s nothing worse than having to leave a charging station before your car has the charge it really needs.

Plan extra time to ensure you've got enough charge.

Plan extra time to ensure you’ve got enough charge.

Allowing extra time also allows you to be prepared for someone else using the charging station when you arrive. In some situations, this will mean you’ll have to wait for them to finish charging before you can plug in. In others, it will mean you can plug in — but that your charge rate may be proportionally reduced in order to prevent overloading the charging station.

While this may sound like advice that only lengthens your trip, we’ve discovered a ‘buffer’ of at least ten minutes helps avoid any disappointment or stress along the way. And if your trip involves more than one rapid charge, be sure to add extra time for each stop.

How to share rapid charging

Finally, we think it’s worth noting some basic rapid charging etiquette, especially if the type of rapid charging station you’re visiting can only accommodate one car at a time. Even if you’re a Tesla Model S driver however, the same rules come under the general coverage of  what the Internet calls the “Wil Wheaton rule.”

For rapid charging, it comes down to a few basic points.

First, don’t hog the charging station. If you really only need a 60 percent charge and someone else turns up, let them charge and move on. It’s also nice to be prepared to let someone else charge first, or grab a quick emergency charge, even if you were there first. If you have the time and you think they don’t — be nice. Similarly, if you don’t need to charge, don’t plug in.

Camping out at rapid chargers is considered bad practice: charge up and move on!

Camping out at rapid chargers is considered bad practice: charge up and move on!

Second, stay near your car. You don’t have to stay with it for the entirety of the charging session, but leaving a friendly note in your car’s window saying when you’ll return or offering a contact number is a nice touch. In the UK, it’s possible at many locations to head into a warm coffee shop which overlooks the rapid charging stations, allowing you to keep an eye out for other people who may need a charge too.

Third, don’t charge all the way to full unless you have a really good reason to do so. Beyond 80 percent full, charging slows to a snail’s pace, and camping out a charging station until your car is completely full isn’t a nice thing to do, regardless of you being in a Nissan LEAF, a BMW i3, or a Tesla Model S.

Finally, if you arrive at a charging station and someone else is already there, don’t be afraid to park next to them with your car’s charge port door open. A universal way of saying “hey, could you plug me in when you’re done?,” it lets the other driver know you’re waiting for a charge. Ninety-nine percent of the time, you’ll find someone has helped you out and plugged you in when you return.

Any tips?

Are you a hardened plug-in car rapid charger user? Do you have any tips we’ve forgotten?

Leave them in the Comments below.


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