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On the first long trip we took in our Nissan Leaf, we charged to 100 per cent at every DC fast charger because we wanted the extra charge as a backup. But we noticed that the fast charging was only fast until the battery reached 80 per cent – that took about 40 minutes, but then it slowed down and took at least another 45 minutes to reach 100 per cent. Now we only charge to 80 per cent or less at each stop – as long as there’s another charger within that range. Is this normal, though, or is it a problem with our car? If it’s normal, why does the charging speed taper off after 80 per cent? – Jill, Burnaby, B.C.

Typically, fast charging is only fast to a point.

That’s usually because your electric vehicle (EV) has asked the charger to slow things down because it’s getting too hot.

“EV fast charging slows down as the battery gets closer to fully charged, and this is done to prevent the battery from overheating; extreme heat will stress the battery and degrade its capacity,” said Michael Stanyer, a spokesman for Plug In BC, a Vancouver-based not-for-profit EV education program. “The charging speed will begin to taper off before 80 per cent, but 80 per cent is a common mark for when charging slows down significantly.”

First, a quick review of fast charging. Out of the three levels of charging, fast charging is, well, the fastest.

Depending on the speed of the fast charger and the maximum charging speed your EV will accept, you can recharge to 80 per cent in 20 to 45 minutes.

Fast chargers are the most convenient option when you’re on a cross-country road trip. Recharging at a Level 2 charger, the kind you would typically install in your garage, could take five or six hours.

On most cars, when you plug in, it will tell you how long it will take to get to a full charge.

“An EV is in constant communication with the charging station and the EV requests more or less power based on battery temperature and capacity according to how it has been programmed,” Stanyer said.

Early EVs were more susceptible to battery damage – which could shorten a battery’s life and reduce the maximum charge it could take – from frequent fast charging, he said. “Modern EVs have more sophisticated battery-management systems [that use] fans and liquid cooling/heating, for safer fast charging with less battery degradation.”

On many EVs, the software that helps to keep the battery cool gets updated regularly, which means you could see faster charging speeds – and even slightly greater range if the update tells the car to tap into extra battery room that it hasn’t been using.

“It’s not unusual to have the battery management tweaked for better charging speeds through over-the-air updates or dealership visits, depending on the manufacturer,” Stanyer said.

While fast charging typically slowed down once the battery reached 80 per cent on older EVs, that’s starting to change on newer EV models with faster charging speeds.

A few years ago, the fastest fast chargers – outside of Tesla’s Supercharger network – provided up to 50 kilowatts. Now, there are faster chargers – the fastest is 350-kilowatts – but most cars can’t handle that speed. EVs have a top rate they’ll charge at, no matter how fast the charger is.

For instance, a 2022 Hyundai Ioniq 5 can charge at up to 350 kilowatts, a 2022 Porsche Taycan can charge at up to 270 and a 2022 Audi E-tron SUV can charge at up to 150.

So, if you’re at a 50-kilowatt station in a car that can take a faster charge, it might not slow down at 80 per cent, Stanyer said.

“Almost every new model of EV can charge above 50 kilowatts,” Stanyer said. “I’m seeing that EVs designed to charge at 250 kilowatts or more will happily take 50 kilowatts above 90 per cent before tapering off.”

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