A Tesla is charged using a wall connector charger.Jeff Chiu/The Associated Press
This summer, we finally got the electric vehicle we’d ordered late last year. For now, we’re charging at home on a regular 120-volt outlet. But we recently used a fast charger when we drove to Kelowna, B.C. I was surprised to see that it wasn’t charging all that fast. We were at a 150-kilowatt charger and I believe our car has a charging speed of 90 kilowatts, and yet the computer showed it was charging at less than 70 kilowatts. It was a hot day – above 35 degrees Celsius. – Kris, Richmond, B.C.
Electric vehicles are a lot like Goldilocks – when fast charging, they don’t like it too hot or too cold.
“For an EV to hit its [maximum] charging speed, it needs two things,” said Michael Stanyer, a spokesman for Plug In BC, a Vancouver-based not-for-profit EV education program. “First, it needs to be using a high-powered station that can supply [its] max charging rate. … Second, the battery must be at the correct temperature.”
If your car can charge at a maximum speed of 250 kilowatts, to get that speed, you’ll need to be at a station that supplies at least 250.
But if it’s not charging that fast, the culprit might be the weather.
To charge at the fastest possible speed, batteries need to be at an optimal temperature. It varies by battery chemistry and design – but it tends to fall somewhere between 25 and 35 degrees Celsius, Stanyer said.
That’s the ideal temperature of the battery – not the temperature outside.
While extreme temperatures slow down charging, a cold snap tends to be worse than a heat wave, he said.
“Technically, yes – the hot summer temperatures can affect charging speed, but not as drastically as cold weather would,” Stanyer said. “Fast charging naturally heats the battery up – so the car will use its thermal management system to cool the battery, even during moderate weather.”
Because fast charging is so fast – from 5 to 32 kilometres a minute compared with 5 to 8 kilometres an hour at a standard 120-volt outlet – it heats the battery.
Most newer EVs have built-in systems that cool or heat the battery to keep it in the ideal zone for charging.
“You will notice fans running [to cool batteries] when the car is fast-charging, even in cooler weather,” Stanyer said.
Some newer EVs also will start bringing the battery to the right charging temperature before you even get to the charger.
It’s called battery preconditioning and it “makes a huge difference,” Stanyer said.
“Cars with this feature will use the thermal-management system to heat the battery [or cool] when a fast-charging station is entered in the navigation system,” he said. “When a Tesla owner selects a Supercharger in the car’s navigation, the system will heat [or cool] the battery to the correct temperature before arrival.”
More room to charge?
There are other things that can slow down fast charging.
One is your battery’s state of charge – if it’s closer to empty, it will charge faster initially and then begin to slow down.
“A helpful analogy is finding a seat in a movie theatre,” Jeremy Whaling, an engineer at EVgo, a U.S.-based charging network, wrote on the company’s website. “When your [state of charge] is low, it’s much easier for electrons to find empty ‘seats’ to fill; when your [state of charge] is high, it takes more time, and your charging speeds will be slower.”
But that’s decided by your car’s computer, Stanyer said.
“The battery-management system adjusts the charging speed in order to charge as fast as possible while protecting the battery from overheating,” he said. “When the battery is close to empty, it can take a much more powerful charge than when it is nearly full – so you will see the best charging speeds in the lower end of the battery’s capacity.”
As you get closer to full, your car tapers off the charging speed – especially once you charge past 80 per cent. That’s to keep your battery from getting too hot.
So, stopping at a fast charger to top off a battery that’s at 79 per cent might actually take more time than charging that battery from 20 to 80 per cent, Stanyer said.
“For that reason, it saves time to drive until the battery is below 20 per cent and only charge up to about 80 per cent on a fast charger,” Stanyer said.
If the conditions are ideal for charging, but you’re not getting the advertised speed, it might be an issue with the charger, he said.
“There can be issues with the fast-charging stations that prevent them from delivering full power, but those are nearly impossible for a driver to do anything about in the moment. Just like any other equipment, fast-charging stations have parts that will wear out and lose performance,” Stanyer said. “There have been a couple of situations where the charging speed was slow at one station, so [I] stopped charging and plugged into the station right beside it for a better speed.”
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