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We just bought a 2021 Chevy Bolt (our previous vehicle was our beloved 17-year-old minivan that we’re scrapping for $6,000). We’re planning a summer trip into the mountains with two adults, two teens and a dog. We’ve heard not to charge the battery past 80 per cent to preserve its life. But can it really hurt once in a while, especially for a long trip? – Noah, North Vancouver

When charging your electric car, 100 per cent isn’t always 100 per cent.

The car’s computer will tell you the battery’s full, but it’s actually charging to less than the maximum capacity to help preserve battery life.

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“The auto manufacturers do have smart systems in place that don’t let it charge to 100 per cent or deplete to zero,” said Greg Keoleian, director of the Centre for Sustainable Systems at the University of Michigan. “There’s a built-in buffer but they won’t say what it is – if I had to guess, I’d say maybe it’s 90 per cent.”

Keoleian, the lead author of a 2020 study looking at the best ways to prolong battery life, said that, ideally, a battery shouldn’t drain to below 20 per cent or charge past 80 per cent.

But since your EV has that built-in buffer, you shouldn’t worry too much if you regularly recharge your car to 100 per cent, Keoleian said.

Why does it matter? Like cellphones and laptops, electric vehicles use lithium-ion batteries.

They degrade faster when they’re drained to zero and then fully charged. That’s a full charging cycle. As the battery degrades, it loses its maximum charging capacity and has a shorter life.

The advice from carmakers varies. For instance, Ford and Volkswagen said you should only charge to 100 per cent if you need your EV’s full range for a longer trip.

VW recommends charging to 80 per cent for daily driving, while Ford recommends charging to 90 per cent.

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But General Motors and Nissan said there’s no problem charging their EVs to 100 per cent every time they’re charged.

Tesla didn’t respond to questions, but the 2020 Tesla Model 3 owner’s manual doesn’t mention avoiding a full charge.

If you decide not to charge your EV to full, you don’t have to run out and unplug your EV the second it hits 80 per cent.

Many EVs, including cars from GM, Ford and VW, have settings that let you set them to charge to a specific percentage.

So, if you set it at 90 per cent, for instance, it won’t charge past that – even if you leave the car plugged in all the time.

Can’t take the heat or cold?

There are worse things for EV batteries than charging to full.

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Heat waves, sub-zero temperatures, letting your car sit unplugged with an empty charge and charging regularly at a fast charger all sap your EV battery’s life, Keoleian said.

“You don’t want high temperatures or low temperatures,” Keoleian said. “Plug it in on hot days and when the temperature is below zero. Parking your car in the shade is helpful.”

Most EVs have built-in systems that keep the batteries from getting too hot or too cold, but your car needs to be plugged in for them to work.

Plus, if you plug into a fast charger – which can get your battery from full to 80 per cent in 30 to 40 minutes – every time you charge, you’ll lose some battery life, Keoleian said.

Using one on weekend trips shouldn’t be a problem, though.

“If you’re on a trip, you need to fast charge,” Keoleian said. “We don’t know exactly how much degradation there is – there’s a lot of controversy – but I wouldn’t want people to worry that a single fast charge would ruin their batteries.”

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So how concerned should you be, overall, about premature battery death?

“People shouldn’t be overly worried about damaging batteries,” said Jim Vanderwal, director of climate change programs with the Fraser Basin Council. “Most vehicles have longer warranties on the batteries and charging management systems.”

Chevy’s Bolt, for instance, has an 8-year/160,000 km warranty on the battery.

The general rule is that when a battery hits 80 per cent of its original capacity, you should change it, Keoleian said. But even with less-than-ideal charging habits, batteries are lasting years longer than expected.

“The studies are showing that the batteries are going to outlast the car,” said Cara Clairman, president and CEO of Plug n’ Drive, a not-for-profit that promotes EVs. “There are a lot more cycles in the batteries than anyone thought.”

Have a driving question? Send it to globedrive@globeandmail.com and put ‘Driving Concerns’ in your subject line. Emails without the correct subject line may not be answered. Canada’s a big place, so let us know where you are so we can find the answer for your city and province.

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