The 80% Rule

By Craig Cole, Senior Editor, EV Pulse

There are rules for everything. Pay your taxes… wear pants… don’t drive on the sidewalk … and if you own an electric vehicle, you need to be aware of the “80% rule.” Why is 80% an important number if you own an electric car, crossover, or pick-up?

There are two reasons: charging performance and battery longevity. Most of the time you should only charge an EV to 80% because charging rates slow down dramatically past the 80% mark. And two, the long-term health of your vehicle’s battery pack is improved when kept below 100%.

What does all this mean?

Concerning charging rates, a good example is the Hyundai Ioniq 5 with the optional, long-range battery. This hatchback-like crossover can DC fast charge from 10 to 80% in an incredibly quick 18 minutes. But it needs an additional 32 minutes to go from 80 to 100% – almost twice as long as it took to go from 10 to 80.

Why? Charging is not linear. Instead of batteries taking in energy at a constant, predictable rate, the rate actually changes based on a myriad of variables, though most importantly, the battery’s state of charge. Simply put, the fuller the battery is, the slower it absorbs energy. Imagine if a conventional car’s gas tank took longer and longer to fill up the closer it got to being full. It’s kind of crazy.

The best analogy I’ve heard for why charging slows down is that batteries are like theater seating. When you’re one of the first people to enter, it’s quick and easy to find a chair – you can sit anywhere – but as the theater fills up, it takes a lot longer to snag a spot and sit down. In the Li-MAX Cineplex above, the electrons are climbing over each other and spilling popcorn everywhere!

It’s important to know about the “80 % rule” if you’re on a long-distance drive in an EV. When it’s time to charge, it’s often smarter to stop at 80% and then get back on the road, instead of waiting for the battery to completely fill up. Doing so maximizes your use of time.

For example, if your EV has 300 miles of range when fully juiced up, that means it can go about 240 miles with an 80% state of charge. (Obviously, you’re going to stop and power up before hitting zero miles, but let’s keep things simple and say 240.) If the 0-to-80% recharge time is 40 minutes, you can hit the road in little more than half an hour. If you want to fully replenish the battery, it could realistically take an additional 90 minutes to go from 80 to 100%. In the time it took you to gain that extra range, you could be a hundred miles or more down the road and in the vicinity of another charger. That’s why stopping at 80% usually makes the most sense (though that is something YOU have to determine).

There are, of course, instances where you’ll want to wait longer to hit 100%. Maybe there are huge distances between DC fast chargers, and you need every bit of range you can get. It could be the dead of winter and you have range anxiety about making it to your destination. Or you’re towing a car or boat, and the extra weight means you need the additional kilowatt-hours to get you to the next charging station.

The other reason to avoid going all the way to 100 is because it can help preserve battery life. Whether it’s a phone, cordless drill, or your car, batteries simply don’t like to be full. Keeping them topped to the brim means, over time, the maximum kilowatt-hours they can hold shrinks faster than it would otherwise. Always concerned about warranty costs, automakers even suggest limiting how much you charge. Car companies make this easy to do with an infotainment system that allows you to set your preferred charge level – even when you’re not at home.

It’s important to note that you can charge your EV to 100%, but it’s just that for optimal battery life over the long haul, charging to a lower percentage is a good idea. It’s like changing engine oil in an old-school vehicle. You can follow the manufacturer’s recommendation, but doing it more frequently is never a bad idea, especially if you plan on keeping your car or truck for years and years.

Craig Cole is Senior Editor at EV Pulse. He brings 15 years of experience to EV Pulse and is a proud member of the Automotive Press Association and the Midwest Automotive Media Association. Check out the EV Pulse YouTube channel here.