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June 20, 2020

Why Charge to 80 Percent?


There are two situations where it's wise to charge your EV only to 80% State-of-Charge (SOC). The first situation is to prolong battery life. The second is as a courtesy to other drivers.

Regular Charging at Home

Lithium-ion batteries are sensitive to extremes: extreme heat and cold, and extreme charge-discharge cycles. Frequent charging to 100% and driving your car down to 1% SOC (or 1% battery capacity) can decrease the overall amount of electricity your battery can store over time. Expressed another way, frequent extreme charge-discharge cycles increases the degradation of the traction battery's ability to store electricity.

For this reason, most EV manufacturers recommend (yes, read the manual) regularly charging at home on Level 2 to 80% of your battery's capacity. It doesn't have to be exactly 80%, it can be 85% or even 90%, simply don't charge to 100%.

Our 2017 Bolt EV has a charging feature called "Hill Top Reserve" that stops charging the car on Level 1 or Level 2 at 89%. Chevy's intent is to allow sufficient unused capacity in the traction battery so that full regenerative braking is available if you live on a hill. When you drive off in the morning after charging to 89% the Bolt has full regenerative braking and will have the same driving characteristics as normal.

In newer Bolts, you can set the charge limit at the level you want.

When you're leaving on a trip or otherwise need the capacity, go ahead and charge to 100%. Don't fret about it. This isn't hurting the battery.

We have only one car and it is the Bolt EV. When we take a road trip, naturally we take the Bolt. I charge it to 100% the night before. I drive it until it's convenient to charge again or the SOC drops to 10% to 15% capacity. Some well-known Bolt owners drive down to 5% or even 1% battery. One driver has put nearly 150,000 miles on his Bolt and still regularly makes 1,000-mile roundtrips. His driving and charging style hasn't obviously hurt the performance of his Bolt.

If you're leasing the EV, this isn't something you need to worry about. The leasing companies don't bill you for battery degradation when you return the car, dents and dings, yes, but battery degradation, no. For the length of most leases, the battery degradation over three years will be minor anyway.

DCFC Charging on the Road

When you're on a road trip and need to stop at a DC Fast Charge station, it's good charging etiquette to stop charging when the battery reaches 80% SOC. This is particularly important if there's someone waiting to charge their car. Don't be a charge port hog.

The charging rate begins to decline dramatically when your car reaches 80% capacity. For a Chevy Bolt, you may start off charging your car at a DCFC station at 55 kW. By the time the car reaches 80% charge, you may be charging at only 15 kW. And it goes down from there.

Because the charge rate drops, it takes longer to add a kWh than it did when you first plugged the car in. In most cases you can get the charge you need to get to your destination in the first 20 to 30 minutes.

It takes almost as much time to charge the car from 80% to 100% as it did from 20% to 80% because the charge rate is so low. In the old days--like a few years ago--many DCFC stations had a 30 minute limit and specifically said to limit charging to 80% so the station could be used by the next person.

Of course, if no one is waiting and you need to charge to 90% or even 100%, go ahead and do so.

If someone is waiting, charge to 80% on the DCFC kiosk and then move out of the way. If there's a Level 2 at the station, you can move to the Level 2 and continue charging as long as you want. 

 

Both of the situations calling for charging to 80% are highly contentious. There are several heated threads on popular news groups about both topics. Take it with a grain of salt. Don't believe everything you read.

For example, one driver noted that on the Bolt Facebook page someone wrote that you charge only to 80% at a DCFC station because it costs more to charge the last 20%. While this may or may not be technically true depending upon the DCFC network you're using, that's not the reason for stopping the charge at 80%.

You stop the charge at 80% so someone else can use the charger. If there's no one waiting, and you want to charge more, the extra cost--if any--is pennies. It will take more time, but it doesn't cost that much more.

It's like everything else in life, be courteous, know your charging station etiquette, and don't worry about the small stuff.


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