Remember Michael DeGusta, the guy who indadvertedly ‘bricked’ his oh-so-expensive Tesla Roadster after storing it in an almost-empty state for nearly two months?
In the case of DeGusta’s Tesla, the car used most of the remaining charge trying to keep itself warm, but the story did remind us all of one simple thing: like any other rechargeable battery, an electric car’s battery pack will slowly discharge when left unplugged.
So, with some very crude scientific endeavor, we set out to see just how much range a 2011 Nissan Leaf would lose as a consequence of being parked up for eight days with a partial charge.
An electric car battery pack turns electrical energy received during recharging into chemical energy. Then under use, it converts that chemical energy back to electrical energy to provide power to the car.
Disconnect a regular rechargeable battery to store it, and slow, internal chemical reactions take place within the battery which gradually discharges it. Leave an electric car unplugged for a while, and the same happens.
The rate of discharge depends on the type of battery. Typically for lithium-based battery packs, self-discharge is around 2 to 3 percent per month--though it varies with the specific cell chemistry used. For nickel-based battery packs, self discharge can be as much 30 percent per month.
2011 Nissan Leaf SL
On paper that might sound easy to predict, but temperature, state-of-charge when unplugged, and battery age also affect the rate at which a battery self-discharges.
On to our test case. After a 30-mile drive, we parked our Leaf at our local airport with an indicated 51 miles of remaining range. Over the course of the next eight days, we regularly checked the car’s state-of-charge remotely using Nissan’s Carwings service, gradually watching the predicted range drop.
During that time, daytime temperatures dropped from 68 degrees Fahrenheit down to the high 50s, while nighttime temperatures averaged 40 degrees.
When we returned to our car, it told us we had enough charge to drive 45 miles, representing a 6-mile drop in predicted range over 8 days.
While that might seem more than the suggested 2-3 percent drop per month, self-discharge wasn't the only thing responsible the 6-mile drop.
Lithium-ion battery pack of 2011 Nissan Leaf, showing cells assembled into modules
As with any car when it is switched off, some of the Leaf’s 12-volt accessories continue to consume power. Everything from the alarm to the battery monitoring and telematics systems consume power when the Leaf is switched off.
Over time, this drains the Leaf’s on-board 12-volt lead-acid accessory battery, requiring the car to periodically charge it as needed through the 24-kilowatt-hour traction battery and DC-DC converter.
What have we learned?
In most circumstances, leaving your Leaf unplugged for a week or so with a moderate amount of charge will lose you a small amount of range.
But if you’re going away for longer, you should follow Nissan’s own advice and plug in the car, to make sure that you keep your Leaf’s battery warranty valid.