Range Confidence: Charge Fast, Drive Far, with your Electric Car

By David Herron

Battery pack age, and capacity loss

A wise person observed long ago that all things turn to dust. An aged electric car battery pack holds less energy than a sprightly young pack fresh from the factory. The same happens with laptops, cell phones, and other gizmos with lithium-ion battery packs. As the pack ages, it holds less energy, and the gizmo doesn't run as long on the battery pack.

At least a laptop can be kept plugged into a power adapter. You can't do that with your electric car, because it's designed to roam far beyond the length of any extension cord.

A car whose pack has lost capacity will not give as much range as it should. This is simple, the car cannot store as many kiloWatt-hours as it did previously. All the equations shown elsewhere dictate that fewer kiloWatt-hours of energy means less range.

This is different from the leadfoot, whose high energy consumption driving habits diminish their total driving range. The problem in this case is lost energy capacity, not high energy consumption.

Range is energy capacity divided by energy consumption. A car with a 24 kiloWatt-hour pack might diminish to 20 kiloWatt-hours, over time. A fully charged range of 84 miles would become 67 miles maximum range, if the pack diminishes to 80% of original capacity.

Capacity loss is expected, and at the "normal" rate of loss it would take several years to reach 80% loss. Unfortunately the Nissan Leaf, in 2010-2014 model years, in some cases suffered extreme capacity loss. This became a large controversy during the summer of 2012. Nissan developed technical fixes in the 2013 and 2015 model years, with 2015 seeing the introduction of the Lizard battery that's not affected by heat.[BatteryCapacityLoss]

The primary culprit seemed to be heat, either from ambient temperature or from frequent fast charging. Many of the affected cars were in Phoenix Arizona. Whether a given battery pack is vulnerable to heat depends on its chemistry, with some chemistries being more vulnerable than others. The lithium-manganese chemistry used in the Nissan Leaf is among the most vulnerable to heat.

Recommendations

These recommendations are thought to avoid capacity loss, and promote longer battery life:

  • Avoid deep discharges Taking the State of Charge too low risks battery pack damage. Simply avoid driving the car to 0%, and instead keep it above 20%. Most electric cars start warning you to find a charging station at 20%.
  • Avoid keeping the car charged to 100% for a long period Daily charging to 100% is fine. It becomes a problem if the car stays fully charged for weeks at a time. If you're going to leave the car sitting that long, like when taking a vacation, drive it until the pack falls to 80% then park the car.
  • Some suggest we avoid charging to 100%, period, because it's thought packs degrade more when charged to 100% than when charged to a bit less than 100%. The real concern is the length of time the battery pack is held at 100% state of charge, which is why we recommend storing the car at 80%. However, when charged to 100% there is risk of some cells getting overcharged.
    • The battery management system (we're getting way down in the weeds here) will make sure battery cell voltages stay balanced, but might not completely avoid overcharging individual cells.
    • Some cars let you adjust the charging target, so the car will never be charged over 80% or wherever you set the threshold.
  • Avoid frequent fast charging Fast charging can cause battery pack heating, and heat is thought to degrade battery pack capacity. Some electric cars have active thermal management (heating and/or cooling) to take care of heat build-up in the pack, which should minimize any damage that might occur from fast charging.
  • Avoid leaving the car parked in a hot parking lot on a sunny day Again, heat is thought to harm battery packs. Hot parking lots amplify the local temperature, due to all that asphalt. A heat-sensitive thing, like an electric car battery pack, needs the shade - find a tree to park under, or inside a parking garage, anything.

These recommendations aren't precise because the automakers haven't told us clear best practices. Instead some, like Nissan, have changed their position over time. Initially Nissan told drivers to do fast charging as frequently as we like, but after the capacity loss controversy hit the company switched to recommending infrequent fast charging.

Another risk with fast charging

Heat isn't the only risk factor with fast charging. Simply charging at a high rate is thought to damage a battery pack, and again this depends on the battery chemistry. Measuring the risk requires understanding the "C" rate.

Battery engineers call "C" the rate of discharge (or charge) that will completely drain (or charge) the battery in one hour. A 1C rate discharges it in one hour, a 2C rate discharges it in 30 minutes, a 3C rate discharges it in 20 minutes, a C/2 rate discharges it in 2 hours, a C/4 rate discharges it in 4 hours, and so on. The same is true for charging, with a 1C charging rate giving a complete recharge in one hour.

That is - with a 10 kiloWatt-hour pack, a 10 kiloWatt charge (or discharge) rate is 1C, a 5 kiloWatt rate is C/2, and a 20 kiloWatt rate is 2C.

Each battery manufacturer tests battery capabilities, will determine the maximum safe charge or discharge rate, and the capacity degradation over time. The actual maximum rates will depend on the battery chemistry, and the manufacturing process. That hasn't stopped many from developing a rule-of-thumb saying that 2C is the maximum safe charging speed.

On a 24 kiloWatt-hour electric car, like the Nissan Leaf, a 50 kiloWatt charging rate is about 2C. The BMW i3, with a slightly smaller pack, 50 kiloWatts is slightly over 2C. The Kia Soul EV with a slightly larger pack, 50 kiloWatts is slightly under 2C.

For the 90 kiloWatt-hour Tesla Model X, the Tesla Motors Supercharger maxes at a 130 kiloWatt charging rate. That's about 1.5C, and on a 50 kiloWatt charger the rate is about C/2.

In other words, the manufacturers show a tendency to limit fast charging to a 2C or less rate. Suggesting there might be some truth to the rule of thumb to keep the charge rate less than 2C.

The manufacturers also limit the maximum charge while fast charging. Usually, the car will stop at, say, 83% or 94% while fast charging. The car will also decrease the charge rate dramatically as the battery fills up. Both behaviors are clearly meant to avoid overcharging some battery cells.

Footnotes
1. [BatteryCapacityLoss]: (www.electricvehiclewiki.com) Battery Capacity Loss The Electric Vehicle Wiki has a compendium of information that's heavily influenced by the issue of Nissan Leaf capacity loss.

Range Confidence is Copyright © 2016-17 by David Herron

Battery pack age, and capacity loss

(disqus.com) comments powered by Disqus