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

By David Herron

Battery pack age, and capacity loss

How long will an electric car battery pack last? We all know batteries wear out over time, and of course the battery pack in an electric car wears out over time. This is called "capacity loss" meaning the battery pack holds fewer kiloWatt-hours than when it was new, and that means the electric car will have less driving range. In this article we'll go over this in more detail.

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. In a gasoline car the 12 volt battery wears out making it hard to start the car. As a battery pack ages, it holds less energy, and the gizmo doesn't run as long on the battery pack.

At least a laptop or cell phone or whatever we can easily replace the battery when it doesn't hold a charge, or it can be kept plugged into a power adapter. With an electric car whose battery capacity is gone, it obviously doesn't work to just keep the car plugged in, and it's harder to replace the battery pack.

The impact of a battery pack in an electric car is that it will not give as much range as it should. The reason for this is obvious from the range equation:

range = kWh in pack x miles/kWh

Remember, this calculation is approximate, but the main determiner of driving range is battery pack capacity, in kiloWatt-hours. The technical effect of a weak battery pack is that it holds fewer kiloWatt-hours.

This is different from the "lead-foot" example we talked about elsewhere. The aggressive driver simply uses more energy to drive as they gun the car around town. This case is about lost energy capacity, not high energy consumption.

Capacity loss is expected as the battery pack ages. The "normal" rate of loss should take several years to reach 80% capacity loss. Typical electric car manufacturer warranties cover the car retaining 70% or 80% capacity after a number of years -- typically over 100,000 miles or 8 years. The car can still be driven with a weak battery pack, just not for as far of a distance.

Apparently the manufacturer battery pack warranty is designed to cover the pack for the useful life of the car.

This phenomena is not unique to electric cars, of course. Gasoline or diesel engines wear out, as do transmissions, and other parts. Because gasoline/diesel vehicles have so many more moving parts than electric cars, the opportunity for things to break are higher.

Recommendations for achieving long electric car battery lifetime

We understand the problem - over time an electric car loses energy capacity. How do we stave off the decline? Like anything else, we must treat the battery pack properly.

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%. To help remind you, the car makers usually install a warning system that starts nagging the driver when the remaining range runs too low.

Bottom line, if your daily commute is 100 miles, do not buy an electric car rated for 105 miles range thinking you'll be okay. That car will be deeply discharged every day and the battery pack capacity will decline quickly.

Instead use your electric car so it always has a comfortable margin of remaining range. Arrange your trips incorporating charging sessions to keep SoC high enough.

Battery pack size A related choice is getting a car with a large battery pack. The average driver travels 40 miles or less per day. In a car with 100 mile range, 40 miles is 40% of the capacity, but in a car with 200 miles range that is 20% of the capacity. That means the typical daily commute in a car with a small battery pack is a deep discharge, and it is a shallow discharge for the car with a large battery pack.

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 a long time. Most electric cars can be set to a maximum charge level, which can be overridden as needed.

If you're going to leave the car sitting for weeks or months, like when taking a vacation, park the car with the battery pack below 80%.

For the times one must charge to 100%, such as a trip at the far end of the driving range with no chance to stop to charge along the way, then charge to 100% just before the trip. Also, don't fret about it too much. An electric car sitting at 100% for a few hours is not a problem, what we're pointing at is a car kept at 100% for days or weeks at a time.

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. A road trip would involve back-to-back fast charging, which would maintain hot conditions in the battery pack.

In cold weather try to charge the battery pack while it's hot This recommendation is imprecise because most of us have no way to know how hot the battery pack is. Because battery packs store electricity as a chemical reaction it stands to reason that charging a cold battery pack will be less successful than charging a hot battery pack. Some electric car thermal management systems include a heater for the battery pack, just for this purpose. Another method to heat the battery pack is to drive the car fast for several miles. A high discharge rate also generates some heat.

Yes the surrounding recommendations are warning against heat. But in cold weather heat is good because it helps the battery pack operate better.

Avoid driving the car at high speed for a long time in hot weather Returning to the "heat is bad" theme, on a hot day avoid heating the battery pack even further. Such as driving fast.

Avoid leaving the car parked in a hot parking lot on a hot/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.

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.

Rapid battery capacity loss -- Nissan Leaf 2011-2014

An issue for the 2011-2014 Nissan Leaf models was early battery degradation. It was a big controversy that Nissan at first refused to acknowledge. After independent testing by Leaf owners, Nissan admitted to the problem and began offering buyouts or battery pack replacements.

Ground zero for the problem was the Phoenix Arizona area, but Leaf owners in other areas also suffered from lost capacity. The primary culprit seemed to be heat, either from ambient temperature or from frequent fast charging.

Nissan developed a technical fix for the 2013 model year, and an even better one for the 2015 model year. The 2015 Leaf's had what was called the Lizard battery which wasn't affected by heat.[BatteryCapacityLoss]

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. Another factor is whether the car has an active thermal management system for the battery pack, or a passive system. The Nissan Leaf has a passive thermal management system. An active thermal management system has a heating/cooling system just for the battery pack.

Technical considerations with fast charging electric car batteries

Every OEM electric car has what is called a battery management system or BMS. As the name implies, this system takes care of the battery pack recording data about the pack, watching its operation to ensure it is kept above the minimum State of Charge, and watching the charging process to ensure it is kept below the maximum SoC.

The State of Charge is the capacity currently in the battery pack. Often the infotainment system shows how fully charged the car will show a percentage state of charge.

There are four main factors to consider in caring for the battery in an electric car:

  • Overheating
  • Overcharging or high voltage
  • Deep discharge
  • High discharge or charge current

Heat, either as high ambient temperature but more likely as heat generated by charging or keeping the battery at high-voltage (fully charged), reduces the capacity of a lithium battery.

This list warns against both over-charging and over-discharging. The battery management system in electric cars should prevent both from occurring, but we do need to understand this effect.

Lithium-ion cells have a safe range for State of Charge. Take it too low or too high, and the battery cell is damaged, and if this happens too often the battery cell is ruined. Again, the battery management system is supposed to prevent both from occurring.

A reason we recommended earlier to not keep the car charged to 100% is the risk the battery management system will allow slight overcharging in battery cells.

Charge rate and discharge rate in electric car battery packs

The last issue from the four main factors deserves its own section.

There is a measure, called C, that is used to discuss battery pack charge and discharge rate.

A "1C" rate is when charge or discharge, in Amps, is equal to the capacity, in Amp-Hours, of the battery pack. That is, a 100 Amp-Hour battery pack discharged at 100 Amps is expected to be completely discharged after 1 hour, and is therefore being used at a 1C rate. Likewise a 100 Amp-hour pack charging at 100 Amps is expected to be fully charged after 1 hour, and is charging at a 1C rate. Discharging or charging that 100 Amp-hour pack at 200 Amps is 2C. Discharging or charging at 10 Amps is C/10.

Each battery pack design has a maximum C rate for discharge, and a different maximum C rate for charging.

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.

Many folks believe there is a rule-of-thumb that 2C is the maximum safe charging speed.

For example when the 24 kiloWatt-hour electric cars were the best we could get, the manufacturers developed 50 kiloWatt fast charging stations. That's a 2C rate. The Tesla Supercharger system started at a 90 kiloWatt charging rate, which is 1.5C charging for the 60 kiloWatt-hour cars. They've since moved to 120 kiloWatt charging and 100 kiloWatt-hour cars are more common in Tesla's fleet, and this is still slightly over 1C.

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.

But as the manufacturers move to support 250-350 kiloWatts charging rate, they're moving beyond the 2C boundary just mentioned. The manufacturers will not have done this without first testing their battery packs at higher charge rates and determining it to be safe.

In some cases if a car is fast-charged in quick succession, the car will purposely limit the charging rate. This will be done to keep the battery pack from overheating while charging. We understand it is frustrating for charging to be rate-limited, but don't you want the battery pack to last for as long as possible?

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

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