Berkeley Lab/UC Berkeley study shows EV batteries meet daily travel needs of 85%+ of US drivers even after 20% energy capacity fade; calls for new EOL criteria
31 March 2015
A new study by researchers from Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley shows quantitatively that EV batteries can continue to meet daily travel needs of drivers well beyond the 80% floor for remaining energy storage capacity that is commonly assumed. An open access paper on their work, which applied detailed physics-based models of EVs with data on how drivers use their cars, is published in the Journal of Power Sources.
The study also sheds light on a number of other factors concerning battery use and energy and power fade, including that even EV batteries with substantial energy capacity fade continue to provide sufficient buffer charge for unexpected trips with long distances; that enabling charging in more locations, even if only with 120 V wall outlets, prolongs the useful life of EV batteries; and that EVs meet performance requirements even down to 30% remaining power capacity.
Energy capacity fade impacts the range capabilities of EVs; power fade impacts the driving performance of EVs in terms of acceleration, gradeability, and maximum charging during regenerative braking or charging events. Put another way, that final finding illustrates that energy capacity fade is a more limiting factor governing retirement than power fade.
The issue of battery degradation amplifies both the range and cost challenges for EVs. Batteries experience capacity fade with time and with usage, and causing EVs to lose driving range capability over their lifetime. If a vehicle battery pack degrades to sufficient levels it will need to be replaced, possibly resulting in substantial additional cost for the car owner to replace the battery pack. Prior research in battery degradation, economics of EVs, and second life applications of EV batteries have assumed that batteries must be retired from their vehicle application once they have 70–80% of their original energy storage capacity remaining. In order to offset the high costs of batteries, many studies have explored the use of these degraded batteries in a second life where they are used as stationary batteries to offer grid services.
Although it is a persistent criterion in nearly all studies, the use of a 70–80% remaining capacity threshold has not been questioned. However, a recent study showed that batteries with 80% remaining capacity continue to meet the needs of a vast majority of drivers. Quantitative analysis is needed to determine when drivers are likely to retire their vehicle batteries, based on when these batteries no longer meet the daily travel needs of the driver. The ability of EV batteries to meet driver daily travel needs is influenced both by energy storage capacity (which affects EV range), and by power capacity (which affects acceleration, gradeability and regenerative braking capabilities)—the present study examines both energy and power fade in terms of satisfying driver needs. Redefining the threshold in remaining energy and power capacity for battery retirement has the potential to redefine the economics of EVs as batteries may last longer in their first life (in vehicles), and therefore enter their second life with much lower levels of remaining energy and power capacity than has been assumed in prior analyses. This paper presents quantitative analysis to understand the levels of remaining battery energy and power capacity that meet the daily travel needs of drivers, thereby redefining what level of energy and power capacity batteries may have remaining when they are retired from their vehicle life.—Saxena et al.
To conduct the study, the researchers took nearly 160,000 actual driving itineraries from the National Household Travel Survey conducted by the Department of Transportation. These are 24-hour travel itineraries showing when a car was parked or driving, including both weekend and weekday usage by drivers across the United States.
The researchers then assumed all itineraries were driven using a vehicle with specifications similar to a Nissan LEAF, which has about 24 kWh of energy storage capacity, similar to many other EVs on the market, and 400 kW of discharge power capability, which was based on battery cell-level measurement data for the chosen vehicle.
This data was fed into the team’s simulation tool, V2G-Sim, or Vehicle-to-Grid Simulator. Developed by lead author Samveg Saxena and other Berkeley Lab researchers, V2G-Sim quantifies second-by-second energy use while driving or charging for any number of different vehicle or charger types under varying driving conditions.
Then for each of the itineraries, they changed different variables, including not only the battery’s energy storage capacity, but also when the car was charged (for example, level 1 charger [standard 120V outlet] at home only, level 1 charger at home and work, level 2 charger [240V outlet] at home and level 1 charger at work, and so on), whether it was city or highway driving, whether the air conditioner was on, and whether the car was being driven uphill. More than 13 million individual daily state-of-charge profiles were computed.
As the battery continues to degrade down to 50% of its original energy storage capacity, the research found that the under a base case, daily travel needs of more than 80% of US drivers can still be met, and at 30% capacity, 55% of drivers still have their daily needs met even under the most unfavorable charging scenario. (The base case does not necessarily capture the full range of vehicle use by drivers.)
The researchers also modeled the impact of power fade on a vehicle’s ability to accelerate as well as to climb steep hills and complete other drive cycles. They found that power fade for the chosen vehicle does not have a significant impact on an EV’s performance, and that a battery’s retirement will be driven by energy capacity fade rather than by power fade.
In fact, our analysis showed that the battery pack we studied, the Nissan Leaf, has a large margin of extra power capability. Energy capacity fade is really the limiting factor for this vehicle, not power fade.—Samveg Saxena
The combined results suggest that EV batteries will continue to meet the daily travel needs of drivers significantly longer than has been assumed in prior literature; the team suggested that two underlying facts drive these results:
Electric cars are more energy efficient than their conventional internal combustion (IC) engine counterparts. The energy conversion efficiency of batteries and motors taken together is significantly higher than that of IC engines. Thus EVs need far less energy storage capacity than conventional vehicles in order to meet drivers’ daily travel needs.
The way that people use their cars on a daily basis seldom requires driving range in excess of what an EV provides (even if its battery has experienced substantial levels of energy capacity fade), and given that vehicles spend a majority of their time being parked there is ample time for EVs to be charged.
Most importantly, the results in this paper show that EV batteries will continue to meet driver needs much longer than current literature suggests. A standard metric to define the retirement time of EV batteries is when the battery degrades to have 80% of its original rated capacity. This paper conclusively shows that EV batteries continue to meet the daily travel needs of a majority of drivers well beyond 80% remaining capacity. As a result, researchers, analysts, automakers and battery manufacturers should consider new criteria to define the time when EV batteries are retired. One proposed criteria is to define retirement of a battery once the daily travel needs of an individual driver are no longer met.
… A second important implication from this study’s results is that the useful life of EV batteries can be extended by enabling EV charging in more locations where vehicles are parked. … A third implication from this study is that the second life and EV economic analysis literature needs to be re-examined using an end-of-life metric that considers retirement of EV batteries to occur when the batteries no longer meet the daily travel needs of individual drivers. … A final implication arising from this study’s results are that degraded vehicle batteries may have a secondary use in vehicles that are rated for shorter range trips (e.g. intra-city travel).—Saxena et al.
In sum, we can lose a lot of storage and power capability in a vehicle like a Leaf and still meet the needs of drivers.—Samveg Saxena
This research was funded through the Laboratory Directed Research and Development (LDRD) program at Berkeley Lab. V2G-Sim is available for licensing through Berkeley Lab’s Innovation and Partnerships Office.
Samveg Saxena, Caroline Le Floch, Jason MacDonald, Scott Moura (2015) “Quantifying EV battery end-of-life through analysis of travel needs with vehicle powertrain models,” Journal of Power Sources, Volume 282, Pages 265-276 doi: 10.1016/j.jpowsour.2015.01.072
I think they are pushing it, at least for drivers in colder climates where the range takes a hammering in winter.
Stuck by the edge of the road in January in Michigan, it ain't much help to philosophise that on average, at average Michigan temperatures, you would have made it.
Range has to have a pretty good reserve for non-average days to be adequate.
Of course, countering that the next generation of bog-standard BEVs will probably have somewhere around 40-50kwh of energy, and those really will be useful down to low capacity.
In moderate climates old Leaf 24kwh cars should still be useful for some, too.
Posted by: Davemart | 31 March 2015 at 02:20 AM
You've always got to know better, haven't you, despite not even living in the US or being at all qualified to critique these scientists, or perhaps you do have such qualifications and have neglected to tell us. The methodology employed in this study as reported suggests to me that the rather short life previously predicted for EV batteries probably has had a lot to do with the general low uptake of this technology, and seems to have taken pretty firm root in your thinking, too. Myself I prefer to listen to the scientists.
Posted by: Peterww | 31 March 2015 at 02:47 AM
Dear me, now you're pushing it at Dave. You shouldn't be so harsh to him. After all he may not be a scientist but certainly has other attributes as his name implies Dave(s)mart.
Posted by: yoatmon | 31 March 2015 at 03:29 AM
The problem is that people are not rational and the advertising industry exists to increase this irrationality.
If you can do 85-90 % of your journeys in an EV, they will focus on the 10-15% you cannot do.
For these, you need access to another car, probably some kind of ICE. [You could hop along from charger to charger, but after the first time, it would probably get a bit stale.]
Thus IMO, the EV companies and governments need to make it as easy as possible to swap to a long ranged ICE (or HEV) when you occasionally need it.
The simplest thing would be to be able to drive to a rental office and switch cars and use your own insurance on the car you rent (to keep the cost down). The insurance details could be worked out in advance for a couple of rental companies so the switch becomes very quick.
Or you could have an informal "car swap/lend" club with both EV and ICE cars in it, but that might end in tears if you get a difficult person in it.
Of course, if you have a second car, you are OK. Perhaps "they" could make it cheap to tax and insure a "backup" ICE if you have an EV already.
There are lots of "administrative" solutions to the EV range problem, you do not need to wait for a technological one.
Posted by: mahonj | 31 March 2015 at 04:38 AM
Presumably nowhere outside of the US ever has cold weather, and the performance of batteries there is very different to the degradation graphs I have seen for batteries in general.
So what is you scientific training telling you is the range of a Leaf when it is depleted by a certain percentage, and at what temperature?
The data given does not seem to refer to the temperature at all, but I eagerly await your analysis.
Posted by: Davemart | 31 March 2015 at 06:33 AM
Your advanced scientific training seems to have skipped some of the details, like how the scientific method works.
Evidently to your disappointment, it does not involve comparing qualifications and positions.
I think you are getting it confused with being an archbishop, or something.
It is simply a case of addressing the argument made, which you have not done.
All you need to do to prove me in error is to show that:
The study's range estimates were in respect of low temperature use.
Range is not affected by low temperatures, and all the stuff about their losing range in the cold is ill informed FUD
Good luck with either of those.
Perhaps you should have a look into what scientific debate consists of, or ask for a refund on whatever you paid for your qualifications, as you do not seem to have grasped the basics.
Posted by: Davemart | 31 March 2015 at 07:25 AM
+ they should use a diesel heater in EVs for cold places.
Diesel is only 35% efficient (or whatever) as an automotive fuel, but it would be close to 95% efficient as a heater.
+ quicker than waiting for an ICE to warm a car.
It isn't as pure as a full EV, but at least you could maintain the range and not waste battery KwH heating the interior of the vehicle.
Posted by: mahonj | 31 March 2015 at 07:29 AM
DaveMart makes a couple of valid points.
DM>Range has to have a pretty good reserve for non-average days to be adequate.
Here's where daily practical experience will trump number crunching. I do a lot of spreadsheet analysis on battery capacity and range for stories I publish but have frequently found that the number on the dashboard at night and in the cold look oddly different than they did when I was in the warmth and comfort of my study.
DM>Of course, countering that the next generation of bog-standard BEVs will probably have somewhere around 40-50kwh of energy, and those really will be useful down to low capacity.
Absolutely. This problem pretty much goes away when you have more than 200 miles to start. Then the only trips that might be impacted are those long range out of town trips for which you are likely planning both route and time more carefully (and perhaps have more flexibility in time).
Having adequate range to complete a daily commute is a good start. But for EVs to be widely adopted, more of those edge cases need to be satisfied. And that means bigger buffers. It's less irrational than you might imagine. Call it intuitive worst case scenario planning.
3 BEVs in the family. One ICE that hasn't been driven in two years except to move it out of the way, smog and register it. It's for sale.
Posted by: electric-car-insider.com | 31 March 2015 at 08:36 AM
Davemart, you're missing the point.
The point being that having simple 120V plugs available everywhere one parks significantly improves usable range of today's EVs today and tomorrow.
I'd love to add addition scenarios adding 5, 10, 20 and 30 minute Quick Charge stops, too.
I imagine that the biggest bang for the buck comes from the first 5-10 minutes of QC when charge rates are the highest.
Posted by: Dave R | 31 March 2015 at 08:52 AM
I have nothing against BEV users getting as much use out of their cars as they possibly can, and certainly they will continue to suit some, perhaps even in cold climates, until they reach very low capacity.
For instance some older people may only want to use them for a once a week trip to the shops, and so a cheap s/h BEV with very low maintenance would be ideal.
I would also suggest that there might be a modest trade, modest because the value of the product would be low by that time, in selling used Leaf cars from cold areas in warmer places where the remaining range would not be so hard hit by the winter, and where you ain't going to freeze if it does run out.
I was primarily simply pointing out two things though, that the cold is going to have a big effect on how useful an old BEV still is, and that simple averages don't tell enough of the story.
You always have to have a margin, as 50% of distances driven may end up being above average! ;-)
Posted by: Davemart | 31 March 2015 at 09:06 AM
Boy, some of you guys and the allure of "scientists"...
One of my kids is a scientist, BTW, in the biosciences field, so I am not walking into this with a bias. But I'd be very happy to have some lab coat-wearing, sit-in-front-of-an-LCD all day PhD tell me that an 8 or 9 bar Nissan Leaf is OK for almost all his driving needs after he actually has to live with it for a few weeks under a variety of conditions. I drive a 12-bar 2012 and it can get frightening in the dead of winter.
DaveMart is right, and if he weren't then the 40-50kWh mark for the putative "200 mi" EV would not be in everyone's sights.
Posted by: Herman | 31 March 2015 at 10:34 AM
What Davemart said. As a transportation appliance, a car needs to get you where you are going *every* time. The average driver expects to be able to get in his/her car at the drop of a hat and go wherever without planning ahead or worrying about getting stuck. That is what our existing liquid-fuelled car/road/filling station infrastructure allows, and that is the standard of convenience and functionality that EVs need to achieve for mass adoption.
Posted by: Nick Lyons | 31 March 2015 at 10:47 AM
As an electrical engineer and sometime electrician, I have my eyes open for existing infrastructure which allows installation of "chargers of opportunity". There is a LOT of 208/240 VAC wiring already run into parking areas, and much of it is rated for the demands of HPS or even incandescent lamps rather than the LEDs now replacing them (at 70% savings). All the capacity now rendered excess could be taken up by chargers, and 100% of capacity during daylight.
If charging is ubiquitous, even at Level 1 ratings or just barely more (e.g. 208 VAC 10 A), the range limits of EVs are greatly relaxed. So is the winter range penalty, because a battery that's being charged stays warm. All you have to do is solve the "last-yard" problem, from the base of the light pole to the vehicle with the bumper almost against it.
Diesel heaters are so yesterday. Gasoline is now cheaper per BTU in the USA, and a sustainer engine is a much better use in the cold than just a heater. If you want instant heat, add a heat exchanger to the catalytic converter.This is even more true for PHEVs than EVs.
Posted by: Engineer-Poet | 31 March 2015 at 11:10 AM
Perhaps because I have never lived anywhere where it gets really, seriously cold I may have an undue respect for conditions where it does.
OTOH I am a belt and braces guy, and I have done a little sailing etc where it pays to have respect for the elements.
So whilst the AER might take a hammering in very cold weather and it might cost me more in petrol, I would be insouciant about using a PHEV in any conditions even if the battery capacity was not what it was.
Not so a BEV if it might strand me in the cold.
I like having warm feet, and a beating heart for that matter.
Posted by: Davemart | 31 March 2015 at 11:27 AM
Problems with BEVs' extended range (specially in cold areas) will persist as long as the following changes are not incorporated:
1) use affordable higher performance 3X to 4X batteries.
2) equip BEVs with 100+ kWh battery pack per 2000 lbs.
3) reduce extended range (500+ Km) BEVs total weight by 30% to 40%.
4) reduce (air) drag from 28-32 to 20-22.
5) reduce tire and drive train resistance.
6) reduce cabin eating requirements by 50%
All above changes are possible by 2020 or shortly thereafter.
Posted by: HarveyD | 31 March 2015 at 11:40 AM
Ref: 6) should read ......cabin heating......
Posted by: HarveyD | 31 March 2015 at 11:42 AM
I like warm feet too, but that doesn't stop me from playing the hypermiling game even in the dead of winter.
This doesn't work so well when it's so cold that the software fires up the engine the instant the key is turned.
Posted by: Engineer-Poet | 31 March 2015 at 04:07 PM
I should add that I have not seen the degradation curves for the Leaf battery, but if it follows the normal pattern then it will slow down a heck of a lot after it hits 70% or so, and take absolutely ages to drop to around 40%.
In my view 24kwh is too small to make that very generally useful, but if you buy a BEV in around 2017 it seems likely that its perhaps 50kwh pack which is anyway likely to be based on a tougher chemistry is going to allow useful for many range pretty much forever, or at least for the life of the vehicle.
Low maintenance means that should provide a great option as spare car etc, and in my view the likeliest cause of their eventual scrapage, accidents aside, will be the cost of insurance, as in the post 2030 or so world insurance companies are likely to frown on those who want to drive non-robotic cars on the public highway.
So the car would be obsolete rather than the battery not having fairly decent range.
Posted by: Davemart | 01 April 2015 at 12:08 AM
You guys are still missing the point of the study.
I don't think anyone is saying that MOAR RANGE!!!! wouldn't help.
But look at the charts. Now imagine that 50% remaining usable capacity is really you have left in winter conditions.
Now look at how many more miles you can make in two bands:
1. Workplace charging (L1 vs L2 not a huge difference).
2. Ubiquitous L1 charging.
In short, the range you have (whether due to capacity loss or weather conditions), the more charging infrastructure you have, the more you can drive your EV.
Another way to look at is that with just L1 charging everywhere, with 50% of the range you can drive almost as many miles as you can with a 95-100% battery and charging at home.
From personal experience I can tell you that I've had many scenarios where if I had even L1 workplace charging it would have enabled quite a few more trips without worry.
Posted by: Dave R | 01 April 2015 at 12:18 AM
Yeah, more away from home chargers may help, depending on who pays for them, as where they are not subsidised the costs per kilowatt hour or per mile are sometimes very steep.
There is no point economising by not buying a new car if that sticks you with routine expensive charging.
I still don't think running the present generation of 24kwh cars forever is too practical for most, the next generation will be a different matter.
Posted by: Davemart | 01 April 2015 at 07:33 AM
Davemart, that begs the question of how much it costs to build new EVs versus adding infrastructure to get more out of the ones already built.
I'm not sure why "routine EXPENSIVE charging" should be an expectation. If something is expensive, people are going to avoid it. There is nothing inherently expensive about a power cord with a connector and some electronics. They could be slapped all over the place where wiring already goes (I already have sketches for add-ons) and if mass-produced they should be cheap. If the wiring goes through a business you have some way to do "net billing" where the charger power draw is subtracted and handled separately. This is a few lines of computer code in the accounting system.
There's no good reason for any of this to be expensive, so why do people think it will be?
Posted by: Engineer-Poet | 01 April 2015 at 08:27 AM
I have not run the numbers in any detail, and they are going to vary quite a lot from country to country and even region, as some pretty substantial part of them is going to be the cost of the property, rates etc, especially where they are most useful such as in high transit city areas etc.
The other big factor is utilisation rates.
BEV owners constantly assure us that charging away from home is a rare event, and the abysmally low utilisation rates of some of the chargers installed at public expense sure bear that one out.
In the absence of doing some work investigating it extensively, which I am allergic to, the primary reason I say it is expensive is because that is what they are charging wherever there is no subsidy.
Now maybe that is because they are capitalist criminals, but that is capitalism for you, and I assume that the likely future rate in unsubsidised locations, will be in the same ball park as at present.
Posted by: Davemart | 01 April 2015 at 10:13 AM
Public charging, as a fee-for-service, is generally uneconomic. There are plenty of studies out there that will step you through the math. Add UCLA and UC Davis to your search query if needed.
But that's speaking broadly. WiFi in coffe houses is no longer thought of as a viable fee-for-service in most regions, but that doesn't stop a trade show from charging $70 per day for it.
The coffee houses provide it nonetheless.
Posted by: electric-car-insider.com | 01 April 2015 at 10:47 AM
The Leaf's 24-kWh battery pack may do fine as a commuter vehicle with twice-daily charging, both at home and at work, even when the battery capacity will be reduced to below 70%.
Solar-car-port charging should be encouraged at work to increase use of RE for transportation, because it is not possible to do solar charging at night at home.
With promise of much lower price of solid oxide fuel cell in the near future, the work place will have solar panels backed-up by fuel cell running on natural gas to provide both power and heating. With increasing use of ultra-efficient LED lighting, there will be excess of electricity power vs heat if the fuel cells will be run to provide heat. What to do? Either using heat pump to absorb some of this excess power, or allow for at-work charging of PEV's even during winters, while heating the building at the same time. Far into the future, use Hydrogen to replace the NG.
Likewise, homes that are equipped with FC's can also use NG for both power to charge EV's at night as well as heat to keep the house warm. Far into the future, use the hydrogen to replace the NG.
The vehicles of the future will most likely be FC-PEV to be able to take advantage of direct RE charging whenever RE will be available, and take advantage of "waste electricity" whenever the NG or H2 FC will be used to provide heat, and only use the FC on the board the vehicle to provide heat in the winter or for long trips. In this way, the round-trip efficiency of utilization of future H2 will be very high and can rival the efficiency of Li-ion battery, while the excessive consumption of materials and energy during the construction of a large battery pack for long-range BEV can be avoided when FC-PEV's will use much smaller battery packs of 10-15 kWh instead of 60-100-kWh packs for long-range BEV's.
Posted by: Roger Pham | 01 April 2015 at 11:26 AM
I suspect that other factors include inconvenient location, and simple lack of knowledge. Many locations where people spend time have no chargers nearby. My local Ford and BMW dealerships have one each, but none of the local shopping centers do. If you want to go shopping, it's very hard to plug in anywhere close enough to return with purchases.
People who drive BEVs probably arrange their travel to avoid having to stop to charge, assuming they ever get near their range limits. If they could charge at their usual stops it would be different, but IME that is rarely the case. PHEVs are different; they are regularly at their range limits.
Part of that is probably the high cost of commercial stand-alone chargers, and the infrastructure they need (dedicated cabling). I think this could be changed if they were designed for low cost, to connect to existing wires, and mass-produced; when the major components are a relay, a circuit breaker, a ground-fault sensor and a relatively simple enabler circuit, there seems to be a lot of room for economy. It also changes if there are lots of PHEVs, especially hybrids converted to micro-PHEVs. If half of all Priuses ever built were suddenly clients for J1772 or just NEMA outlets attached to metered parking, the utilization could jump overnight.
The real issue for the moment is what we can do with minimal cost for planning and construction. Cheap measures which piggyback on existing features can expand much faster than anything else, and jump-start the next stage of growth in the vehicle fleet.
Posted by: Engineer-Poet | 01 April 2015 at 03:17 PM