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
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