Useful Life of EV batteries

Redefining the Useful Lifetime and the Start of EV Battery 2^nd Life

Note: This case study is in the process of being revised to include results quantifying the impacts of battery power fade in addition to the capacity fade results that are presented below.

Overview

Electric vehicles enable clean and highly efficient transportation decoupled from oil, however concerns about range anxiety and battery degradation hinder the adoption of EVs. EV batteries are commonly thought to provide a useful life in vehicles until they degrade to 80% of their remaining capacity and there has been little systematic analysis to support or challenge this retirement threshold. We apply detailed physics-based models of EVs with data on how drivers use their cars to quantify how battery capacity fade impacts the ability of EVs to meet the mobility needs of U.S. drivers. We show that EV batteries continue to meet the needs of U.S. drivers well beyond degradation levels of 80% remaining capacity. For instance, we show that over 85% of U.S. mobility needs continue to be satisfied at 80% remaining capacity, while over 65% of U.S. mobility needs continue to be satisfied at 50% remaining capacity. Further, we show that EV batteries with substantial levels of battery degradation continue to accommodate provide sufficient buffer charge to accommodate unexpected trips with long distances, for example exceeding 50 km. We show that the effects of battery capacity fade upon the ability to satisfy mobility needs can be mitigated by enabling charging in more locations, even if the additional chargers are only 120 V wall outlets. The findings in this study conclusively show that EVs satisfy the majority of the mobility needs of drivers, even when EV batteries lose substantial amounts of their energy storage capacity through naturally-occurring degradation.

Specific Objectives

This study quantifies how EV battery energy capacity fade impacts the ability of EVs to satisfy the mobility needs of U.S. drivers. The vehicle-to-grid simulator (V2G-Sim) is used to predict the battery SOC profile for vehicles driven according to trip itineraries specified by the National Household Travel Survey. To understand the impact of battery capacity fade, simulations are run with a parametric sweep of gradually lower levels of usable battery capacity levels. The fraction of U.S. mobility needs being satisfied under each level of usable battery capacity is quantified as the fraction of vehicles that can complete their travel itinerary without running out of charge. Specifically, this study is conducted with the following objectives:

Quantify the impact of battery capacity fade upon the ability of EVs to satisfy the mobility needs of U.S. drivers.
Demonstrate that vehicle batteries can have useful life within a vehicle beyond today’s commonly used metric of retiring batteries once they have lost 20-30% of their rated battery capacity.
Propose a new metric for defining the predicted retirement of vehicle batteries to occur when individual driver needs are no longer met, and present data to quantify the magnitude of driver needs that are no longer met at various levels of battery capacity fade.

Results

The National Household Travel Survey (NHTS) includes a large sample size of 24 hour vehicle usage itineraries for drivers across the United States. These NHTS travel itineraries are simulated in V2G-Sim assuming that each vehicle has specifications resembling a Nissan Leaf. The simulations include different scenarios for where vehicles are charged, and what type of charger vehicles plug into in different locations (e.g. L1 or L2 at home or work, etc.). The V2G-Sim simulations predict the vehicle battery SOC throughout a full travel day using the given travel itineraries, and these SOC results are used to identify how many vehicles are able to satisfy their travel itineraries (i.e. complete planned trips without running out of charge). Parametric sweeps are run in the V2G-Sim simulations to identify the fraction of mobility needs that are satisfied as each vehicle loses more of its battery energy storage capacity.

Figure 1 summarizes parametric simulations that quantify how battery energy capacity fade impacts the ability of EVs to satisfy the mobility needs of U.S. drivers. As expected, the results show that as vehicle batteries lose more of their capacity they are able to satisfy the mobility needs of fewer drivers. Two important results are apparent in Figure 1 which have not been quantified in prior literature. First, a sizable fraction of U.S. mobility needs continue to be satisfied by EVs even after they have experienced substantial levels of battery energy capacity fade. For instance, in the L1 Home, no Work charging scenario, over 85% of mobility needs continue to be satisfied even after the battery has degraded to 80% of its remaining battery capacity (the commonly accepted threshold for retirement of EV batteries). Second, the useful life of EV batteries can be extended by enabling charging in more locations. In this context, useful life is defined as the time span and level of battery capacity fade in which a battery continues to meet a driver’s mobility needs. For instance, in the L1 home no work charging scenario 85% of travel needs are satisfied when a battery has degraded to 80% remaining capacity. This same level of satisfying 85% of travel needs is possible down to 57% of remaining capacity if L1 charging is added in work locations as well.

Figure 1: Impact of battery capacity fade on the ability of EVs to satisfy the weekday

mobility needs of U.S. drivers for several scenarios of charger availability.

What Figure 1 tells us: Results show that a high fraction of U.S. weekday travel needs continue to be satisfied beyond the commonly accepted battery retirement threshold of 80% remaining capacity, and making chargers available in more locations enables a substantial extension to the useful life of a vehicle battery.

Figure 2 presents similar results as Figure 1, but for weekend travel. The overall trends of the weekend results are identical to the weekday results, however the fractions of travel needs satisfied under various levels of battery capacity fade are slightly higher suggesting that weekday travel is more likely to represent a limiting case. For weekend travel, it can also be seen that vehicle batteries continue to meet the needs for sizable fractions of U.S. travel needs even beyond degradation to 80% of remaining energy capacity.

Figure 2: Impact of battery capacity fade on the ability of EVs to satisfy the
weekend mobility needs of U.S. drivers for several scenarios of charger availability.

What Figure 2 tells us: Results show that a high fraction of weekend travel needs continue to be satisfied beyond the commonly accepted battery retirement threshold of 80% remaining capacity.

From Figures 1 and 2 it is important to note that even when vehicle batteries degrade down to 30% of their original energy storage capacity, over 55% of U.S. travel needs continue to be satisfied. A common concern by prospective EV buyers is that they will have to replace a battery pack well before the end of a vehicle’s useful life. However, the results in Figures 1 and 2 suggest that vehicle batteries can continue to satisfy sizable fractions of driver needs even after experiencing substantial levels of energy capacity fade. With these results, it can logically be concluded that vehicle batteries can continue to provide the energy storage needs to satisfy driver mobility requirements for the full lifetime of an electric vehicle. Although some degradation will occur in these batteries, the results suggest that this degradation need not necessitate replacement of a vehicle battery.

The results presented in Figures 1 and 2 showed that sizable fractions of U.S. mobility needs can be satisfied by EVs even after the EV batteries experienced substantial levels of energy capacity fade. These results, however, did not consider whether vehicles batteries that have experienced energy capacity fade will still be able to accommodate unexpected trips by drivers. This section quantifies the ability of EVs to accommodate unexpected trips after varying the EV batteries have experienced varying degrees of energy capacity fade.

Figure 3(A) through Figure 3(F) present contour plots that quantify the fraction of EVs that will have different amounts of buffer range for unexpected trips. Each contour plot is created by using the battery SOC projections from V2G-Sim results to identify the minimum SOC value that each vehicle will encounter during its travel day. The minimum SOC value is converted into an EV range that would be available if a driver makes an unexpected trip at the time of day when the vehicle is at its minimum SOC. The contour plots in Figure 3 summarize the results of this procedure by showing the fraction of EVs that will have different levels of buffer range for unexpected trips. For example, in the 90% remaining energy capacity case in Figure 3(A), the results show that 16% of EVs will have between 100 and 110 km of buffer range if an unexpected trip occurred at the time that each vehicle had its minimum SOC.

Several important results can be seen from the contour plots of Figure 3. First, for each charging case the contour plots show a bar of high concentration with a linearly decreasing slope as batteries experience more capacity loss. This trend in each plot indicates that as batteries experience more energy capacity fade the magnitude of buffer range for unexpected trips decreases. The y-axis magnitudes of these high concentration regions are important to note, as they show that even under extreme degradation cases, large fractions of EVs will be able to accommodate unexpected trips of substantial distance. For example, the results in each plot show that with only 60% remaining energy capacity, the substantial fractions of EVs will have over 50 km of buffer range for unexpected trips. Second, for each charging case the contour plots show a region of high concentration emerging near zero buffer range (bottom of y-axis) for the cases with the greatest levels of battery capacity fade (right side of x-axis). This trend indicates that the fraction of vehicles that are unable to accommodate any unexpected trips sharply increases as battery capacity fade approaches 30% of remaining energy capacity.

Figure 3: Contour plots of the fraction of EVs that have different levels of buffer in remaining

EV range if an unexpected trip occurs at the time when vehicles have their minimum SOC in a day.

What Figure 3 tells us: Each plot includes a parametric sweep (x-axis) for different cases of battery capacity fade, and each of the different plots shows a different case of where EVs are charged. Overall results show that as vehicle batteries lose greater capacity (moving rightwards on each plot), vehicles tend to have lower EV range available for unexpected trips and larger fractions of EVs have no buffer at all for unexpected trips indicating that they ran out of charge at some point in a travel day.

The contour plots in Figure 3 showed that as EV batteries fade to the lowest capacity levels simulated (30% remaining capacity) there are many vehicles (up to 40%) that will not have any buffer to accommodate unexpected trips. However, the results also show that even with substantial levels of battery capacity fade (e.g. down to 60% of remaining capacity), the largest fractions of EVs can still provide substantial buffer range (e.g. above 50 km) for unexpected trips. These results show that EV batteries can indeed continue to meet the needs of EV drivers even beyond the commonly accepted battery retirement threshold of 80% remaining capacity.

Implications of these Results

The results presented in above have several important implications impacting the economics and perceived utility of EV batteries during their first life in vehicles and in their second life for stationary storage. 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 needs of a majority of EV 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 needs of an individual driver are no longer met. The results of Figures 1 and 2 quantify fraction of drivers who’s needs will no longer be met at various levels of battery degradation, and this can be taken as an indicator of how many batteries may be retired at different levels of remaining usable capacity.

A second important implication from this study’s results are that the useful life of EV batteries can be extended by enabling EV charging in more locations where vehicles are parked. For instance, Figures 1 and 2 show that EVs that are charged at home and work locations continue to meet driver needs to much greater levels of battery capacity loss. Charging in secondary locations need not require build out of expensive charging infrastructure, simply adding a standard 120 V outlet in secondary locations has a dramatic impact on extending the useful life of an EV battery. In fact, only limited benefits are observed from deploying L2 chargers in work places over and above the benefits from adding 120 V outlets for charging.

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 needs of individual drivers. A majority of prior literature on second life potential and value for EV batteries assumes that EV batteries are retired from their vehicle life when 70-80% of the original rated capacity remains. This paper showed that EV batteries continue to satisfy the needs of a majority of drivers well beyond 70-80% remaining capacity and as a result, the vehicle life of batteries is likely longer than is used in prior analysis while the second life will be shorter.

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). For example, if an EV battery is retired from its first life when it has 60% of its original capacity remaining, that battery can continue to meet the mobility needs of over 75% of drivers. As a result, these used EV batteries may be utilized in an entirely new market of shorter range vehicles potentially enabling the deployment of short range EVs at substantially reduced cost.