Hawaii study finds vehicle-to-grid discharge detrimental to EV batteries
15 May 2017
Results of a study by a team at the Hawaii Natural Energy Institute, SOEST, University of Hawaii at Manoa, suggest that the additional cycling to discharge vehicle batteries to the power grid in a vehicle-to-grid (V2G) scenario, even at constant power, is detrimental to EV battery cell performance. This additional use of the battery packs could shorten the lifetime for vehicle use to less than five years, the researchers said in a paper published in the Journal of Power Sources.
By contrast, the researchers found that delaying the grid-to-vehicle (G2V) charge in order to reduce the impact on the power grid had a negligible impact on the cells at room temperature, but could be significant in warmer climates.
Given electric vehicles (EVs) projected penetration into the market, their batteries have the potential to provide significant energy storage for the electric power grid in the future by allowing the grid to give and take energy from the batteries when needed. This bidirectional charging is known as Grid-to-vehicle (G2V) and Vehicle-to-grid (V2G) respectively. … The potential of using EV battery capacity for grid storage has been the subject of much discussion in recent years but few studies have tested grid impact on EV battery degradation. … Most battery-related work published so far focuses on modeling. Although some studies stand out, the models unfortunately do not represent realistic battery degradation.
Understanding the real impact of bidirectional charging on batteries is essential to weigh their viability as a grid support service. Indeed, V2G will induce more usage of the cells by discharging to the power grid and having to recharge the battery again for transportation. Controlled G2V on the other hand will allow the cells to rest at a different state of charge (SOC) compared with charging immediately upon arrival, and therefore might induce a different calendar aging. Since battery degradation is path dependent, any changes in usage can lead to different degradation mechanisms and drastically influence the reliability of the batteries.
—Dubarry et al.
The Hawaii team performed laboratory testing on commercial Li-ion cells to investigate the impact of bidirectional charging on Panasonic 18650 NCA batteries. The researchers investigated the effects of V2G/G2V combined with different charging schedules (1 or 2 charges a day, immediate or delayed charging) and different charging currents (level 2 or fast charging). Further, the effect of calendar aging at different temperatures was also investigated in a second set of experiments.
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| Extrapolation of model parameters for (a) the cycle aging experiment and (b) the calendar aging experiment. Errors bars represent 95% confidence bounds on fitting parameters for both models plus the experimental observed maximum variation between cells. Dubarry et al. Click to enlarge. |
The team concluded that a V2G step twice a day increased battery capacity loss by 75% and the resistance by 10%. This step once a day accelerated the capacity loss by 33% and the resistance increase by 5%. Forecasts based on the measurement results indicated that that V2G implementation would decrease the lifetime of the battery packs to under 5 years.
The team also found that calendar aging influenced the cells little enough that it was beneficial to charge the cells twice a day instead of once. Charging twice per day resulted in 5% less capacity loss and similar resistance increase compared to once per day.
Resources
Matthieu Dubarry, Arnaud Devie, Katherine McKenzie (2017) “Durability and reliability of electric vehicle batteries under electric utility grid operations: Bidirectional charging impact analysis,” Journal of Power Sources, Volume 358, Pages 39-49, doi: 10.1016/j.jpowsour.2017.05.015
They used non-automotive grade cells in a lab to make conclusions about BEVs. LOL. Why is green car congress even reporting this crap.
Posted by: Account Deleted | 15 May 2017 at 02:43 AM
This will get revisited as batteries can accept more cycles. If the recent information that Tesla new 2170 cells, used in Model 3 and PowerWall2 and by year end in all other Model S & X cars too, can really support 2X more cycles than previous 18165 cells, thanks to Canadian led enhancements on electrolyte dopants plus coating, then we should see more openings for V2G. The fact that Tesla just allowed "Aggregation" for its Powerwall2, that is Residencial Battery-to-Grid feeding, controlled by the Grid, gives an indication that we may not be that far away to V2G on Tesla side at least. Just missing in my view is that all new Tesla cars announced with 21700 cells and this type of capabilities to get a reduced battery warranty compared to today Model S & X 8Y unlimited milleage battery warranty... May be reduced to 5Y unlimited or 100K Miles only.... Then my bet is they will open V2G for these models only.
Posted by: Patrick Free | 15 May 2017 at 03:44 AM
I told you that batteries were costly and degrade fast and can barely keep up in a small but costly car that is only driven locally with very short rides when outside temperature are between 55f to 85f only.
Posted by: gorr | 15 May 2017 at 06:18 AM
"Results of A study..." is rarely worth much.
https://www.youtube.com/watch?v=0Rnq1NpHdmw
Real conclusions come from compiling multiple independent studies.
Posted by: ai_vin | 15 May 2017 at 08:16 AM
Doesn't it make sense that the more recharges it would decrease the cycle life Even if rated for 10000 cycles ,if it was used and rechaged every night it would reduce to 5000 for auto use. I guess I don't understand the significance of this study.
Posted by: Jimr | 15 May 2017 at 11:38 AM
1: The report is behind a paywall which on principle sucks. This is a whole other matter and has a long history of affecting disemination of information for public or peer criticism. This is a study and not peer reviewed. No free speech or sharing of non commercial information that is in the public interest.
It is not consistent with previous laboratory studies which are categorical in stating that cycling is a secondary cause of battery degradation compared to actual age and also the charging discharging strategy.
These have been and are known as the primary causes of battery degradation.
Talk about fake news this reeks.
Posted by: Arnold | 15 May 2017 at 03:16 PM
Sounds like a study that anti battery trolls would come up with - barely comprehensible.
Posted by: Arnold | 15 May 2017 at 03:18 PM
Who cares about the Study. Failure increases as you reduce the cycle times. Look at HVAC Compressors. Short-cycling compressors is a No-No if you want longevity of the compressor. Its no different for these batteries.
Posted by: Dr. Strange Love | 16 May 2017 at 07:10 AM
Not sure why people are hating on this study. Some responses to the critics here:
1. This is indeed a peer-reviewed study. The Journal of Power Sources (JPS) is one of the pre-eminent journals in the field of energy storage (among other, well, power sources).
2. Who says that the cells are "non-automotive grade"?
3. It's true that multiple studies will be required to draw overarching conclusions. This doesn't change the observation that the notion that V2G won't be detrimental to the lifetime of an ESS has at the very least been challenged. This speaks to the significance of this study, in that proponents of V2G have hypothesized that V2G would have a negligible effect on the ESS lifetime.
4. No way that this is "fake news" or a "barely comprehensible". Just because the results are different than previous studies (and it's important to note that the study may be different than the previous ones in methodology and electrochemistry, among other ways) doesn't make it invalid. This is how science progresses.
5. @Arnold: I would be interested to see your sources for the assertion that calendar life is more important than cycling when it comes to degradation (note that charging/discharging strategy is part of cycling).
6. @DRSL Failure does not increase as you reduce cycle times. You are conflating the mechanical meaning of "cycling" with the meaning as it relates to an ESS.
This study has some important implications for V2G usage, if the results are shown to stand up to scrutiny and are applicable across the Li-ion chemistries. May have to attempt a replication myself!
Posted by: mrrumbles | 30 June 2017 at 03:07 PM