Recent vehicle-to-grid (V2G) studies have found that regular bi-directional cycling to the grid hurts and helps EV batteries
One of the most common and persuasive arguments for greater EV adoption is the flexibility large amounts of stored energy could offer the grid. With GWh of energy stored in vehicles across the grid, storing and stabilising power flows could be made much more effective. However, the effects of bi directional charging as part of so-called vehicle to grid (V2G) process on EVs themselves has been debated.
Many contend that using li-ion batteries in this way will severely impair performance and shorten their lifespan, although recent studies have found mixed results.
In May, a University of Hawaii at Manoa study concurred with the above. In a paper the authors, led by Matthieu Dubarry, noted that: “Additional cycling to discharge vehicle batteries to the power grid, even at constant power, is detrimental to cell performance. This additional use of the battery packs could shorten the lifetime for vehicle use to less than five years.”
Dubarry’s contention was that most V2G studies so far had relied heavily on modelling. Instead, his team used Panasonic 18650 NCA cells, combined with different charging schedules of either 1-2 charges a day with immediate or delayed charging, and various charging currents from level 2 to fast chargers.
They concluded that a V2G step twice a day increased the expected battery capacity loss by 75%, and increased resistance by 10%. Once per day resulted in a 33% increase in capacity loss and a 5% increase in resistance.
However, they also found that calendar ageing could be somewhat mitigated by charging the batteries twice per day, resulting in a 5% reduction in capacity loss.
Conversely, another recent study from the University of Warwick found markedly different results. The research published by Dr Kotub Uddin, together with staff from the University’s Energy and Electrical Systems group and Jaguar Land Rover, suggests that V2G could in fact improve vehicle battery life by around 10% over a year.
A two-year analysis of a number of commercial EV battery packs has produced what the group says is one of the most accurate battery degradation models in the public domain. Using this model, Uddin developed a ‘smart grid’ algorithm which calculates how much energy a vehicle requires to carry out daily journeys, and how much energy can be taken from its battery without negatively affecting it. They concluded that “battery degradation is more complex – and this complexity, in operation, can be exploited to improve a battery’s lifetime.”
Uddin notes in his abstract that: “If a daily drive cycle consumes between 21% and 38% state of charge, then discharging 40%–8% of the batteries state of charge to the grid can reduce capacity fade by approximately 6% and power fade by 3% over a three month period.”
Dubarry’s caution should be noted here given that much of these results are based on simulations. However both studies do indicate that careful cycling can be beneficial – exactly how beneficial may be more clear with further V2G pilots.