Can a vehicle-to-grid extend battery-life of electric cars by ten percent?

In the previous post, I wrote about a report calculating the expected electricity price in a vehicle-to-grid system and the assumptions that went into it. One of the difficulties that was detailed in the report was the aging of the battery used in a vehicle-to-grid system. In the meanwhile, I read this 2017 article from the Dutch sustainability website wattisduurzaam.nl contradicting this. The author of the article writes that it is contra-intuitive, but that research from the University of Warwick revealed that a vehicle-to-grid system can even extend the lifetime of lithium-ion batteries…

I could somehow understand “minimize”, but a vehicle-to-grid system that extends battery life is a very strong claim.

Although the article was written in a cheering mode, it also acknowledges that battery degradation is a problem in current vehicle-to-grid systems, but that this research achieved an extended battery life. Not just a tiny extension, a whopping 10 percent extension of battery life by operating in the vehicle-to-grid system.

Let me first say that I could somehow believe that in specific circumstances it should be possible that hooking a car battery to a (smart) grid could minimize or maybe even extend battery life. In a situation where car drivers abuse their battery, it could be a good idea to hook it up to a smart management system that can make the charging/discharging decisions for them. For example, if a battery is not used for an extended period of time and/or often allowing deep discharge, then such a smart system might prevent faster than average degradation. It then depends what the researchers actually compared.

The article didn’t give much information how the research was done or what the researchers compared exactly. It however did reveal that the number of cycles a battery goes through is not the most important factor that causes battery degradation, but also temperature of the battery, current rate and level of discharge are important. I could certainly agree with that. Were these the factors that the researchers played with?

There is a video about a vehicle-to-grid initiative in Utrecht (the Netherlands) embedded in the article. One of their goals of that project is to test how to charge the electric cars and use it in such a way that the efficiency of solar energy increases. Not sure why the author use that video as an example, it doesn’t mention the 10% better battery life nor the Warwick study. Maybe that system from Utrecht wants to use the results of that research from the University of Warwick to improve battery life of the electric cars that operate in their vehicle-to-grid system? That is entirely possible. The author wrote that this findings would make the vehicle-to-grid even more attractive, so it would make sense to have an algorithm in place that would improve battery life of the cars connected to that vehicle-to-grid system.

There is some backpedaling in the last paragraph. The author admitted that this 10% gain was only achieved in lab conditions and that implementation in the real world could turn out challenging, but then reassuring that even if only a part of this 10% would be achieved in practice, the research still would have quite an impact.

These weasel words shows me that there is more to it than meets the eye. Now I definitely wanted to have more information about that research! I had to read that paper to find out what the researchers actually did and what their actual conclusion were.

Unsurprisingly, the paper was much more nuanced than the article on wattisduurzaam.nl. The authors of the paper wrote that this 10% gain could only be reached under specific circumstances. Okay, that is interesting. Which circumstances exactly lead to this result?

The researchers developed a battery degradation model and validated it using 63 battery cells under lab conditions. They determined their initial state, then they put them in controlled lab conditions and performed various usage scenarios on them (one of those scenarios is not using the batteries for an extended period of time, which doesn’t come as a surprise). Every 8 weeks the researchers then characterized the battery cells for degradation. The total time of the experiment was somewhat more than one year. Finally, the model was integrated in a smart grid algorithm and fed with real-world data, simulating load balancing of a building. The researchers concluded that the algorithm reduced the battery capacity fade of the electric vehicles in the simulation by up to 9.1% and power fade by up to 12.1%.

Before I had read the paper, I assumed that the authors might have compared an abused battery with a perfectly conditioned battery. This is not the case. They compare the batteries conditioned by their model with those that were not, so basically those batteries that were in the vehicle-to-grid system and those that were not.

But then you would ask: how did they managed to extend battery life by 10%? That is where it gets interesting. When I read the assumptions that the researchers used, it became perfectly clear how they got to their 10% extension. This is their main assumption (my emphasis):

… an aggregative framework where the principal functionality of vehicle-to-grid energy transfer is to minimize battery degradation

That is interesting. This means that the decision whether to charge or to discharge will depend on the state of that individual battery. The algorithm calculates the expected cost of the lifetime caused by discharging against the expected cost of staying in the same state. When the expected cost of discharging is higher than the expected cost of staying in the current state, then no transfer is done. Otherwise the battery will discharge to the grid.

In other words, their main assumption is that the electric cars only supply the grid with energy on the condition that doing so does not further degrade the battery. The best result was in the scenarios where the batteries were charged just in time to be ready for driving.

This allows them to increase the lifetime of the battery, but that is as far from a real-life vehicle-to-grid system as you can get!

That a vehicle-to-grid can extend battery life by 10% is therefor not “contra-intuitive” as the author of the article claims it to be, the research is just based on a different approach. The starting point of the University of Warwick is that the battery management system only allows transfers that don’t cause more battery degradation. When comparing this with the same situation where those transfers are allowed, then gaining battery life is entirely possible. That is however different from real vehicle-to-grid systems having the goal to balance grid load, gain financially from energy arbitrage or consume electricity from intermittent sources in an efficient way.

For example, the principal functionality of the energy transfers of the vehicle-to-grid system that was used as an example in the article is to use the electricity generated by solar panels as efficient as possible. To be able to do that, a charging station was developed that can as well charge (from solar panel production during the day) as discharge (provide this energy to a building during the night). Implementing the techniques tested in the paper will have a negative impact on that efficiency. Electric cars that autonomously decide whether they want to charge or discharge will lower this efficiency, so I really doubt that the managers of that system will be keen on implementing the fruits of the research from the University of Warwick.

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