Fast Charging Vs. Slow Charging: Study Reveals (little) Difference In Range Degradation

[Mart]

Just read this article and was quite surprised:

Fast Charging Vs. Slow Charging: Study Reveals Difference In Range Degradation

One of the thoughts that go through the mind of an EV owner has to do with the impact that DC fast charging will have on the vehicle’s high-voltage battery in the long run. In other words, how much will the battery degrade over time if fast charging is used predominantly?

Well, according to a recent study from Recurring Auto that analyzed charging data from over 12,500 Tesla vehicles in the United States, the rate of range degradation is statistically similar when compared to AC charging.

The company, which provides vehicle and battery analysis reports for EVs, compared cars that fast charge at least 90 percent of the time to cars that fast charge less than 10 percent of the time, and the results show little to no difference between the two charging methods.

Not sure if this applies to all makes and models of BEV's, as the article stresses the benefits of the pre-conditioning of batteries that Tesla's do, as you head to a spercharger location. Also interesting to see how the degradation seems to level off, quite a bit, around the 90% SOH point.

But ...... anyone know the relevance of the x-axis label 'battery days'? Rather than just age in days.

I did look to the report but can't spot an explanation.

[openspaceman]

I would like to know the answer to Mart's question too.

Also can anyone explain if the charging efficiency drops as the battery loses capacity?

[Ken]

I am thinking that battery days is the No. of individual days the battery was charged because these are actual customer stats. In a lab they would force the process along time wise to the point of charging say twice per day and refer to charging cycles. In the field somebody might only charge twice per week and therefore incur 2 "battery days" so therefore 1000battery days might equate to nearly 10yrs for some people.

However i am not so sure that this data is meaningful because it does not distinguish the state of charge or for how long the car is kept fully charged.
With rapid DC charging the car may be charged to a high charge but is immediately driven away wheras a AC charge could be sitting around at 100% by many.

[Mart]

Hi Ken, good point about the SOC. I'm only speculating (guessing) here, but I'd assume that most supercharging is only to 80%(ish), since charge speeds will be falling by then, and with the extensive Tesla SC network, you could pretty much bet, another SC location will be available further down the road.

Of course, folk may be charging to 90%+ at the SC's, but on average, I'd be willing to bet the vast majority will be less than 90%.

[Swwils]

Over time as batteries cycle they tend to form a solid-electrolyte interface (SEI) on the anode. This SEI layer acts as an impedance to the flow of ions between the electrode and the electrolyte, making it less efficient for the battery to charge (i.e., more energy is wasted as heat). As the impedance of the battery increases, the charging efficiency decreases. The other way to mess up your charging is with imbalanced cells as the BMS will need to compensate.

Teslas, come with a really advanced thermal management system that keeps the battery temperature within an optimal range during fast charging. This prevents the battery from overheating, which is one of the main culprits of accelerated battery degradation along with full depletion of cells.

Also, there may be a fundamental flaw in the study, I would say you couldn't "frequently" fast charge even if you wanted... e.g. more than once a day.

[dan_b]

Over on YouTube TeslaBjorn recently did a video looking at the battery health of a Tesla Model 3 taxi that had been charged exclusively on Superchargers multiple times a day as a workhorse and the battery was not in a good way.

Of course just an n=1 case, and an extreme case too.