Thomas More study: ‘EVs can lose 8% of battery energy per year’

Not all battery packs in EVs age at the same level. According to research by the Future Mobility department of Thomas More High School, the variation can mount up to 8% annually on specific packs. This is much worse than the rule of thumb prescribing 70% capacity after usage of eight years and could severely impact residual values.

For two years, researchers from Thomas More followed the so-called “state of health” (SoH) of their electric vehicle fleet. The ten-car fleet, specifically the state of health of the batteries, was systematically checked as the researchers wanted to answer questions about the longevity of the battery packs in these electric cars.

A loss of 64%

Though battery packs are often sourced from not a tremendously vast selection of cell manufacturers, the results showed irregular patterns. While some electric vehicles barely sacrificed their battery capacity, others performed much worse.

Some showed annual losses of up to eight percent in battery capacity and driving range. In those cases, the pack has lost 64% of its capacity after the guarantee period ends. The researchers didn’t correlate performance directly to cell manufacturers.

“The most expensive part of an electric car is the battery pack,” comments Luc Claessens, Future Mobility researcher and lecturer at Thomas More. “Replacing it usually costs more than the vehicle’s value, especially in used vehicles. So it’s better to be sure about an electric vehicle’s ‘state of health’ before buying.”

Tesla claims great retention

Battery packs in EVs must comply with a separate guarantee of eight years or 160 000 kilometres. There’s no binding rule for the remaining capacity, but the consensus among carmakers who benefit from strong residual values is that degradation shouldn’t exceed 30% after eight years.

That’s a loss rate of 3,7% per year. Challenging the competition, Tesla claims that its battery packs only lose 12% of their capacity after 320 000 kilometres driven – the figures are valid for the Panasonic cells in the Model S and X.

“That not every battery ages in the same way was already clear to us and is now confirmed,” adds Claessens. His team compared its own measurement method with those of the brand importers of the vehicle fleet. “And we also had four specialized commercial companies do a measurement.”

Europe’s Car Remarketing Association CARA, overseeing the used car market in the region, is also aware of the deviations in the battery state of health. It has set up a certification scheme, the first of its kind, to test and evaluate battery pack retention in second-hand cars.

The scheme, primarily based on the battery management systems from the automakers, must leverage transparency so that the residual capacity can be compared to the initial value at the moment of purchase.