What if your electric battery never used its full potential, even when new? This is the question that haunted two research teams, one in China and the other in Sweden, after sifting through three years of real-world data from 133 electric vehicles.
- A single cell aging faster than the others is enough to condemn the entire pack, even if the rest works perfectly.
- Up to 170,000 km the gaps between cells stay small, but beyond this threshold some cells degrade much faster than their neighbors.
- Buses (LFP batteries) lose more usable energy than cars (NMC), with 72.9% and 80.7% of the energy potential actually used, respectively.
- The weak link dragging down the entire pack
- The 170,000-kilometer turning point
- Buses vs cars: why fleets take the hit
- What these figures mean for the future of batteries
A single worn cell is enough to condemn an entire battery. This stark finding comes from two laboratories, in China and Sweden, after sifting through three years of real-world data from 133 electric vehicles. Cars and buses combined, up to 27 % of stored energy will never be used. The culprit: an invisible weak link.
The weak link dragging down the entire pack
An electric vehicle battery is not a monolithic block. It is made up of many cells connected in series, a bit like train cars. Except in this train, if a single car slows, the whole convoy must brake. This is precisely what the work of Professor Chen Zhongwei, from the Dalian Institute of Chemical Physics in China, and Professor Zou Changfu, from Chalmers University in Sweden, reveals. Their research highlights what they call the weakest-cell effect: as soon as a cell ages faster than the others, it ends up pulling the entire pack out of service, even if most of the remaining cells still function perfectly.
In concrete terms, these aging gaps result from several cumulative factors: the quality of the materials used, manufacturing imperfections, poor grouping of cells during assembly, or temperature differences related to each cell’s position within the pack and the design of the cooling system. Add to that daily mechanical stress and vibrations, and you get a cocktail of imbalances that, cell after cell, creates a genuine energy bottleneck.
The 170,000-kilometer turning point
The most interesting aspect of this study is precisely that it observed this phenomenon in real-life use over time, not just in the lab. Researchers identified a genuine tipping point: up to about 170,000 kilometers, health gaps between cells within a single pack of a passenger car remain generally small and not too worrying. But beyond this threshold, the situation changes dramatically. Some cells begin to age much faster than their neighbors, and the differences become noticeably more pronounced, with the severity varying significantly from vehicle to vehicle.
This finding also resonates with observations from the Austrian battery-diagnostics company Aviloo, which had already noted that electric vehicles generally retain a sizable portion of their initial capacity even after 150,000 kilometers. In other words, it isn’t the entire battery that collapses abruptly, but rather a few isolated cells that, aging badly, drag the rest down with them.
Buses vs cars: why fleets take the hit
Not all batteries age the same way. When comparing data from passenger cars equipped with nickel-manganese-cobalt (NMC) batteries to buses using lithium-iron-phosphate (LFP) batteries, the study reveals notable gaps. In cars, cell-to-cell inconsistencies reduce the usable health of the pack by 6.2%, versus 7.5% for buses. The overall pack lifespan declines by 17.7% for cars and climbs to 22.8% for buses, relative to the theoretical average lifetime of the individual cells.
Power-capacity also follows the same troubling trend: down 12.9% for cars and down 15.1% for buses, due to differences in internal resistance between cells. The final result, over the full operational life of the vehicle: car packs only utilize 80.7% of their potential energy resources, while buses cap at just 72.9%. In other words, up to 27.1% of the energy stored in buses will never be used.
What these figures mean for the future of batteries
What this research highlights is the need to rethink the control of cell coherence from manufacturing onward through the vehicle’s life. It is no longer enough to ensure that each cell works individually: we must also guarantee that they all age at roughly the same rate, or the entire pack will pay for the faults of a few failing elements. On the bright side, the imbalance tied to state of charge remains limited to less than 2%, thanks to balancing strategies already integrated into current battery-management systems.
For the automotive industry and for fleet operators, these results open a concrete path: better latent defect detection before assembly, improving the grouping of cells, and refining cooling systems could recover a significant portion of energy that is currently lost. This is far from a trivial issue, at a time when every kilowatt-hour counts—for both vehicle range and the resources required to manufacture them.
At heart, the story of the weakest cell recalls an old mechanical lesson: the strength of a system rarely comes from its average, but from its weakest link. It remains to be seen whether manufacturers will draw the lessons from these findings to design batteries that are more homogeneous and capable of finally exploiting their full energy potential.
