Among the cars we test on Car-Shooters, a growing number — the Tesla Model S, the Polestar, the Hyundai IONIQ, the Volkswagen ID. Buzz, the Volvo XC40 P8 — share the same organ under the floor: a battery pack that is, by cost and by complexity, the single most important component of an electric car. Yet for most owners it remains a black box, understood only through two numbers on a screen: range, and an increasingly anxious question — how much has it aged?
From cell to pack
An EV battery is built in three layers. The base unit is the cell, a sealed pouch, prismatic can or cylindrical can containing two electrodes separated by a porous membrane soaked in liquid electrolyte. Most cells on the road today use one of two lithium-ion chemistries: NMC (nickel-manganese-cobalt), which favours energy density and is common on premium and long-range models, and LFP (lithium-iron-phosphate), which trades some density for lower cost, better thermal stability and longer cycle life — the reason it now equips a growing share of standard-range EVs.
Cells are grouped into modules, wired in series and parallel to reach the working voltage (typically 350-800V), and modules are assembled into the pack, a sealed structural unit that today often doubles as part of the car’s floor and chassis stiffness. Around the cells sits the part that actually decides how the battery behaves and ages: the Battery Management System (BMS), which measures voltage and temperature of every module, balances charge between cells, and enforces the safety limits the chemistry can’t enforce on its own.
How a battery actually ages
Unlike a mechanical part that wears out through friction, a lithium-ion cell ages through chemistry, along two parallel paths. Calendar ageing happens simply with the passing of time, driven mainly by temperature and by the state of charge at which the battery sits idle: a passivation layer called the SEI (solid electrolyte interphase) slowly and irreversibly thickens on the negative electrode, consuming usable lithium. Cycle ageing instead depends on use: how deep the charge/discharge swings are, how fast the current is (notably during DC fast charging), and how hot the cells get while working. Push any of these too far — charging at very high power in already-hot conditions, for instance — and a faster, more damaging mechanism called lithium plating can appear, depositing metallic lithium instead of intercalating it, which both eats capacity and can create safety risks.
The result is capacity fade (a fully charged pack simply holds less energy than when new) and power fade (internal resistance rises, so the same current produces a bigger voltage drop, which shows up as reduced fast-charging speed and acceleration). Most manufacturers warranty the pack down to 70-80% of original capacity over 8 years or ~160,000 km — a threshold, not a cliff: a battery below it still works, just with reduced range.
State of Health: a number harder to pin down than it looks
The industry calls the ageing indicator State of Health (SoH), usually expressed as the percentage of original capacity still available. The problem is that SoH cannot be read directly the way a fuel gauge reads a tank: it has to be inferred, and every carmaker’s BMS infers it with its own proprietary algorithm, from voltage curves, coulomb counting and internal resistance estimates gathered during ordinary driving. A 2025 review published in npj Clean Energy made the point explicit, calling for a standardised SoH measurement procedure for EV battery packs and proposing energy- and capacity-based metrics precisely because today no such standard exists at the vehicle level — which is also why the SoH percentage shown by two different EVs, or even by two apps on the same car, can disagree.
The metrology angle: measuring ageing is a research problem, not just an engineering one
Getting a trustworthy, comparable SoH number is, at its root, a metrology challenge — the science of measurement itself — and it is exactly what a cluster of EU-funded projects under the European Partnership on Metrology (coordinated by EURAMET, the network of Europe’s national metrology institutes, together with the EU’s Horizon Europe programme) has been working on:
- LiBforSecUse (2018-2022, coordinated by Germany’s PTB institute) tackled the fact that today’s residual-capacity tests are “too slow or too imprecise to be economically viable”: it developed a fast, impedance-based method to estimate how much capacity an ex-EV pack has left — the same measurement problem that decides whether a used battery gets a second life in home or grid storage instead of the scrapyard.
- OpMetBat built a metrological framework for “operando” analysis — measuring a cell’s chemistry and structure while it is actually charging and discharging, rather than pulling it apart afterwards — specifically to trace how and why degradation mechanisms like the ones above actually unfold inside a working cell.
- HyMetBat (2025-2028, €3.49M, also PTB-coordinated) extends this to the sustainability side of the same question: traceable methods to quantify contaminants and purity in recycled battery materials, needed to know whether a second-life cell is really as good as it claims to be.
On the industrial side, BATT4EU — the EU public-private partnership (driven by BEPA, the Batteries European Partnership Association) that channels up to €925M of Horizon Europe funding into Europe’s battery value chain — is backing 2026 Horizon Europe calls for proposals asking explicitly for “accelerated multi-physical and virtual testing for battery aging, reliability and safety evaluation” and for harmonised measurement and diagnostic methods that go beyond what any single carmaker’s BMS can offer today. Put together, this research points at the same destination: a battery’s health should eventually be as trustworthy and comparable a number as a car’s odometer — it just isn’t there yet.
What this means for an owner, today
Until that standard exists, the fastest lever an owner has is behaviour, since calendar and cycle ageing both respond directly to it:
- Keep daily charging around 20-80% state of charge, reserving 100% only for the trip that actually needs it — both ends of the range stress the chemistry more than the middle.
- Use DC fast charging when convenient, not as a daily habit — the high current is precisely what the research above links to accelerated ageing and, in the worst case, lithium plating.
- Avoid leaving the car parked at a high state of charge in the heat for long periods — calendar ageing is driven by exactly that combination.
None of this changes the bigger picture: a modern EV battery, used reasonably, is built to outlast the car around it. But knowing what is actually happening inside the pack — and why measuring it precisely is still an open scientific question in Europe’s own metrology labs — is a better way to read that ageing percentage than treating it as a mystery.
Photo: © Car-Shooters