Battery health is the new mileage
For most of the automobile's history, one number did the heavy lifting. Mileage told you, at a glance, roughly how much life a car had left in it. It set the asking price, framed the negotiation, and let a buyer who knew nothing about engines feel confident enough to hand over the money. The odometer was a shared language, and the whole used-car market was built on top of it.
Electric vehicles quietly broke that language. The motor barely wears. There is no timing belt to snap, no oil to neglect, no clutch to burn out. The single most valuable and most fragile part of the car is the battery, and the odometer tells you almost nothing about its condition. Two identical models with identical mileage can be worth thousands of euros apart, and the number everyone has trusted for a hundred years can't see the difference.
Why one number was enough
Mileage worked because it was a decent proxy for wear on a combustion car, and because it was universal. Every car had it, every listing showed it, every buyer understood it, and it was hard (though not impossible) to fake. That combination is rare. A used market needs a number that is cheap to read, comparable across brands, and trusted enough that strangers will transact on it. Mileage cleared that bar for decades.
Crucially, mileage compressed a complicated reality into something simple. A car's true condition depends on how it was driven, serviced, stored and repaired. Nobody could inspect all of that, so the market agreed to lean on one observable figure and price the uncertainty around it. It was imperfect, but it was shared, and a shared imperfect number beats a perfect number that only one side of the deal can see.
The number that stopped working
On an EV, the engine is no longer the thing that ages. The battery is. And battery wear does not track mileage in any reliable way. A car that has covered modest distance but spent its life fast-charging in a hot climate and sitting at 100% can be in worse health than a higher-mileage car that was charged gently and kept in a garage. Same odometer, very different asset.
This is the heart of the problem. The number the market still reads, mileage, has come loose from the thing the market actually cares about, which is how much usable energy the battery can still hold and how long it will keep holding it. The shorthand survived into a world where it no longer means what everyone assumes it means.
State of Health is to an electric car what mileage is to a combustion car, except it's invisible until someone measures it.
What State of Health actually measures
State of Health, usually shortened to SoH, is the honest replacement. At its simplest, it expresses the battery's present usable capacity as a percentage of what it had when new. A pack reading 91% can store roughly 91% of its original energy, which translates fairly directly into range and resale value. It is the closest thing the EV market has to a universal condition score.
SoH is not a marketing badge or a gut feeling from a quick test drive. It is a measurement, derived from data the car itself records: cell and module voltages, temperatures, charge and discharge behaviour, and the number of cycles the pack has been through. Read correctly and normalized properly, it produces a figure that means the same thing on a Tesla, a Hyundai and a Renault, which is exactly the property mileage used to have.
It is worth being precise about what SoH is not. It is not range on the dashboard, which fluctuates with temperature and driving style. It is not the age of the car, since two cars of the same age can differ widely. And it is not the manufacturer's original spec sheet. It is a present-tense reading of a specific pack, on a specific day, by a specific method.
What makes a battery age
Batteries degrade through two broad mechanisms, and understanding them explains why mileage is such a poor guide. The first is cycle ageing, the gradual loss of capacity each time the pack is charged and discharged. The second is calendar ageing, the slow decline that happens simply with the passage of time, even in a car that barely moves.
On top of those, a handful of stress factors accelerate the decline. Heat is the big one: a battery that spends its life in high temperatures ages faster than one kept cool. So does habitually charging to 100% and leaving it there, draining to near zero, and relying heavily on high-power DC fast charging. None of these stress factors leave a mark on the odometer, which is precisely why a single distance number can't capture the state of the pack.
The good news is that all of this leaves a measurable signature inside the battery management system. A pack that has been treated harshly shows it in its cell balance, its internal resistance and its capacity readings. The history is written into the hardware. The only question is whether anyone reads it before money changes hands.
The trust gap, and what it costs
Today, in most transactions, nobody reads it. The seller says the battery is fine. The buyer has no independent way to check. So the buyer does what any rational person does in the face of hidden risk: they assume the worst, or they discount the price, or they walk away. Economists have a name for this. When the seller knows more than the buyer and the buyer can't verify the claim, good products get dragged down to the price of bad ones, and confident buyers stay home.
For a young secondary market, that friction is expensive. It slows sales, compresses prices, and makes financing and leasing harder to underwrite because nobody can put a defensible number on the asset. Every party in the chain pays for the missing measurement: the seller in a lower price, the buyer in anxiety, the financier in risk premium. The gap doesn't make the uncertainty go away. It just makes everyone pay for it.
How a battery is actually read
The reassuring part is that closing the gap does not require new hardware in the car. The data already exists. Every modern EV runs a battery management system that continuously monitors the pack down to the module level. Reading it is a matter of connecting to the vehicle, pulling those values, and interpreting them correctly.
In practice the flow is short. A standard diagnostic connection to the car exposes the battery management system. Software reads the per-module voltages, the temperatures and the cycle history, then runs them through a normalization layer that maps each manufacturer's raw readings onto a common scale. That last step matters: different makers report capacity and degradation differently, so without normalization a 90% on one brand isn't comparable to a 90% on another. With it, the output is a single, comparable State of Health figure and a letter grade derived from it.
Done properly, the whole reading takes minutes rather than hours, and it produces something far more useful than a verbal assurance: a number with a method behind it, the same method applied to every car, every time.
From private reading to public proof
A measurement only closes the trust gap if the other side can rely on it, and that requires two more things beyond the reading itself: independence and verifiability. A score that the seller can edit is worth no more than the seller's word. A score with no way to confirm it is just a screenshot.
That is why the measurement has to become a certificate, not just a number. The reading is taken by an independent method, the score is locked so it can't be quietly adjusted afterwards, and the result is published to a page anyone can check, typically reached by scanning a code on the document. The buyer no longer has to trust the seller. They trust the method and verify the proof. The certificate is time-bound on purpose, because a battery's health changes, so an honest record reflects a recent reading rather than an ancient one.
Regulation is about to make this non-negotiable
This shift is not only a market convenience. It is becoming a regulatory expectation. The EU Battery Regulation introduces a digital battery passport: a record of a battery's identity, provenance and health that travels with the vehicle through its life. For the used market, the health portion is the part that bites at resale, because it turns a nice-to-have into something buyers, regulators and recyclers will increasingly expect to see.
The practical implication for anyone in the trade is simple. Ad-hoc claims about battery condition will not satisfy a regime built around verifiable, standardized records. An independent, signed, versioned certificate will. Building that habit now, before the deadline forces it, means being compliant early and capturing the sales benefits in the meantime rather than scrambling later.
What this means if you buy, sell or inspect EVs
If you sell used EVs, a State of Health certificate is the fastest way to stop leaving money on the table. A verified score lets a strong battery command the price it deserves instead of being discounted to the level of an unknown one. It shortens negotiations because the central question, what condition is the battery in, already has a credible answer attached to the listing.
If you inspect vehicles, battery certification is a natural extension of the trust you already provide, and an EV-ready service line as your market shifts away from combustion. If you run a marketplace or a leasing book, a standardized health figure is the missing input that makes verified stock searchable, residual values defensible and risk easier to price. And if you are buying, the lesson is blunt: stop reading the odometer as if it still tells the whole story. Ask for the State of Health, and ask whether it can be independently verified.
The bottom line
Markets reorganize themselves around whatever number they can trust. For a century that number was mileage. For electric vehicles it is State of Health, and the only thing holding the market back is that, for now, most participants still can't read it. The data is already inside every car. The methods to read and normalize it exist. The regulation is arriving to make it standard.
The transition from mileage to battery health is not a question of if, but of who gets there first. The sellers, inspectors and platforms that start certifying now will be the ones setting fair prices, closing deals faster and earning the trust of a market that is quietly rewriting the rules. That is the gap BattGrade was built to close.