SOH is the new mileage: why the odometer no longer tells you what an EV is worth

SOH is the new mileage

For more than a hundred years, buying a used car started with a ritual: walk up to the dashboard and look at the odometer. Those five or six digits were the summary of a whole mechanical life. Low miles, healthy engine. High miles, time to haggle. It was the first thing a buyer checked.

With an electric car that ritual breaks. Two identical EVs, same year, the same 30,000 km on the dash, can be worth thousands of dollars apart. The reason? The one thing the odometer can't see: the real state of the battery. And that's measured by an acronym you're going to hear more and more: SOH, for State of Health.

In 30 seconds

• The odometer measures distance. In an electric car, what defines the value is the health of the battery (SOH), not the miles.
• SOH wears down mostly from time, heat, and how you charged — not from the distance you drove.
• A healthy battery is sellable only if you can prove it. SOHpro turns that hidden data into a verifiable score that travels with the VIN.

What SOH is

In its most common definition, SOH is the percentage of capacity a battery has left compared with what it had from the factory. A battery at 100% is brand new. At 90%, it has lost a tenth of its usable energy. At 70%, it has crossed the threshold where most manufacturers consider the warranty exhausted and where range starts to bite in everyday driving.

The important thing is that this percentage does not drop in a straight line. A battery loses a little faster at first and then settles into a long plateau where wear is slow and predictable — so slow that most electric cars spend more than a decade above 70%. What really matters isn't today's number, but the slope: whether your battery is cruising along that calm plateau or fading faster than it should. (We dig into this in How your EV's battery really ages.)

For the curious — the chemistry behind the number Why does a battery lose capacity even when it isn't used? Because of the growth of a microscopic layer on the negative electrode — the solid electrolyte interphase, or SEI — that keeps trapping lithium that no longer circulates. Electrochemists call it "loss of lithium inventory," and it's the mechanism the literature identifies as the main driver of calendar aging (Keil et al., J. Electrochem. Soc., 2016). Every ion locked away in that layer is a little bit of range that doesn't come back.

The key: SOH doesn't wear out with the miles

Here's the counterintuitive part. SOH wears down mostly from time, heat, and the way you charged — not from distance.

The largest fleet study available — Geotab, covering 22,700 vehicles — proves it with real data. An average electric car loses around 2.3% of capacity per year, leaving a typical battery near 81.6% after eight years. But the average hides the most interesting part: how you charge changes the result entirely. Cars that use fast charging sparingly hold near 88% at eight years; those that lean on high-power fast charging drop to 76%. Same amount of time, same brands — different fates.

That's why an EV with 80,000 km charged patiently in a cool garage can be healthier than one with 25,000 km that lived under the tropical sun plugged into fast chargers. The odometer knows nothing about that. SOH does.

"An EV's mileage is like the age a person looks: it gives you a hint, but it doesn't tell you how their heart is doing. SOH is the heart exam." — Alonso Aguilar, Founder & CEO

Why everyone's suddenly talking about this

It's no accident. Electric vehicle sales in Latin America grew by about 75% in 2025, according to the International Energy Agency's Global EV Outlook, driven by Chinese brands like BYD, MG, and GWM. That first wave of cars — the ones sold between 2021 and 2023 — is already reaching the used market. And with it came the question no one knew how to answer: how do I know this battery is still good?

In Brazil, the press already says it without hedging: battery health has become "the new mileage" of the used electric car. And the economic weight is enormous:

The battery accounts for between 20% and 50% of the car's total value. — Swiss Re, in its electric-mobility analyses (cited by the regional financial press)

Guessing at the state of the battery means betting half the price blind. In Mexico, reports describe a distrust that is sinking resale value. Uncertainty, not degradation, is the real enemy: a battery at 85% is perfectly sellable if you can prove it. The problem is that, until recently, no one could.

The odometer can be read; SOH had to be guessed

Here's the trap. The mileage is right there, in plain sight, on the dash. SOH lives hidden inside the battery's brain — the BMS, or Battery Management System — and only shows up when someone plugs in a diagnostic tool and knows how to read what comes out.

That left buyers and sellers at a disadvantage. The honest seller had no way to prove the battery was healthy. The wary buyer had no way to verify it. And the result was the same as always: the price sank on doubt, not on facts.

SOHpro turns that hidden data into something anyone can read: a SOHpro Score from 0 to 100, calculated from the battery's real diagnostic. And, crucially, it doesn't just take the raw figure from the BMS and parrot it back. That number is the patient's own opinion — sometimes exact, sometimes optimistic — so SOHpro cross-checks it against other signals: the voltage spread between cells, the temperature, the charging efficiency, and, when there's history, the real rate at which that battery is degrading.

From a number to a story

But a single day's SOH is a snapshot. What really builds confidence is the film. That's why each scan in SOHpro doesn't stand alone: it's saved in your electric's passport, a record tied to the car's VIN that accumulates every visit and travels with the car when it's sold. The next owner doesn't inherit a promise; they inherit the full story of how that battery aged — including the most valuable data point of all: whether the curve is still on its healthy plateau or degrading faster than it should.

"Anyone can hand you a single score. The hard part, and the valuable part, is the timeline: seeing how that battery behaved visit after visit. That's what turns a diagnostic into a passport." — Alonso Aguilar

What a real health check looks at (and what you should ask for)

A good report isn't a single number: it's a handful of signals that cross-check each other. The ones below are universal — any serious diagnostic can show them — and they're the ones that matter most when buying. Ask to be shown them:

• Capacity and its trend (SOH). Today's state of health and, above all, more than one scan to see where the curve is heading. Below ~70% you enter the attention zone — it's the floor for most warranties. Cross-check it with the warranty status.
• Cell balance (ΔV). That no cell is falling behind the rest — the fingerprint of the "weak link." Healthy: below ~10 mV between the highest and lowest cell.
• Temperature and thermal management (ΔT). That the pack operates in a safe range (0–45 °C) and that there's no hot spot (difference between sensors below ~5 °C).
• Fault codes (DTCs). Zero stored faults is what you want to see.

One detail that separates an honest check from an inflated number: it also tells you what it couldn't measure. If the shop's scanner doesn't expose a reading, a good report flags it as "not available" instead of making it up.

All those signals are universal. What SOHpro does is gather them into a single easy-to-read number — the SOHpro Score, from 0 to 100 — and save them in your electric's passport, so you don't depend on interpreting voltages by hand and so the story travels with the car. (How that score is built, in Anatomy of a score.)

And if you're going to sell, flip it around: a battery health certificate stops being a luxury and becomes the proof that defends your price. And it's not just intuition. In markets where battery certification is already common, the data confirms it: in the UK, the manufacturers' association SMMT reported — using data from the firm Generational — that used cars with a battery health certificate sell about a week faster than those without one.

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Want to see what an electric car's state of health looks like? Try the SOHpro simulator or ask your trusted shop for a scan to start your electric's passport.