Read the SoH or Measure It? What the Garage Figure Can and Cannot Tell You
“Just have the SoH read out.” The line turns up in every second forum thread about buying a used EV, and it sounds straightforward: plug in, read the number, done. Franchised garages now offer it as a document of its own, and and dongle apps show the same figure. What comes out is a useful figure. It is simply not a measured one.
That read-out value is the subject here. Some testing providers do something else: they pull the same signals through the diagnostic port but do not adopt the battery computer's SoH figure, recalculating it from those signals and a fleet database. That is a different procedure with its own sources of error, and not the subject of this article.
What happens during a read-out
Every EV has a battery management system, or BMS. It watches cell voltages, temperatures and currents, and from those it keeps a running estimate of how much capacity the pack still has. That estimate sits as a number in the control unit. The diagnostic tester in the garage asks for it and prints it on paper. Manufacturers have long had a dedicated test routine for this in their diagnostic systems, and its output is the SoH figure the garage certifies.
This is quick, cheap and genuinely well suited to one question: is the pack still above the warranty threshold? Most manufacturers warrant their batteries for eight years or 160,000 km with at least 70 per cent usable capacity left, and the garage document confirms, in essence, that the threshold has not been crossed.
Why the figure can still be off
The BMS does not measure capacity directly. It infers it from voltage curves, current integration and model assumptions, and that calculation drifts when a car has not been charged to full for a long time, spends its life in the middle of the state-of-charge range, or uses a chemistry with a flat voltage curve (LFP). The figure can then sit unchanged for months while the pack ages, or jump after a single full charge.
How large the effect is in practice was examined across 1,114 vehicles from five manufacturers over 375 days (preprint, 2026). Agreement between BMS SoH and actually measured capacity ranged from weak to moderate depending on the manufacturer; within identical models the real capacity spread by 12 to 25 per cent, and a third of the cars returned no SoH figure at all. The read-out value is a self-report with small print, as the article on the SoH value sets out.
The second route: counting capacity
The alternative to a self-report is to count the energy that actually goes into the pack and comes back out, and to relate it to the state of charge. If a car charges from 20 to 80 per cent and takes in 45 kWh, the pack holds roughly 75 kWh of usable capacity. Across many charges that becomes a trend with a scatter band, and the trend does not depend on whether the BMS happens to be well calibrated right now.
Notably, manufacturers take this route themselves, though only for business customers. At least one large group sells fleets, dealers and insurers a state-of-health figure per vehicle, calculated in the background from charging cycles, temperature, usage patterns and battery age. It requires AC charging with a swing of at least 40 percentage points. The stated accuracy is up to plus/minus 10 percentage points away from actual remaining capacity. Private owners cannot order it.
That number matters because it sets a scale. A band from 78 to 98 per cent answers the question “warranty, yes or no”, but not the question of whether the car is worth €1,500 more or less. Anyone negotiating on battery condition needs a narrower band, and needs to see it on the document.
The three routes side by side
| Read-out (garage, dongle) | Manufacturer calculation for business customers | Akkubrief | |
|---|---|---|---|
| What is behind it | the BMS's self-report | a calculation from charging behaviour | energy counted from the charging history |
| Result | a percentage, essentially “above 70%” | a percentage per vehicle | capacity in kWh, SoH with a scatter band |
| Stated accuracy | none given | up to ±10 percentage points | the band is printed on the certificate |
| Who can get it | anyone, with a garage appointment | fleets, dealers, insurers | private owners, online |
| Effort | an appointment, a few minutes | runs in the background | connect the car; the trend builds as you drive and charge |
| Verifiable by the buyer | paper from the garage | a figure inside the customer's system | signed, verifiable in the browser |
A battery history built up over time helps put individual day-to-day deviations in context and additionally shows how the measured condition has developed. Whether it is more accurate than a read-out in any single case can only be answered against an independent reference measurement. Akkubrief is having this tested in a pre-registered study and prints the band on every certificate rather than promising a fixed number.
What this means for sellers
Certified listings are becoming the norm: the share of documented EV offers rose from just under 8 to 19 per cent within a year, according to the Initiative Sicherer Autokauf, a German road-safety and used-car initiative. Before long a buyer will not ask whether a certificate exists, but which one. Three things are worth doing then:
- Take the garage document if you have an appointment anyway. It costs little, carries the brand logo and answers the warranty question.
- Add a counted figure for the negotiation. Capacity in kWh with a band is the measure that appraisers and valuation houses work with.
- Insist on verifiability. A document the buyer can check for authenticity ends the argument about whether the PDF is genuine.
Which kinds of battery certificate exist, what they cost and how to recognise a good one is covered in the overview “Battery certificate for an EV”.