Akkubrief

Methodology version: v0.1

How an Akkubrief is made — our methodology

Updated: 19.07.2026

No provider in this market discloses how its battery score is calculated. We do — not as a marketing gesture, but because a certificate is only worth as much as the traceability behind it. This page is versioned; whatever changes is recorded in the changelog below. Critical scrutiny is explicitly welcome.

1. How the data reaches us

You connect your Tesla account through the official Fleet API — with read-only permissions. We cannot control, open or change anything. And one rule is hard-coded on our side: we never wake your car. No sneaky wake-ups for readings, no phantom drain caused by us; if the vehicle sleeps, we wait.

Two paths deliver the raw data: preferably Fleet Telemetry, where the vehicle streams energy level and state of charge directly — every data point then is its own capacity estimate. Where telemetry is not available (yet), we reconstruct charging sessions from regular status reads.

2. How we calculate

Tesla's API does not provide a ready-made SoH figure. But every charging session reveals capacity:

capacity ≈ energy added (kWh) / (Δ state of charge / 100)

Charge from 20 to 80 percent while adding 45 kWh, and your battery holds roughly 75 kWh of usable capacity. A single estimate like this is noisy — temperature, charging losses and measurement resolution all scatter. So we combine many of them with robust statistics: a weighted median against outliers, MAD as the spread measure, a Theil–Sen estimator for the trend.

From charging sessions to an SoH range: noisy capacity points, robust trend, uncertainty band

The result is deliberately a range with a confidence statement — say "91 to 93 percent" — instead of pseudo-precise decimals. If the data is not sufficient for a robust range, we issue no certificate. No number beats a shaky one.

3. Why history beats a snapshot

A test on a single day measures exactly that: one day — with its temperature, state of charge and measurement errors. A history across weeks and months averages these effects out and shows what buyers actually care about: how this battery ages, not just where it stands today. Jumps caused by BMS drift are obvious in a history; in a snapshot they look like truth.

4. Limits — and what an Akkubrief is not

Honesty is part of the methodology: every figure is an estimate with a stated uncertainty, not a lab measurement. The Akkubrief is not an official government document and not an inspection by a testing organisation, and it does not warrant the vehicle. Our calibration against reference measurements is in progress: a validation study with a community fleet is the next step — the published error metrics will be linked here once available.

5. Tamper-proof by signature

Every certificate is signed with Ed25519. The certificate ID is the hash of the signed content — ID, statements and signature are inseparable, and any subsequent change fails verification. The verification page runs entirely in your browser (WebCrypto): check the signature, recompute the certificate ID, optionally match the VIN — which, for privacy, appears only masked and as a SHA-256 hash inside the certificate.

6. Version & changelog

The current methodology version is shown at the top of this page; every certificate references the version it was calculated with.

Who's behind this

I've driven electric since the Model 3 first existed. The questions this page answers are exactly the ones I ask myself with every used-car purchase.