Whoever Measures Has a Stake: What Makes a Battery Certificate Independent

One used car, two figures for the same battery: one from the manufacturer's garage, one from an independent provider. Both are properly recorded, both are on paper. They are still not equivalent, and the reason has nothing to do with measurement technology. It has to do with who pays the bill when the figure comes out low.

A conflict of interest is not an accusation, it is arithmetic

Auditors are not allowed to audit the accounts of their own employer. The reason is no suspicion of fraud: nobody should assess what they themselves pay for. A manufacturer's battery certificate creates that constellation in two places, and it does so for every manufacturer.

First, the warranty. The threshold is the same order of magnitude across the industry: Tesla, Volkswagen and Hyundai guarantee 70% remaining capacity, usually over eight years or 160,000 km, and a purchased Renault Zoe comes with 66%. Below that, the manufacturer repairs or replaces the battery at its own expense. The figure that decides such a case comes from the battery management system of the same manufacturer. Whoever gives the promise also measures whether it has been kept.

Second, residual values. Almost every large car maker owns leasing and finance companies that take back off-lease cars and work with calculated residual values. A battery figure widely documented as low pushes those residual values down, for the group's own vehicles and in the group's own books. Whoever measures the value also carries the risk.

There is a third and more obvious party: the seller. A dealer who wants to sell you the car and certifies its battery in the same appointment is biased too. With a dealer, everyone spots it immediately.

How accurate is the manufacturer's figure anyway?

The question is rarely asked, and most manufacturers do not answer it. Volkswagen does, which makes it the transparent case here. The group sells fleets, dealers and insurers a data product that calculates battery health from its online services: from the amount of energy charged and the charging curve, on the condition that the car has been AC charged across a swing of at least 40 percentage points. That is essentially the same route we take.

On accuracy, Volkswagen writes that the result "may deviate by up to +/- 10 percentage points from the actual residual capacity". Ten points is a lot. A stated value of 88% then means anything between 78% and 98%, which is the difference between an unremarkable car and a worrying one.

Two things follow. First, manufacturers run the same arithmetic as independent providers, so there is no fundamental advantage in the method itself. Second, and more important: an SoH figure without a range is incomplete. Whether a number is accurate to within 2 or 10 points decides what you can do with it. Ask for it. With us the range is printed on the certificate, and how we derive it is in the methodology.

Some manufacturers solve this themselves, by bringing in a third party

The conflict is well understood, and individual makers draw the conclusion. Since 2024 Hyundai has offered battery certificates for used EVs together with mobile.de, and they are issued not by the manufacturer but by the independent test provider Aviloo after a diagnosis at the dealership. Mercedes-Benz includes a battery certificate with its certified used cars.

Both point the same way: a documented battery state now belongs to the sale. The open question is only who answers for the figure, and whether that party gains anything from it coming out high.

What an independent issuer does differently

A provider who only sells the document has no stake in any particular number. It carries no warranty obligation, holds no cars in stock and calculates no residual values. Its only asset is that the figure holds up.

The incentive even points the other way from what many people assume. A flattering value is exposed by the next test or the first disappointed buyer, and it devalues every other certificate from the same issuer along with it. A cautious figure that buyers later find confirmed costs nothing.

With the Akkubrief the calculation is also in the open: the methodology is published, the result is tied to the VIN, the document is signed, and any interested buyer can verify it in the browser. And it builds up over weeks from many real charging sessions, a battery history rather than a snapshot.

What remains, even with an independent certificate

One point applies to every provider, ourselves included: the certificate is usually ordered and paid for by the seller. A poor result therefore ends up in a listing less often than a good one.

No promise helps against that, only the way the document is built. The value comes from the vehicle's own data, not from an agreement, and changing it afterwards breaks the signature. Since 1 November 2025, mobile.de, one of Germany's two large vehicle marketplaces, also requires sellers who flag a listing as having a battery certificate to upload the document.

As a buyer this gives you a simple lever: ask for the certificate, not for the number. Anyone who quotes a value but does not show the document has already answered you.

Four questions for any issuer

• Is the issuer selling the vehicle? Then it is rating its own stock.
• Does the issuer belong to the manufacturer? Then it is checking its own promise.
• Does the issuer bear costs when the value is low? Warranty, buy-back, residual value: every yes is a stake in the outcome.
• Does the issuer publish how the figure is calculated? A number without a readable methodology can be neither checked nor challenged.

A yes to the first three does not make a certificate wrong. It makes a second opinion worth having. The most informative case is both numbers side by side: if the manufacturer's figure and an independent certificate agree, the question is settled. If they diverge, you know what to talk about.