Battery tests for electric cars: Do you understand SOH?

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Germany generally buys cars second-hand: in 2024, there were 6.5 million so-called owner registrations for every 2.8 million newly registered cars. This is the term used by the Federal Motor Transport Authority (KBA) for second-hand purchases. Just 173,034 of these were electric cars. This includes 140,364 plug-in hybrids, which also have a traction battery. This component is one of the most valuable in the entire car. Nobody wants to buy a used car with a defective or weakened traction battery. There are certificates to minimize the risk. However, not all aspects of such a test report are easy to understand – What is really important?

Even if it should be clear to almost everyone, a used electric car naturally does not wear out exclusively around the traction battery. Other components, such as the chargers, can also break down. Of course, tires, and chassis also wear out on an electric car. So there are plenty of aspects that a prospective buyer should reasonably take a close look at beforehand. The electrochemical storage system is generally quite durable, but there are, of course, exceptions here too.

Marked with traffic light colors

Aviloo currently offers what is probably the best known and most widely used battery test. It stands as an example. The Aviloo certificate is a two-page document with figures and abbreviations. Everyone can immediately understand the overall result because it is marked with the colors green for everything is okay, orange for there is a real problem, and red for a massive defect. The chassis number (VIN) of the respective car is entered on the certificate. Trust is good, control is better: The VIN should be compared with the real vehicle by the prospective buyer.

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State of Health bei Elektroauto-Traktionsbatterien (7 Bilder)

The values in the certificate are based on Aviloo's flash test, which only takes a few minutes and in which the battery management system is read out in detail. This has to be done so quickly because the experts for leasing returns usually have less than ten minutes to assess a used car as a whole. The leasing business is dominant for new cars and continues to grow. The fact that this flash test works has to do with the extensive premium test, which is also carried out thousands of times: Extensive data records are collected in a cloud and compared. Deviations are immediately apparent.

The State Of Health (SOH) counts

The State Of Health (SOH) is particularly important. This refers to the actual amount of energy in kilowatt hours (kWh) that can be drawn from the traction battery. This is set in relation to measurements of the new condition. For example, an SOH of 92 percent means that 92 percent of the original amount of energy and the correspondingly reduced range is still available.

At Aviloo, there is a point of conflict at this point: the company takes the amount of energy as the value that is shown to the driver in the display from 100 to zero percent charge level. Any buffer that goes beyond this is not considered. This can lead, for example, to a VW ID.3 with a net capacity of 58 kWh specified by the factory only releasing 54 kWh in the 100 to zero percent charge level window. If 51 kWh of this is still available, this value is set in relation to 54 kWh; the SOH would be 94 percent here. A tip: Pay attention to the absolute numerical value in kilowatt hours that can still be taken. This tells many electric car drivers more than an SOH in percentage points.

What is the benchmark?

Benchmarking is a new feature of the Aviloo certificate. It indicates how the specific vehicle performs "in comparison to comparable vehicles" (yes, that's what it says). For electric cars that have been built frequently, hundreds or thousands of very similar vehicles are available for benchmarking. This is the case, for example, with a VW ID.4 with 77 kWh. The benchmarking graphically indicates whether the specific electric car performs above or below average.

This could be different for an Alfa Romeo Junior Elettrica, for example. It is sold in small numbers. The drivetrain, on the other hand, is identical to several electric cars in the Stellantis Group. These are likely to be the benchmark for benchmarking. Almost all electric cars on the market can be checked by Aviloo https://aviloo.com/de/fahrzeugabdeckung.

Differences in the cell voltage

The most exciting part of the Aviloo certificate is the cell voltage diagram. It shows each individual cell with the corresponding numerical cell voltage and also graphically with the traffic light colors green, orange and red. A battery system is made up of many individual cells. In the Renault Zoe 40, for example, there are 96  cells. If one or more cells are weakened or defective, this can be seen directly from the difference in cell voltage.

The lower the voltage difference in the battery system in millivolts (mV), the better. The reason: the weakest cell in the system slows down performance. A battery system should therefore be as homogeneous as possible. The Aviloo certificate shows exactly which cell may be worn or defective. In the Renault Zoe 40, as in most electric cars, several cells are combined in one module; a repair would therefore involve replacing the module with the defective cell or cells. This structure of individual cells and modules is representative of many electric cars in the fleet.

Special cases and mistrust

As always, there are special cases with battery certificates. One example is the 800-volt system from Hyundai. It only outputs the data for the cell voltage in steps of 20 mV, whereas other manufacturers only output one to two mV. This can give the impression that a battery system is flawless, even though there are small differences. Nevertheless, even with this lower resolution in the measurement data, a minimal difference in cell voltage is synonymous with a healthy battery system.

Used car buyers should be skeptical and suspicious if an older electric car is offered by the dealer with an SOH of 100 percent. Such dream results are usually the result of the manufacturer's own readings of the battery management system, which are generously interpreted positively. Traction batteries always degrade – just like any other car wears out through use.

Unfortunately, this is not (yet) a matter of course

Many car dealers have realized that electric cars without a credible certificate are harder to sell than those without. This is especially true if the factory warranty has expired or is no longer valid. Against this background, it is mysterious that the relatively inexpensive measuring devices have not yet gained widespread acceptance. It is not a bold assumption that this could soon change: battery certificates will probably become commonplace.

With a car with a combustion engine, it was possible to check whether the maintenance history was complete, whether there were any unusual noises or whether the oil looked reasonably good. Ultimately, however, there was still a lack of clarity, and a defect could easily be covered up. The electric car offers the advantage that the most important component can be measured precisely. This transparency is good for buyers – and can make some sellers sweat.

(mack)