The Secrets of Battery Health (SoH) - A Look Behind the Scenes

We've never seen that before!

During our daily work, we have investigated various cases. One electric vehicle that we tested in July 2023 stands out here and remains unforgotten!

Nikolaus Mayerhofer

The safety of all road users is a top priority. In the case of electric vehicles and plug-in hybrids, this depends significantly on the battery, or more precisely on its State of Health (SoH). During our daily work, we have studied several cases. One electric vehicle that we tested in July 2023 particularly stands out here and remains unforgotten!

Through our AVILOO testing, there were clear signs of serious battery degradation. First, the FLASH test was performed, which triggered a red alert, warning our customer of a major problem. In this case, it was a cell problem that triggered the red alarm. The spread of the cell voltage was an incredible 463mV. For a more precise clarification and to provide the repair shop with as many details as possible, the PREMIUM testwas performed afterwards. This once again confirmed our fears, as the SoH value was only 42% and cell defects were also detected. Although we had expected a poor SoH value, this result was extremely surprising, as the SoH value specified by the vehicle manufacturer was 89.87%. The differences in the results can have various causes, which can be read here .

We have performed a more detailed analysis for this particularly interesting case. The technical analysis of DI Nikolaus Mayerhofer, AVILOO CTO can be found below:


Error pattern: Low SoH, cell defect à Red alarm on FLASH test

Summary of analysis:

The vehicle shows a "felt" strong reduction in range. The manufacturer's SoH readout is an unremarkable 89.87%. However, a flash test performed shows a red - and therefore very noticeable - result and indicates a massive problem in the battery. The PREMIUM test shows a SoH of only 42%. This confirms 2-fold that the battery has a defect and must be urgently repaired in a specialist workshop.

The following test report contains all the diagrams and analyses proving these fault patterns, which provide the workshop with direct assistance in identifying the cause and rectifying the fault.

To record the data, the AVILOO Box was plugged into the vehicle for 3 minutes and a FLASH test was performed. This gave a red result (= battery defect). In addition, the customer drove his vehicle within the PREMIUM test from 100 to 10% SoC.

The AVILOO box remained plugged in the following day. This also automatically recorded a charging process. The permanent problem was also visible during the charging process.


After plugging in the AVILOO Box the range of all cell voltages is shown in a light blue band on the following diagram.

As an example, some cell voltages are highlighted in dark blue. The lowest blue line represents cell 25, whose values deviate massively from the other cell voltages. This cell is extremely conspicuous and therefore it can be concluded that this is where the battery's problem lies.

The subsequent charging process with only 10 kW also shows that the cell voltage spread is very high and increases with increasing state of charge:

A look at cell voltage Min Max output by the BMS also shows the same picture:

An evaluation of the cell number with the minimum cell voltage showed that it is always the same cell with the number 25.

It is also noticeable that despite constant charging power, the state of charge experiences a bend in its characteristic curve at a state of charge of approx. 25 %.

Enclosed is the FLASH Test Report:

The front page of the report indicates a massive problem in the battery area.

The back page points out a problem in cell voltage spreading (highlighted in red).

Also, the manufacturer SoH on the back is shown to be 90% which is definitely wrong. The correct SoH is 42%, which was exactly determined in the PREMIUM test.

On the next page the PREMIUM test is shown, which certifies a SoH of only 42%. The cause is definitely the problem at cell 25.


There is at least one defect at cell 25. Whether the cell itself, the voltage measurement of this cell or the BMS at this cell is defective cannot be clearly determined. In any case, the vehicle must be taken urgently to the specialist workshop and examined more closely, especially at cell 25.

Presumably, the module in which cell 25 is integrated must be replaced. It can be assumed that a SoH of approx. 90% will be available after module replacement.

In any case, it makes sense to perform another PREMIUM test after repair to verify the successful repair.

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