SOH value electric car: indicator of battery health

The SOH value is an important indicator of the condition of the vehicle battery. But what does it actually say? How is it determined and why is it important to know it? In this article, we provide you with the answers.

1) What does SOH mean?

SOH stands for "State of Health" and refers to the battery's state of health. This indicates how powerful or healthy the battery is in relation to its original condition.

The parameters that determine the health of a battery are

• its nominal voltage in volts (V)
• its capacity in ampere hours (Ah)
• and the resulting amount of energy that can be stored in kilowatt hours (kWh)

The SOH value therefore indicates how much energy a battery can still store and transfer. The higher the value, the more powerful the battery and the greater the range it can provide.

In short, the SOH value reflects the ageing of the battery and its current condition.

2) What SOH phases does a battery go through?

Similar to an ageing person, the battery goes through several phases:

• Begin of Life (BoL) - 100% SOH

At the beginning of its life, it starts its existence with full battery power. It is fully charged and provides its full capacity.

Side effect: The range is optimal and can be used without restriction.

• Slightly reduced performance - 90 % SOH

After a while, the initial euphoria subsides somewhat and the battery becomes a little quieter. Not much, but noticeably. In this phase, it has an SOH value of around 90 % and only a slightly limited range.

Tip: Experts recommend looking for an SOH of at least 90% when buying a used electric car.

• Significantly reduced performance - < 80 % SOH

The older the battery gets, the weaker it becomes. It can then absorb and transfer significantly less energy, which further reduces the range.

It also finds it difficult to retain the remaining energy, so that it discharges increasingly quickly. Even when it is not being used.

Note: If you do not drive long distances and are happy with a limited range, there is nothing to stop you from continuing to use a battery that is running low.

• Old age - greatly reduced performance

At some point, even the best battery can no longer cope. Charging thousands of times, intensive use, endless hot summers and bitterly cold winter nights all leave their mark. And then?

Then it goes into retirement and spends the second half of its life as an energy storage device in stationary operation or as a back-up buffer for the emergency power supply of hospitals or fire stations.

• End of Life (EoL) - SOH 0%

As the SOH value of the battery decreases linearly, it will sooner or later reach zero. This officially ends the life of the battery and it can no longer be used. At least not as a power source or energy storage device. As a valuable supplier of raw materials, however.

3) What effect does a high or low SOH value have?

• High SOH - long range, infrequent charging stops

A high SOH indicates that the battery has hardly aged and has largely retained its original capacity. It can store and transfer a lot of energy so that it can also cover longer distances without stopping to charge. This ensures that the electric vehicle has a long range.

• Low SOH - limited range, more frequent charging

A low SOH, on the other hand, indicates that the battery can only store and transfer a small amount of energy compared to its original state. It therefore discharges more quickly and needs to be recharged more frequently. The range of the electric vehicle therefore decreases.

4) How can the SOH be determined?

The SOH value can be both measured and calculated. However, there is not yet a standardized formula for calculating the SOH. According to Aviloo, the formula could be as follows:

Current range divided by new range

The result would then be given as a percentage. The catch is that this would have to be based on a standardized driving style. However, as every driver has an individual driving profile, this is rather unrealistic.

In addition, external influences, such as temperature, also affect the battery condition. It is therefore difficult to determine a reliable result with this equation. According to Aviloo, this equation would be more realistic:

WLTP current divided by WLTP new condition

The problem: in order to determine the current WLTP value, the vehicle would also have to be driven according to the WLTP scale. This is an effort that is not feasible for normal drivers, which is why the simplest thing to do at the moment is to measure the SOH value.

Various diagnostic devices and/or special software are used to measure the SOH value. Here, too, only approximate values are usually determined, as external influences such as the type of charge and temperature can affect the result.

• Battery Management Systems (BMS)

The BMS is part of the battery system of electric vehicles. It controls various parameters of the battery, such as voltage, current, temperature and state of charge (SOC). It also monitors the condition of the battery and provides early warning of a loss of capacity.

Although it cannot measure the SOH value directly, it provides important data that can be used to calculate the SOH. By analyzing this data, experts can assess the condition of the battery and indirectly derive the SOH.

• Diagnostic devices

Manufacturers and workshops for electric vehicles often have special diagnostic devices to determine the SOH. These can communicate directly with the BMS and thus provide precise information about the battery performance.

These include OBD-II dongles and battery testers, for example.

• Software solutions

There is now also special software for determining the SOH value, such as battery analysis tools, vehicle diagnostics software and open source software. These are usually only available from manufacturers, dealers or service partners.

5) Why is it important to know the SOH value?

• Estimate range & plan replacement

The SOH value of a vehicle battery shows you how fit the battery (still) is. This makes it easier to estimate the range of a vehicle and, if necessary, plan to replace the battery in good time.

• Determine residual value more realistically before purchase

If you want to buy a used electric vehicle, it is also advantageous to know the SOH value. This way, you don't "buy the battery in a bag", but know exactly how powerful it actually is. This will prevent you from making a rash purchase and being faced with expensive surprises.

After all, the battery is the most expensive component in an electric vehicle and can even exceed the hundred thousand euro mark in premium models.

• Taking battery worries away from used car buyers

Conversely, an electric car with a demonstrably high SOH value is also easier to resell. Because if the battery is no longer an incalculable black box, potential buyers are much more willing to opt for a used electric car.

6) SOH value - the battery indicator for owners, dealers & used car buyers

The State of Health (SOH) value indicates how powerful the battery of an electric vehicle is. It goes through several phases and decreases linearly from the original 100% to zero.

A high SOH is associated with a long range and proves that the battery can still absorb and transfer a lot of energy. A low SOH, on the other hand, indicates that the battery has lost a significant amount of capacity and only has limited performance. This has a negative effect on the range and makes more frequent charging stops necessary.

With the help of BMS, diagnostic devices and special software, the SOH can be approximately determined so that battery replacement can be planned in good time. As the SOH value enables a more realistic assessment of the residual value, it is also a reliable decision-making aid when buying a used car and a good argument against possible battery fears.

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