SOH battery and capacity: what to understand

Traction batteries lose capacity over the years, which affects the performance of electric vehicles. This phenomenon is completely natural for lithium-ion batteries and is called aging. v SoH (Health Status) is a reference indicator for measuring the condition of a used battery in an electric vehicle.

SOH: battery aging indicator

Old batteries

 Traction batteries are used to store the energy needed to run electric vehicles. Batteries degrade over time, resulting in reduced range for electric vehicles, reduced power or even longer charging times: this is what aging.

 There are two mechanisms of aging. The first is cyclic aging, which refers to the degradation of batteries when using an electric vehicle, i.e. during a charge or discharge cycle. Therefore, cyclic aging is closely related to the use of an electric vehicle.

The second mechanism is calendar aging, that is, the destruction of batteries when the car is at rest. Therefore, storage conditions are of great importance, given that the car spends 90% of its life in the garage.

 We've written a complete article on aging traction batteries that we invite you to read. here.

Health status (SOH) of the battery

SoH (State of Health) refers to the state of the battery of an electric vehicle and allows you to determine the level of destruction of the battery. It is the ratio between the maximum capacity of the battery at time t and the maximum capacity of the battery when it was new. SoH is expressed as a percentage. When the battery is new, the SoH is 100%. It is estimated that if the SoH falls below 75%, the battery capacity will no longer allow the EV to have the correct range, especially since the weight of the battery remains unchanged. Indeed, a SoH of 75% means that the battery has lost a quarter of its original capacity, but since the car still weighs the same weight as it was left from the factory, it becomes less efficient to maintain an over-discharged battery (the energy density of a battery with SOH less than 75% is too small to justify mobile use).

The decrease in SoH has direct consequences for the use of electric vehicles, in particular the reduction in range and power. Indeed, the loss of range is proportional to the loss of SoH: if SoH increases from 100% to 75%, then the range of an electric vehicle of 200 km will schematically increase to 150 km. In fact, the range depends on many other factors (the vehicle's fuel consumption, which increases when the battery is discharged, driving style, outside temperature, etc.).

Therefore, it is interesting to know the SoH of his battery in order to have an idea of ​​the capabilities of his electric vehicle in terms of autonomy and performance, as well as to monitor the state of aging in order to regulate the use of his VE. 

SOH Battery and Warranties

Electric battery warranty

 The battery is the main component of an electric vehicle, so it is often guaranteed longer than the vehicle itself.

Typically, the battery is guaranteed for 8 years or 160 km at over 000% SoH. This means that if the SoH of your battery falls below 75% (and the car is less than 75 years old or 8 km), the manufacturer agrees to repair or replace the battery.

However, these numbers may differ from one manufacturer to another.

The battery warranty may also differ if you purchased an electric vehicle with the supplied battery or if the battery is rented. Indeed, when a motorist decides to rent a battery for his electric vehicle, the battery is guaranteed for life on a specific SoH. In this case, you are not responsible for repairing or replacing the traction battery, but the cost of renting a battery can add to the overall value of your electric vehicle. Some Nissan Leaf and most Renault Zoe rent out batteries.

SOH, reference

 SoH is the most important element to know because it directly reflects the capabilities of a used electric vehicle and, in particular, its range. In this way, EV owners can learn about the condition of the battery in order to apply or not apply manufacturer warranties.

SoH is also a decisive indicator when selling or buying a used electric vehicle. Indeed, motorists have a lot of concerns about the aftermarket electric vehicle's range because they know that aging and loss of battery capacity are directly related to reduced range.

Thus, knowledge of SoH allows potential buyers to understand the condition of the battery and understand how much range the car has lost, but above all, SoH must be directly taken into account when evaluating the cost of a used electric vehicle.

As for the sellers, SoH points to the still possible use of their electric vehicles, as well as their cost. Given the importance of the battery in an electric vehicle, its selling price should be in line with the current SoH.   

If you want to buy or sell a used electric vehicle, La Belle Batterie Certificate will allow you to transparently indicate the SoH of your battery. This battery certificate is for those who want to sell your used electric car... By being transparent at the time of sale about the real condition of your electric vehicle, you can ensure a fast and trouble-free sale. Indeed, without specifying the condition of your battery, you risk that your buyer will turn against you, noting the low autonomy of a recently purchased electric vehicle. 

Other indicators of aging

First: the loss of autonomy of the electric vehicle.

 As we explained earlier, the aging of traction batteries is directly related to the loss of autonomy in electric vehicles.

If you notice that your electric vehicle no longer has the same range as a few months ago, and the external conditions have not changed, the battery has probably lost its capacity. For example, you can compare year after year the mileage displayed on your dashboard at the end of the ride you're used to, making sure the initial state of charge is the same and that the outside temperature is about the same as last year.  

In our battery certificate, in addition to SOH, you will also find information on maximum autonomy when fully charged. This corresponds to the maximum range in kilometers that a fully charged vehicle can cover.  

Check the SOH of the battery, but not only 

 SOH alone is not enough to determine the condition of a battery. In fact, most manufacturers provide a "buffer capacity" that appears to reduce the rate of battery degradation. For example, the first generation Renault Zoes has an officially installed 22 kWh battery. In practice, the battery is typically around 25 kWh. When the SOH, calculated on a 22 kWh basis, drops too much and falls below the 75% mark, Renault will “reprogram” the computers linked to the BMS (Battery Management System) to raise the SOH. Renault specifically uses the buffer capacity of the batteries. 

Kia also provides buffer capacity for its SoulEVs to keep SOH high for as long as possible. 

Therefore, depending on the model, we must look, in addition to SOH, the number of BMS reprograms or the remaining buffer capacity. The La Belle Batterie certification indicates these metrics in order to restore a battery aging state that is as close to reality as possible.