To function, an electric car is powered by a lithium ion battery. Over time, this showpiece naturally loses capacity and “ages.” This phenomenon has direct consequences on vehicle performance. This is why, in the second-hand market, knowing the state of health of the battery is essential. To assess it, a reference indicator exists: the SOH (State of Health). How to interpret it and where to get it? Our answers.
SOH, crucial information on the condition of the battery
Charging capacity, power delivered, autonomy... When it ages, the battery loses performance. However, this element represents almost a third of the cost of the vehicle! On the second-hand market, its condition is therefore key information to motivate or not a purchase decision. In addition, knowing its capacity makes it possible to estimate the value of a used electric car.
To understand the overall health of the battery, refer to the SOH. Expressed as a percentage, it indicates the ratio between the real capacity and the initial capacity of the car. A new battery therefore has an SOH of 100%, which decreases over time. On average, feedback has shown that an electric car loses 2% SOH per year.
To interpret this rate, here is an example: a 52 kW battery with an SOH of 92% will have a residual capacity of 47.8 kWh (52 x 92%).
When the SOH reaches 75%, it becomes critical. This means that the battery has lost 25% of battery life, or a quarter of its original capacity. Below this rate, the battery no longer has the capacity to allow the vehicle to have a correct autonomy.
Good to know
Knowing the SOH also makes it possible to enforce the manufacturer's warranty. When purchased, the battery is usually guaranteed for 8 years or 160,000 km (these conditions vary depending on the manufacturer). As a general rule, if the battery's SOH falls below a certain percentage — 75% or 70% most often —, the manufacturer has an obligation to repair or replace the battery.
How do you get the SOH from your battery?
The SOH is established with data from the battery's “Battery Management System” (BMS). Unfortunately, this information remains sensitive in the eyes of manufacturers, and therefore difficult to access... especially since there is no obligation to provide it.
However, some manufacturers provide information quite easily. Renault, for example, makes it possible to generate a battery certificate from its customer area. Note that this service is chargeable and only accessible to connected electric vehicles.
If this is not your case, there are two solutions left:
— go to a authorized workshop ;
— connect a special electronic tool to the vehicle's OBD socket.
In the coming years, offers on the second-hand market will multiply. Access to SOH is therefore essential to facilitate the purchase or resale of an electric vehicle. Recognizing this challenge, the European Union plans to change the legislation by 2024 to simplifying access to this crucial data.
In the meantime, some companies are continuing to develop measurement tools. The startup Moba (ex-La Belle Batterie) for example, has developed a small box to be plugged into the OBD socket in order to measure the SOH of its battery. Once obtained, an independent certificate indicating his health condition is issued.
SOH: how to optimize it?
The rate of battery degradation is influenced by the conditions of use of the electric vehicle. Several elements contribute to the reduction of SOH, including:
— sporty driving;
— intensive use (long driving on the highway for example);
— a high number of quick refills;
— an extended stay in the garage;
— extreme weather conditions (cold as well as hot).
Also to read: How do I preserve the battery in my electric car?
