Why 100% on the dashboard (SoC) doesn't mean the battery is healthy

Imagine you are buying a used smartphone. You switch it on, the battery icon lights up in the top right corner, and next to it you see 100 %. Does that mean the battery is in perfect condition and the phone will play video for just as long as when it was brand new? Not even close. We all know it will drain far faster than it did on day one.

Exactly the same trap awaits inexperienced buyers in the used electric vehicle market. You arrive for a viewing, the seller proudly points to the fully charged battery on the on-board computer display and says: "Look, charged to one hundred percent, the range shows three hundred kilometres, there's nothing wrong with the car."

But at that moment you are looking at a figure that says practically nothing about the real health and wear of the battery. Why is that, and what is the difference between the two key abbreviations you need to know before buying?

The difference between two abbreviations: SoC vs. SoH

To avoid being fooled when buying an electric car, you need to understand the difference between the two terms people most often confuse: SoC and SoH.

• SoC (State of Charge): This is the number you see on the instrument panel. It tells you what percentage the battery is currently filled to. It only tells you how many percent are in the "tank" at that moment, regardless of how big that tank actually is.

• SoH (State of Health): You won't normally find this number on the dashboard. It expresses the real condition and remaining capacity of the battery compared to the moment the car rolled off the production line.

The leaky bucket analogy

Think of the battery as a water bucket. A new car has a big, clean 10-litre bucket. When you fill it up, it is 100 % full (SoC) and holds 10 litres of water.

After years of driving, fast charging and hot summer days, however, the battery naturally degrades. In battery chemistry, this means that some of the lithium ions irreversibly "fall asleep" or solidify. Picture it as if hardened concrete had settled at the bottom of your bucket. The bucket's usable volume is now only 7 litres.

When you fill this older bucket to the brim, it is once again at 100 % SoC (State of Charge). The on-board computer proudly reports a full charge. But instead of the original 10 litres, you are now carrying only 7 litres of water. Your tank has simply shrunk, even though the indicator says it is full. That shrinkage is exactly what SoH (State of Health) expresses — in this case, it stands at 70 %.

Why does the range indicator lie too?

Fine, but why does the car's on-board computer still show a high range in kilometres? The answer is simple: the on-board computer is not a fortune teller — it is a mathematician working with inaccurate numbers.

The range indicator calculates the remaining kilometres based on two things: the current state of charge and the driving style over the last few dozen kilometres.

If the seller charged the car before your arrival, wiped the consumption history and drove it a few kilometres in a defensive style — cruising smoothly at fifty through town with the heating switched off — the computer rejoices. It calculates an absurdly low consumption and displays an enormous range. But as soon as you take the car onto the motorway at 5 °C, the range starts dropping twice as fast as the kilometres you actually cover.

How do you find out the real truth?

Blind trust in what lights up on the instrument panel is the shortest route to buying an electric car with a low SoH. No car manufacturer wants owners to see the real capacity loss in percent, which is why this figure is buried deep in the car's menus — or not provided at all.

The only way to determine the battery's real health (SoH) is an independent diagnostic test. A special device connects to the battery management system (BMS) and bypasses the distorted data on the display. It looks directly at:

• the voltage of every single battery cell,

• the ratio of the battery's current capacity to the capacity of a new battery,

• the actual amount of energy in kilowatt-hours (kWh) that has flowed through the battery.

Only when you hold a certificate in your hand showing, for example, SoH: 91 % do you know that you are buying a car whose "bucket" is still almost as big as it was when the vehicle was new. Don't try to read fortunes from the dashboard — bet on exact data.