SoC tells you how much is in the tank right now. SoH tells you how large the tank has become. Neither is measured directly: both are calculated from logged battery data, so different tools may produce different answers for the same vehicle. A health figure means little without an explanation of how it was obtained.
Four terms that are often confused
| Term | What it describes | Unit | How quickly it changes |
|---|---|---|---|
| SoC State of Charge | How much energy is in the battery at that moment. | % | Continuously, both up and down |
| SoH Battery health | How much of the original storage capacity remains under a given load condition. | % | Slowly, almost exclusively downwards |
| Internal resistance | Put simply: how readily the battery can deliver and receive power. As internal resistance increases, SoH decreases because higher internal resistance reduces the usable charge and discharge capacity under a given load condition. | milliohms | Increases with age and is temporarily higher in cold conditions |
| Range | The result of all the above, plus driving style, weather, load and speed. | km | From day to day |
The first two are confused most often. SoC is a snapshot and resets whenever the vehicle is charged. SoH is a measure of battery condition and moves slowly in one direction. A vehicle can display 100% and still have a noticeably worn battery. The display is showing SoC.
SoC is not SoH
The same state of charge, different amounts of energy
Total capacity of 77 kWh
Total capacity of 62 kWh
Think of SoH as the size of the tank and SoC as the amount in it. A 77 kWh battery that has lost 20% of its capacity can hold 62 kWh. At a 60% state of charge, it contains 37 kWh, whereas a new battery at the same 60% contains 46 kWh. The driver sees the same figure on the display but has nine fewer kilowatt-hours available for driving.
This is also why a decline in range rarely presents as a sudden fault. It happens gradually and can easily be attributed to winter temperatures, new tyres or a changed route. Only a measurement can show what has actually happened.
How battery health is measured in practice
There is no sensor that measures battery health directly. Every method combines measurement with calculation. They differ in the time required, the stress placed on the battery and how readily the results can be compared between vehicles.
| Method | How it works | Strength | Points to consider |
|---|---|---|---|
| Full charge cycle | The battery is discharged and fully recharged while the energy is measured. | Direct capacity measurement. The reference method in laboratory settings. | Takes many hours, ties up a charger and staff, and the deep discharge adds wear to the battery. |
| Battery-data readout | Values logged by the battery are retrieved through the vehicle’s diagnostic port. | Takes under two minutes, requires no charging and uses data from the battery’s entire life. | Raw data must be interpreted against reference values for the model. |
| Reference-fleet analysis | The values are compared with vehicles of the same model and model year. | Makes the figure comparable and helps identify anomalies. | Requires a sufficiently large and current reference database. |
| Internal resistance and impedance | The battery’s response to current and voltage pulses is measured. | Captures power capability, among other things. | Indirectly affects SoH and is already partly reflected in the SoH value. Modern battery management systems (BMS) continuously monitor and estimate internal resistance. |
In workshop settings, the combination of a data readout and analysis against reference data has become the practical standard, and for good reason. The battery management system has logged voltage, temperature, charge cycles and energy flows throughout its life. The information is already in the vehicle and simply needs to be retrieved and interpreted. This provides evidence based on years of real-world use rather than a single measurement.
Completing the test in under two minutes is more than a matter of convenience. A test that takes half a day is used only occasionally, on vehicles where a problem is already suspected. A test completed in under two minutes can be run on every vehicle at trade-in. Only then do the figures become comparable across the entire stock.
What constitutes normal degradation
Remaining capacity after 10 years
Predominantly AC charging
Average
Extensive fast charging
Geotab’s analysis of more than 22,700 vehicles from 21 model series provides the broadest picture to date of real-world degradation. The average capacity loss is approximately 2.3% per year. A typical battery therefore retains around 80% after eight years, comfortably above the 70% warranty threshold applied by most manufacturers for eight years or 160,000 kilometres.
There is considerable variation, however. Vehicles charged primarily with AC were around 1.5% per year, while vehicles with heavy use of fast chargers above 100 kW were around 3.0%. Over ten years, this is roughly the difference between a battery at 85% and one just above 70%.
Two patterns are worth knowing. Capacity often declines faster in the first year before the curve levels out. A loss of a few percentage points in the first year is normal and not a warning sign. Towards the end of the battery’s life, the process may accelerate again. In between, degradation is largely slow and predictable.
Two vehicles, the same mileage, different batteries
The same model and mileage
Mostly home-charged to 80%
Frequently fast-charged to 100%
Mileage is the first figure we look at when valuing a used vehicle. For the battery, it is a blunt measure. Age, charging patterns, state of charge while parked and climate together carry more weight than the number of kilometres driven.
A vehicle that was mostly charged at home to 80% and parked half-full in a cool garage can be ten to fifteen percentage points above an identical vehicle that was fast-charged to 100% every day in a hot climate. This is not a theoretical difference. It represents several years of normal degradation and is not visible in the service record.
What an SoH figure does not tell you
Battery health is often summarised in a single figure, but the reality has more dimensions. Comprehensive evidence should also address:
- Variation between cells. A pack with 90% capacity but large differences between its cells is in worse condition than the figure suggests. The weakest cell sets the limit for the entire pack.
- Internal resistance. A battery may retain its capacity while losing the ability to deliver full power, which appears as weaker acceleration and slower charging.
- Temperature and charging history. How the vehicle has been treated says a great deal about how quickly the curve is likely to continue downwards.
- Stored fault codes. Active or historic faults in the battery system can affect value far more than a few percentage points of capacity.
How to interpret a battery report
The ranges below are rules of thumb that help put a figure in context. What is normal varies between models, so compare against the same model and model year wherever possible.
| Measured SoH | Reasonable interpretation |
|---|---|
| Above 90% | In line with or better than expected for most vehicles under five years old. |
| 85–90% | Normal for a vehicle aged five to eight years. No action required. |
| 80–85% | Normal at an older age or higher mileage. Worth asking about charging patterns. |
| 70–80% | Noticeably shorter range. Check warranty status and variation between cells. |
| Below 70% | Often below the manufacturer’s warranty threshold. Should be investigated before purchase or sale. |
Frequently asked questions
Is the information displayed in the vehicle sufficient?
Vehicle data is the starting point, but the display presents a simplified view and many models do not show battery health at all. The underlying values must be retrieved and interpreted against reference data for the model before they can be compared.
Does battery health decline faster at first?
Often, yes. Many batteries lose a few percentage points in the first year before the curve levels out. This is because the SEI layer on the anode forms most rapidly when the battery is new.
Can a worn battery be restored?
Lost capacity cannot generally be recovered. Cell balancing and calibration can, however, help the vehicle display a more accurate figure.
How often should a battery be tested?
At trade-in and sale. Beyond this, one measurement per year is a reasonable routine for vehicle fleets and anyone who wants to monitor the trend.
Sources
Geotab, EV Battery Health: Key Findings from 22,700 Vehicle Data Analysis; Volkswagen Sverige, batterigaranti och underhåll; Toyota Sverige, service och batterigaranti. The stated ranges are rules of thumb and vary between models.
Compare battery health consistently, every time
Optiledger reads battery data through the vehicle’s diagnostic port and interprets the values against reference data for the model. The test takes under two minutes and requires neither discharging nor fully recharging the battery.
How the test works