EV battery state of health: what the numbers mean
Understand battery health, remaining charge and capacity estimates, and learn what to check before trusting an EV health percentage.
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Battery state of health, usually shortened to SoH, describes how a battery’s capabilities compare with a reference condition. For an electric car owner, the most familiar measure is how much energy the battery can hold compared with when it was new.
A useful health number should tell you what was measured, when it was measured and which original value it was compared with. Without those details, two apparently precise percentages can answer different questions.
Charge, capacity and health answer different questions
| Reading | The question it helps answer |
|---|---|
| State of charge, or SoC | How full is the battery now? |
| Full-pack energy in kWh | How much energy does this measurement or estimate describe at full charge? |
| Capacity retention in percent | How does the current capacity compare with a matching original reference? |
| Range in kilometres or miles | How far might the car travel under the assumptions used for that range figure? |
An older battery can still reach 100% charge. That means it is full according to the car’s charge scale; it does not establish that it retains its original capacity.
Capacity is also only one aspect of health. Research into battery diagnostics considers properties such as resistance alongside capacity, and even those measurements do not capture every form of degradation. A capacity percentage alone cannot establish the battery’s overall condition. See the national laboratory research on battery health diagnostics.
How an energy-retention estimate is calculated
For an energy-based comparison:
Estimated retention = current full-pack energy ÷ matching original full-pack energy × 100.
As an arithmetic illustration, 72 kWh compared with an 80 kWh original reference gives 90%. If the appropriate original reference were 78 kWh, the same current reading would give about 92%. These are example values, not measurements from an Odomojo vehicle.
The difference comes entirely from the reference. Reporting more decimal places would not resolve that uncertainty.
Both values must describe the same thing. A nominal pack-energy figure should not be divided by a usable-energy specification without establishing that the definitions are comparable. The unit “kWh” alone does not make two capacity figures interchangeable.
The reference must also fit the installed battery. Model name, model year and trim can help narrow the possibilities, but a catalogue match does not establish a car’s battery replacement history or its individual capacity when new.
Where battery-health numbers come from
A manufacturer may report its own health metric or provide a vehicle test. A service may instead calculate a ratio from a reported capacity and a reference. Another method might estimate capacity from changes in energy and charge level.
These methods should identify their inputs and assumptions. Measuring health during ordinary operation is a different problem from performing controlled laboratory tests; researchers use models to estimate health from operating data. The National Laboratory of the Rockies’ battery lifespan research explains that distinction.
When comparing two results, first compare their definitions, source dates and reference values. Agreement between two calculations using the same vehicle inputs is useful, but it is not an independent measurement of the battery.
A change in the number needs context
Record the method and software version alongside a reading when available. If the method, original reference or installed pack changes, the before-and-after percentages may no longer be comparable.
Our approach is to retain the underlying reported capacity separately from the reference used to calculate a percentage. Changing a catalogue suggestion or a saved original-capacity value should not rewrite the measurements from the car.
A first observation on a used vehicle is a useful starting point for monitoring. Calling it “100% health when new” would lose the distinction between the start of monitoring and the start of the battery’s life.
What to check before relying on a result
- Method: is this a manufacturer-reported result, a test or a calculated estimate?
- Time: when did the vehicle produce the underlying reading?
- Reference: which original battery and capacity definition does the percentage use?
- Coverage: is there a single reading, a series of comparable observations or insufficient data?
- Changes: has the battery been replaced, or has the measurement or reference changed?
A missing reading should remain unavailable. It is not evidence of zero health, and an unsupported car should not be assigned a reassuring default percentage.
Battery health by manufacturer
The Tesla battery-health guide explains Tesla’s own evaluation, the newer nominal-capacity telemetry signal and Odomojo’s estimated capacity retention. For the dated research behind that signal, read what changed with Tesla 2026.32.
Odomojo’s Tesla capacity collection currently has limited availability. Check Tesla compatibility and contact us about capacity reporting for your vehicle; ordinary tracking compatibility alone does not establish battery-health availability.