State of health (SoH) is a figure of merit that expresses the current condition of a battery relative to its condition when new, most often as the ratio of present usable capacity to rated capacity, given in percent. Among EV batteries, it is the standard measure of degradation: a traction battery at 100% SoH performs like a new pack, while one at 80% stores about four fifths of its original energy.
Unlike the state of charge, which changes every minute while driving and charging, state of health changes slowly over months and years. It determines remaining driving range, influences the resale value of a used electric vehicle and decides whether a battery still qualifies under the manufacturer’s warranty.
How state of health is defined and estimated
The most common definition is capacity-based SoH: measured full capacity divided by nominal capacity. A pack rated at 75 kWh that can now deliver 66 kWh has an SoH of about 88%. A second definition is resistance-based SoH, which compares the present internal resistance with the beginning-of-life value; rising resistance reduces available power, increases heat and slows fast charging even when capacity looks acceptable.
No sensor measures SoH directly. The battery management system (BMS) estimates it continuously from cell voltages, currents and temperatures. Typical methods include coulomb counting across a known change in charge, comparing the open-circuit voltage curve at rest, tracking resistance during current pulses and applying model-based filters such as Kalman filters. Manufacturers combine these methods in proprietary algorithms, so two vehicles can report the same physical condition differently.
The table summarizes the main estimation methods and their practical strengths and weaknesses.
| Method | What it measures | Strength | Weakness |
|---|---|---|---|
| Full capacity test | Energy delivered from full to empty | Most direct result | Slow and impractical on the road |
| Coulomb counting | Charge passed between two known states | Works during normal use | Accumulates sensor drift |
| Open-circuit voltage analysis | Voltage at rest versus charge level | Low effort, uses idle periods | Weak on flat LFP voltage curves |
| Resistance measurement | Voltage drop under current pulses | Reveals power fade | Strongly temperature dependent |
| Model-based estimation | Combined inputs in a battery model | Robust over time | Opaque and manufacturer specific |
What lowers state of health
State of health falls through calendar aging, which happens even when a vehicle is parked, and cycle aging, which results from charging and discharging. The main chemical mechanisms in lithium-ion cells are growth of the solid electrolyte interphase, loss of cyclable lithium, cracking of electrode particles and, under unfavorable charging, lithium plating on the anode.
| Factor | Effect on degradation | Practical mitigation |
|---|---|---|
| High temperature | Speeds up side reactions strongly | Park in shade, rely on active thermal management |
| High state of charge at rest | Increases calendar aging | Use a daily charge limit around 80% for NMC |
| Frequent fast charging | Adds heat and plating risk | Precondition the battery, mix with AC charging |
| Deep discharges | Raises mechanical stress in electrodes | Avoid routinely driving near 0% |
| Charging in severe cold | Promotes lithium plating | Allow the BMS to warm the pack first |
A typical capacity curve shows a faster drop in the first one or two years, often a few percent, followed by a slower, nearly linear decline. Modern packs with liquid thermal management commonly retain well above 80% SoH after 150,000 to 200,000 km, although results vary widely with climate, chemistry and usage.
State of health compared with related battery metrics
Battery reports combine several metrics that are easy to confuse. The comparison below sets state of health against its neighbors in the EV battery vocabulary.
| Metric | What it describes | Time scale | Typical unit |
|---|---|---|---|
| State of health (SoH) | Condition versus new | Months to years | Percent of original |
| State of charge (SoC) | Current fill level | Minutes to hours | Percent of usable capacity |
| Usable battery capacity | Energy the car allows to be used | Fixed by design, falls with age | kWh |
| Battery degradation | The process that lowers SoH | Continuous | Percent loss over time |
| State of power | Maximum current available now | Seconds | kW |
Manufacturers also keep a hidden buffer above and below the usable window. As cells age, some systems release part of this buffer, which can make displayed capacity look stable for a while even though the cells themselves are degrading.
Using state of health in practice
SoH matters most at three points: warranty claims, used-car purchases and second-life decisions. Many manufacturers guarantee a minimum of 70% SoH for eight years and a mileage limit, typically around 160,000 km, although exact terms differ by brand and region. The European Union is also introducing durability requirements and battery passport rules that make health data more accessible.
| SoH range | Typical meaning | Recommended action |
|---|---|---|
| 95-100% | Nearly new pack | No action needed |
| 85-95% | Normal wear for several years of use | Plan range with the reduced capacity |
| 75-85% | Noticeable range loss | Check warranty terms and cell balance |
| Below 70-75% | Often a warranty threshold | Request a dealer diagnosis |
| Below about 70% after automotive life | Retired from traction use | Consider second-life storage or recycling |
For a used-car purchase, the most reliable evidence is a standardized battery health certificate or a dealer diagnostic printout, rather than a single dashboard estimate. A controlled full-charge test compared with the range estimate adds useful confirmation.
Common misconceptions
- A displayed SoH is not a laboratory measurement; it is an estimate that may shift by several percent after a BMS recalibration.
- Range loss is not caused only by SoH; cold weather, tires and driving speed often matter more on a given day.
- Charging to 100% is not always harmful; LFP batteries are often recommended to reach full charge regularly for calibration.
- Equal SoH does not mean equal power; a pack with high resistance can show good capacity but charge slowly.
The simple explanation
State of health is the battery’s fitness score. When the car is new, the score is 100%. Over the years, the battery slowly loses some of its ability to hold energy, and the score falls.
Think of a water bottle that shrinks a little every year. State of charge tells you how full the bottle is right now. State of health tells you how big the bottle still is. A score of 90% means the battery holds about nine tenths of what it held when new.
How it is used in practice
A buyer considers a five-year-old electric hatchback with 110,000 km on the odometer. The original battery was rated at 58 kWh usable. The seller provides a dealer battery report showing an SoH of 87%, which corresponds to roughly 50 kWh of usable capacity.
The buyer recalculates expected winter range with the smaller capacity and confirms that daily commuting is still comfortable. The report also shows that the pack remains under an eight-year warranty with a 70% threshold. With that information, the buyer negotiates a price that reflects the reduced range while knowing the battery is degrading normally rather than failing.