State of health (SOH)

State of health describes how much of its original capability a battery still has, most often expressed as remaining usable capacity relative to new. It is a useful indicator, but every SOH figure is an estimate with its own limitations.

What SOH means

In everyday use, SOH usually means the battery's current usable capacity as a percentage of its capacity when new. A battery at 90 % SOH can store roughly 90 % of the energy it held at delivery, which translates into a proportionally shorter range under the same conditions.

SOH can also refer to power capability, because internal resistance tends to rise with age. Higher resistance can reduce peak power and slow down fast charging, even while capacity loss is still modest.

How SOH is measured or estimated

Capacity cannot be read off directly while driving. The BMS estimates it from voltage, current and temperature over time, and that estimate can drift or jump after software updates, recalibration or a full charge and discharge. Independent tests measure capacity more directly through controlled charging or discharging, but still depend on conditions such as temperature.

Researchers use more detailed diagnostics to separate degradation modes such as loss of lithium inventory and loss of active material. These methods explain why two batteries with the same SOH may have aged in different ways, and may continue to age differently.

Why SOH curves are not straight lines

SOH often drops somewhat faster in the first period of use and then declines more slowly. Fleet telematics suggests average yearly losses of roughly 1.5 to 2.5 %, and OEM-reported fleet data indicates that many vehicles retain a large share of their capacity after high mileage. Both are aggregate figures from sources that are not peer reviewed, not predictions for a specific car.

Cell-to-cell variation means some packs age faster than average, and a pack is often limited by its weakest cells. Later in life a knee can appear, after which degradation speeds up, which makes very long-term projections less certain.

SOH in Voltgevity

Voltgevity projects SOH over time from your vehicle, climate and charging pattern, and shows a range rather than a single line. A measured SOH, for example from a dedicated battery test, is a useful reference point, but a single reading carries its own measurement uncertainty.

Key takeaways

SOH usually means remaining usable capacity compared with new, though power capability also declines with age.
Displayed SOH values are BMS estimates and can shift after recalibration or software updates.
Fleet averages provide useful context but say little about a single car.
Treat any SOH projection, including Voltgevity's, as a range with uncertainty.