State of Health

State of health, almost always abbreviated to SOH, is the metric that describes how much of an electric vehicle battery's original capability remains after a period of use. A pack leaves the factory at a nominal 100 per cent, and SOH falls gradually from there as the cells age. Because a traction battery is by far the most expensive single component in an EV, often a third or more of the car's value, SOH has become the single most important figure when judging the long-term worth and remaining usefulness of any used electric car.

In its commonest form SOH expresses the ratio of a battery's present maximum charge capacity, measured in kilowatt-hours or ampere-hours, to the capacity it had when new. A pack that once stored 60 kWh but now holds only 54 kWh has an SOH of 90 per cent. Some battery management systems and diagnostic tools also report a power- or resistance-based SOH, which tracks how the internal resistance of the cells has risen over time and therefore how readily the pack can deliver high currents for acceleration and rapid charging. The two measures need not move in step, but capacity-based SOH is the figure quoted in valuations and warranty discussions.

The practical importance of SOH is straightforward: it directly governs usable range and, less obviously, charging behaviour, since a tired pack may charge more slowly and run warmer. For buyers, a battery health certificate generated by the car's own management system, or by an independent service that reads the data over the diagnostic port, has become a near-essential part of a used-EV inspection. Manufacturers reinforce this by warranting the battery to retain a defined SOH, commonly 70 per cent over eight years or 160,000 kilometres; a pack that falls below the threshold within the term may be repaired or replaced under warranty.

Degradation, the process that erodes SOH, is driven by both calendar ageing and cycling. High temperatures, frequent rapid DC charging, and habitually charging to 100 per cent or discharging to empty all accelerate the loss, which is why thermal management and sensible charging habits matter. The decline is rarely linear: many packs drop a few per cent in the first year or two, then settle into a long, slow plateau, before steepening again late in life.

It is essential to distinguish SOH from state of charge, with which it is frequently confused. State of charge describes how full the battery is at a given moment, swinging from 0 to 100 per cent and back every time the car is driven and recharged. State of health describes the size of the container itself and changes only slowly over years. A perfectly healthy battery at 20 per cent charge has a high SOH but a low SOC. Read alongside related measures of usable capacity and thermal management, SOH gives the clearest single picture of how much life an EV battery has left.