State of Health Explained: What SoH Really Tells You
Anyone buying or selling a used EV trips over three letters straight away: SoH, the State of Health. The number sounds objective — 91 percent, done. In truth it is an estimate with fine print. Know the fine print and you negotiate better.
What SoH means
SoH describes how much usable capacity a battery still has compared with its as-new state. 90 percent SoH on a battery that started with 75 kWh net means: around 67.5 kWh are available today. The figure falls over the battery's life — through calendar ageing and through charge cycles.
Why a single number can deceive
Four things sit behind every SoH figure, and all four can shift the result:
The reference basis. Gross or net capacity? Catalogue value, or what the vehicle actually held when new? Manufacturers also build in buffers, some of which have been changed by software. The choice of denominator alone shifts the percentage.
The measurement method. A full discharge cycle measures differently from a 3-minute pulse test, and both measure differently from the running estimate of the battery management system. Without a stated method, a SoH value is hard to place.
The state during measurement. Temperature and state of charge affect the result. The same battery can perform differently on a cold morning than after a motorway run.
The BMS itself. Many "SoH values" are not a measurement but the vehicle's self-assessment — and that drifts when it has not been calibrated for a long time. LFP batteries with their flat voltage curve are especially prone to this; that is why manufacturers recommend regular full charges for them.
SoH is not range
Range also depends on consumption, weather, driving profile and software. A healthy battery can show little range in winter, a degraded one plenty in summer. To judge battery condition you need capacity data — not the range readout on the dashboard.
The typical curve: steep first, then flat
Fleet and community data show a recurring pattern: in the first year or two, capacity drops noticeably (often into the range of 93 to 97 percent), after which the curve flattens markedly. A three-year-old car at 94 percent is therefore not a warning sign but the normal case. It becomes notable when the trend deviates from the pattern — which you only see if you have a trend.
The 70 percent threshold
Tesla — like most manufacturers — warrants the battery for eight years (depending on the variant, 160,000 to 240,000 km) to at least 70 percent capacity. And the EU is following suit: Euro 7 anchors, for the first time, regulated minimum durability for the traction batteries of new vehicle types. SoH thus moves from a community number to a regulated metric — one more reason to insist on dependable figures when buying used.
Why history proves more than a snapshot
A single measurement says: this is how it was on this day, with this method. A trend over months says: this is how this battery really ages. Jumps, BMS drift and measurement errors average out, outliers stand out. That is exactly what we build on at Akkubrief — calculating battery condition from the charging history across many charging sessions, as a range with honest uncertainty rather than a falsely precise decimal place. How that works is set out openly in our methodology.