Battery health and State-of-Health measurement

State of Health (SoH) — what is it really?

State of Health, or SoH, is a percentage figure that describes how much of the original battery capacity your EV still has. A new battery has 100 % SoH. After a few years and tens of thousands of kilometres the number drops — quickly at first, then more slowly. SoH is the single most important figure that tells you how much life the battery has left, and therefore how much the car is worth on the used market.

SoH is different from State of Charge (SoC). SoC is how full the battery is right now (0–100 %), while SoH is the maximum capacity the battery can hold. A 60 kWh car with 90 % SoH effectively has a 54 kWh battery.

How we measure SoH

Most manufacturers report SoH internally in the car's battery management system (BMS), but this figure isn't always visible from the dashboard. We use specialized diagnostics connected to the OBD port to read the actual value from the BMS. On some brands (Tesla, VW, Polestar, BMW) the value is well documented; on others we have to do a combined readout of cell voltages, balancing and total capacity to give an estimate.

For a thorough measurement we run a combination of:

• BMS readout — read the reported SoH value directly
• Cell voltage analysis — check whether any cell modules are weaker than others (a weak module pulls down the whole capacity)
• Charge cycle test — discharge and charge and measure actual kWh against reported
• Report — you receive a signed report with SoH percentage, number of charge cycles and any anomalies

Expected degradation — what is normal?

Modern lithium-ion EV batteries follow a predictable degradation curve. Typically:

• 0–30,000 km — from 100 % down to about 95 % (rapid initial degradation, especially the first 5,000 km)
• 50,000 km — about 92–94 %
• 100,000 km — about 88–92 % (what you can expect)
• 200,000 km — about 80–85 %
• 300,000 km — about 70–80 %

Numbers below these are signs the battery has been overloaded, exposed to extreme heat/cold for long periods, or has a faulty cell module. Numbers above suggest the car has been driven gently and mostly home-charged at low power.

Warranty — what's covered?

Most EV manufacturers offer 8-year or 160,000 km warranties that the battery will retain at least 70 % SoH. Tesla and VW give equivalent warranties with slightly different thresholds (Tesla 70 %, VW 70 %, BMW 70 %, Polestar 70 %). If SoH drops below this limit before the warranty expires, you have a right to module repair or replacement. An SoH report from an independent workshop is worth its weight in gold if you need to claim warranty work.

When should you measure SoH?

We recommend SoH measurement in four situations: 1) before selling a used EV, 2) if you notice the range has dropped noticeably, 3) when buying a used EV before placing a bid, 4) if you are approaching battery warranty expiry and want to document the status. The measurement takes 30–60 minutes and typically costs NOK 800–1,500 including report. If you are buying a used EV, an SoH report will often save you more than it costs — you can negotiate down the price, or choose another car if the figure is poor.

Book an SoH measurement online or call us on 41 17 32 24.