State of Health Explained: Why SoH Readings Vary
When a customer asks "what's the battery health on this one?", most dealers either don't know the answer or point to whatever the dashboard says. But that dashboard number might be misleading — and here's why.
SoH Is Calculated, Not Measured
State of Health isn't a direct measurement like temperature or voltage. It's a calculated metric that compares the battery's current performance against its original specification. The problem: different calculation methods produce different results.
Two identical vehicles tested on the same day can show different SoH numbers depending on who's doing the testing and how. For a dealer trying to price stock accurately and protect against CRA claims, that inconsistency is a real problem.
Capacity vs Energy: The First Split
Capacity-Based SoH (Ampere-Hours)
The simpler method. It measures how many ampere-hours the battery can store compared to its original rating.
The problem: It ignores internal resistance. As a battery ages, internal resistance increases, meaning less stored capacity actually reaches the wheels. A battery might show 92% capacity SoH but only deliver 86% of its original range.
In older vehicles, capacity-based SoH frequently overestimates remaining range. That gap between the number and reality is exactly where customer complaints and legal claims live.
Energy-Based SoH (Kilowatt-Hours)
This measures actual usable energy — what translates directly into driving range. 90% energy SoH = 90% of original range. More complex to calculate, but far more meaningful.
Professional independent diagnostics use this approach, often standardised against the WLTP cycle at 23°C for consistency. This is what the driver actually experiences behind the wheel.
Pack-Level vs Cell-Level: The Second Split
Pack-Level Analysis
Looks at the battery as a whole unit. Quick and easy, but it's like taking someone's temperature and declaring them healthy. Misses localised problems entirely.
Cell-Level Analysis
Evaluates every individual cell in the battery. A modern EV battery can contain 300+ cells. The critical insight: the weakest cell determines the battery's real-world performance.
A battery averaging 93% SoH might have one cell block at 81%. That weak cell is the limiting factor. It will cause reduced range, inaccurate dashboard estimates, and accelerated aging of neighbouring cells. Pack-level testing would never catch it.
We cover the mechanics of this in detail in our cell-level diagnostics guide.
Why the Dashboard Number Isn't Enough
Every EV has a Battery Management System that monitors the battery. Some manufacturers display SoH. So why not just use that?
- Not independent — the manufacturer has a commercial interest in favourable readings
- Methods vary — every manufacturer calculates differently, making cross-brand comparison impossible
- Precision varies — some BMS readings are rough estimates, not diagnostic-grade
For a dealer stocking multiple brands, you need a single, consistent, manufacturer-independent standard across your entire inventory.
What Good Testing Looks Like
The gold standard combines:
- Energy-based SoH — real-world range correlation
- Cell-level resolution — weakest cell identification
- Manufacturer-independent — consistent across all brands
- TÜV certification — independently verified accuracy
- Visual reporting — heatmaps readable by non-technical staff and buyers
- ±3% accuracy — diagnostic-grade, not estimate-grade
The AVILOO FLASH Test delivers all of this in 3 minutes via OBD-2, covering 95%+ of EVs on UK roads.
Why This Matters for Pricing and Protection
Understanding the SoH methodology gap directly impacts:
- Pricing accuracy — a 92% capacity SoH vehicle and an 86% energy SoH vehicle are not the same proposition. Price the first based on the dashboard number and you're overpaying at acquisition or underdelivering at sale
- Legal protection — under the Consumer Rights Act, you need evidence that the battery was fit for purpose at point of sale. A diagnostic-grade, cell-level reading is evidence. A dashboard screenshot is not
- Customer trust — showing buyers a clear, visual certificate with methodology they can understand builds buying confidence
The dealers who understand the difference between a dashboard number and a professional diagnostic are the ones who'll price accurately, avoid disputes, and build the strongest reputation in used EVs.