EV battery health (SOH) — what the number means and why it matters
Battery state-of-health is the single biggest determinant of a used EV's value. Here's how we measure it, what's normal, and when to walk away.
Buying a used EV in Sydney in 2026 without testing the battery is like buying an ICE car without checking the engine. State of Health (SOH) — the battery's remaining usable capacity as a percentage of its when-new capacity — drives range, charging speed, and resale value. Here's everything you need before you transfer.
What SOH actually measures
SOH is the ratio of current usable kWh to original kWh. A Tesla Model 3 Long Range left the factory with about 78 kWh usable. If today it can only hold 70 kWh, its SOH is 89.7%. That's the number you care about — not the dashboard range estimate, which can be reset and is biased by recent driving style.
How we test it
Battery health testing methods differ; diagnostic access and model compatibility determine what can be assessed:
- BMS diagnostic readout: supported vehicles expose battery estimates and other data through their diagnostic interface. Availability varies by model and software.
- AVILOO FLASH: the manufacturer describes a three-minute test that uses vehicle data and statistical analysis to estimate current SoH. Check compatibility before booking; it is not a partial-discharge driving test.
The available result may include SoH and cell-voltage data, depending on the vehicle and method. A voltage imbalance warrants interpretation, not an automatic conclusion that a module is failing. A test-time result does not guarantee future battery life.
What's normal by age and kilometres
| Age | Typical SOH | Concerning below |
|---|---|---|
| 1 year / <30,000 km | 97-99% | 94% |
| 3 years / 60-80,000 km | 92-95% | 88% |
| 5 years / 100-130,000 km | 87-91% | 83% |
| 7 years / 150-180,000 km | 82-87% | 78% |
| 10 years / 200,000+ km | 75-82% | 70% |
Beyond SOH — the four metrics that matter
- 1.SOH percentage — capacity remaining.
- 2.Per-module variance — should be under 0.04V between weakest and strongest.
- 3.DC fast-charge taper temperature — does it derate at 35°C ambient? (worth knowing in Sydney summers).
- 4.Number of DC fast-charge cycles — high-use cars (Uber/Didi) age 2-3x faster.
What changes price
We've tracked 380+ used EV sales in Sydney across 2025. Each 1% of SOH below the median for a model's age moves price by roughly $400-$600. A 2022 Model 3 with 87% SOH sells for $3,500 less than one with 92%, all else equal.
Battery warranty in Australia
Most EV manufacturers warrant the battery to 70% SOH for 8 years / 160,000 km. The warranty transfers with the car. Before you buy, get the seller to produce the original sale invoice and any service records — that's what the manufacturer will ask for if you claim.
Common questions
Can a 'sleeping' EV regain SOH?
Slightly. A battery that has been at low state of charge for months can show a 1-2% SOH lift after a few full cycles. But this is a measurement artefact, not real capacity recovery.
Does the test damage the battery?
Diagnostic readouts do not require a driving-discharge cycle. AVILOO describes FLASH as a three-minute diagnostic test; ask the tester to confirm the procedure and any vehicle-specific requirements.
Is SOH 88% on a 4-year-old EV bad?
Borderline. We'd want to see DC-charge history (frequent DC-fast = expected). At 88% on city-only AC charging, we'd flag it.
Lock in your inspection
Book a mobile pre-purchase inspection at the seller's address. Same-day slots across Sydney from $199, with a written repair cover on Elite (30 days / up to $3,000).
Mobile inspections near you