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
We use one of two methods depending on the vehicle:
- Direct read from the BMS (Battery Management System) via OBD-II — works on Tesla, BMW, Hyundai/Kia E-GMP, Polestar, MG, BYD.
- AVILOO Flash or Comma test (15-minute partial discharge with controlled load) for models that don't expose SOH cleanly — older Leaf, ZOE, Outlander PHEV.
Both methods produce an SOH percentage and a per-module variance score (how evenly the battery has aged across its cells). The variance is almost as important as the SOH — high variance means one module is failing and dragging the rest down.
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?
No. OBD-II reads are passive. AVILOO Flash uses 15 minutes of normal driving load, the equivalent of a city commute.
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 $249, with a money-back guarantee.
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