EV Battery Health: How to Read SoH and Avoid a $25k Lemon
State of Health is the single biggest hidden risk on a used EV — and the metric most listings don't disclose. Here's how the number is calculated, what's acceptable, and how to test for it before you commit.
When you buy a 6-year-old internal combustion car, the worst hidden risk is a transmission rebuild — call it $5,000. When you buy a 6-year-old EV, the worst hidden risk is a battery pack replacement — call it $18,000 to $32,000 fitted, depending on chemistry and vehicle. State of Health (SoH) is the metric that tells you whether you're buying into that risk.
What State of Health actually means
SoH is the percentage of the battery's original useable energy capacity that is still available. A battery that started life with 75 kWh of useable capacity and now stores 67.5 kWh is at 90% SoH. The pack hasn't 'lost' 10% of its physical lithium — it has lost 10% of its ability to cycle that lithium reliably.
What's acceptable at what age
| EV age | Expected SoH | Negotiation flag | Walk away below |
|---|---|---|---|
| 1–2 years | 94–98% | <92% | <88% |
| 3–4 years | 88–94% | <86% | <82% |
| 5–6 years | 82–88% | <80% | <75% |
| 7+ years | 75–84% | <74% | <70% |
These bands are conservative — they're what we'd accept as 'normal degradation' on cars we test in Sydney. A pack outside the band is either an outlier (unlucky cell variance), abused (heavy DC fast-charging history), or being sold for a reason.
How to actually test SoH
Option 1: trust the dash
Tesla, Polestar, BYD and Hyundai/Kia all let you read approximate SoH from the car's own diagnostics menu or via service-mode screens. Tesla's 'rated range vs original rated range' is the simplest proxy. The catch: the car's BMS can be reset by an unscrupulous seller to make the number look fresher than it is.
Option 2: OBD2 + manufacturer-specific app
Tools like Scan My Tesla, LeafSpy Pro, or the Hyundai/Kia diagnostic apps pull pack voltage data and infer SoH from individual cell balance. Cost: $50 dongle plus the app. Caveat: requires basic technical literacy and the seller's permission to plug into the OBD port.
Option 3: AVILOO-style flash test
What we use. The car is driven from 70% down to 30% under a controlled load while a logger samples cell voltages 100 times per second. The result is a true measured SoH (not a BMS-reported estimate) plus per-cell variance — which is what actually predicts pack failure in the next 24 months.
Charging-history red flags
- 100% of historical charging on DC fast-chargers — pack has been thermally abused. Walk away.
- Long stretches sitting at 100% state-of-charge in summer — common on cars left at airports. Drops calendar SoH faster than cycling does.
- Cars that lived in WA, NT or rural QLD — sustained high ambient temperature accelerates calendar ageing by 30–40%.
- Heavy use cases (rideshare, delivery) — Tesla Model 3s used for Uber routinely show 80,000km cycles in 18 months. The car itself can be fine; the pack is older than the odometer suggests.
What battery replacement actually costs in 2026
| Vehicle | Out-of-warranty pack cost | Refurb option |
|---|---|---|
| Tesla Model 3 LR (75 kWh) | $22,400 fitted | $14,500 (3rd-party refurb) |
| Tesla Model Y LR (78 kWh) | $24,000 fitted | $15,800 |
| BYD Atto 3 (60 kWh LFP) | $16,800 fitted | Limited availability |
| Polestar 2 LR (78 kWh) | $28,400 fitted | $18,200 |
| Hyundai Kona EV (64 kWh) | $18,900 fitted | $12,800 |
| Nissan Leaf (40 kWh) | $14,200 fitted | $9,500 |
Pack warranties vary — most manufacturers cover the high-voltage pack to 8 years / 160,000 km against capacity falling below 70%. That last number matters: if a 5-year-old EV is sitting at 71%, it has zero claim leverage when it drops to 69% next year.
Lock in your inspection
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