How to Check Your EV's Real Battery Capacity (Not Just the Dashboard %)

Your dashboard's state of charge percentage tells you how full the battery is right now — it doesn't tell you how much the battery has shrunk from its original size. A car showing 100% might only have 78% of its original capacity. Here's how to get the real number.

Why this matters

Battery degradation is normal and expected. But the rate varies enormously between vehicles, and if you're buying a used EV or a hybrid, knowing the actual capacity — not the dashboard percentage — is essential for negotiating price and planning for replacement.

A Nissan Leaf at 70% capacity and 100% charge has around 17 kWh of usable energy (from an original 24 kWh pack). That's roughly 130 km of real-world range — fine for a city car, inadequate for anything longer.

Method 1: Leaf Spy (Nissan Leaf)

Nissan Leaf (all generations) is the easiest EV to check because of the Leaf Spy app ecosystem.

What you need:

• A Bluetooth OBD2 adapter (ELM327 compatible, around $20–$50 on TradeMe)
• Leaf Spy Pro app on iOS or Android ($15)

What to do:

1. Plug the OBD2 adapter into the port under the dash (driver's side)
2. Open Leaf Spy Pro and connect
3. The Hx (Health) value shows battery health as a percentage of original capacity
4. The AHr value shows current capacity in amp-hours (original was ~66 AHr for 24 kWh, ~88 AHr for 30 kWh, ~118 AHr for 40 kWh)

A healthy 2018–2020 Leaf should show Hx above 85% and AHr above 100 for the 40 kWh pack.

Method 2: Charge from low to high and calculate

This works for any EV and doesn't require special tools — just patience.

1. Drive until the battery is at 10–15% (or as low as you're comfortable with)
2. Charge to 100% on AC charging (not DC fast charge — it's less accurate for this purpose)
3. Record the kWh added during the charge (most home chargers and public chargers display this)
4. Add the kWh remaining at the start (battery % × original pack size)
5. Compare total to original pack size

Example: A 2016 Nissan Leaf (24 kWh original) starts at 12% and charges to 100%. The charger shows 18.5 kWh added. Remaining at start: 12% of 24 kWh = 2.9 kWh. Total: 18.5 + 2.9 = 21.4 kWh. Capacity: 21.4 ÷ 24 = 89% health. Good for a 2016.

This method has ±3% accuracy. Good enough for a buying decision; not precise enough for warranty claims.

Method 3: Model-specific apps and menus

Several EVs have built-in battery health displays:

Tesla Model 3 / Model Y: Go to the app → Energy → tap the battery icon. This shows pack energy, but Tesla doesn't expose a health percentage directly. Third-party apps (Stats for Tesla, TeslaFi) track this over time. For a point-in-time check, a full charge followed by a range estimate in "rated miles" vs the car's new-car rated range gives an approximation.

Hyundai IONIQ 5 / Kia EV6: No built-in health display accessible to owners. Dealers can read it via GDS (Global Diagnostic System). We can also access this via our diagnostic equipment.

BYD Atto 3: BYD's app shows state of charge but not capacity health. Diagnostic tools required for capacity testing.

MG4: Battery health accessible via the MG iSMART app under Vehicle Health — though accuracy has been variable and updates have improved it through 2025.

Method 4: Professional diagnostic test

For any EV or hybrid where you need a definitive, documented result — especially for:

• Pre-purchase inspections
• Insurance claims
• Warranty disputes
• Deciding whether to repair or replace

A professional capacity test uses calibrated equipment to measure actual pack capacity under controlled load. It produces a cell-by-cell report showing overall capacity, individual module health, and temperature performance.

Cost: $95–$120 for hybrids, $120–$180 for full EVs. Takes 45–90 minutes. You receive a written report you can use for negotiations or records.

What to look for in the results

| Capacity (% of original) | What it means | |---|---| | 90–100% | Excellent — minimal degradation | | 80–90% | Good — normal for age/mileage | | 70–80% | Acceptable — range reduced, fine for city use | | 60–70% | Poor — significant range loss, replacement approaching | | Below 60% | Replace — car is not practical for most NZ driving patterns |

For hybrids specifically, the percentage matters less and cell balance matters more. A 75%-capacity hybrid with good cell balance drives better than an 85%-capacity hybrid with severe imbalance.