Why EV Battery Health Is the Most Important Thing to Check
When you buy a used petrol car, a worn engine is usually detectable — you can hear it, smell it, or see it in the smoke. Battery degradation in an EV is invisible. The car can look perfect, drive smoothly, and still have a battery that’s lost 25% of its capacity.
That lost capacity means:
- Shorter real-world range than advertised
- More frequent charging stops
- Reduced resale value
- Potential for expensive repairs down the track
The battery is also the most expensive single component in any electric vehicle. Replacement costs in Australia range from $8,000 to $30,000+, depending on the model. That’s why understanding battery health before you buy is not optional — it’s essential.
What Is State of Health (SOH) in an EV Battery?
State of Health (SOH) is a percentage that represents how much of the battery’s original capacity it can still hold and deliver.
A brand-new battery is rated at 100% SOH. Over time — through charge cycles, temperature exposure, and use patterns — that figure declines. This is normal and expected. The question is: how much has it declined, and is it within an acceptable range for the price being asked?
SOH is calculated based on:
- The battery's current maximum capacity vs. its original rated capacity
- Internal resistance of the cells (higher resistance = more degradation)
- Charge acceptance rate
- Historical charge and discharge patterns (stored in the BMS)
A battery at 85% SOH on a 400km-range EV means your real-world maximum range is now closer to 340km — before accounting for driving style, temperature, and speed.
SOH vs State of Charge (SOC) — What's the Difference?
These two terms are often confused. Here’s the simple version:
SOC changes every time you charge or drive. SOH changes slowly over months and years.
When a seller tells you “the battery is at 80%” — ask them: is that SOC or SOH? These are very different numbers. SOC at 80% is fine (it just means the car isn’t fully charged). SOH at 80% means the battery has already lost 20% of its total capacity.
What SOH Percentage Is Acceptable When Buying a Used EV?
There’s no single universal answer — it depends on the vehicle’s age, price, and your range requirements. But here are practical benchmarks:
Important: SOH alone is not enough. Two batteries can show the same overall SOH but have very different internal conditions. One may have uniform cell health; the other may have several weak cells dragging the average down. This is why cell-level analysis matters — and why a single SOH number from a basic scanner can be misleading.
Get a Professional EV Battery Health Check Before You Buy
Don’t rely on an SOH number alone - get the full battery story before you buy
An SOH number alone doesn’t tell the full story. A vehicle inspection goes deeper — BMS live data, per-cell voltage analysis, and a signed SOH certificate that tells you exactly what you’re buying.
How EV Battery Health Is Professionally Tested
What Is a Battery Management System (BMS)?
The Battery Management System (BMS) is the electronic control unit that manages every aspect of the battery pack. It monitors each cell in real time and controls:
- Charging and discharging rates
- Thermal management (cooling and heating)
- Cell balancing (keeping all cells at similar charge levels)
- Fault detection and protection
- Communication with the vehicle's main computer
The BMS stores a detailed history of the battery’s operation — including fault codes, temperature extremes, charge patterns, and cell behaviour over time. This data is a goldmine for a trained inspector.
Most generic OBD scanners can only read a small subset of this data. Accessing the full BMS dataset requires specialist diagnostic tools and the knowledge to interpret what you’re seeing.
BMS Live Data Sheets: What They Reveal
When George connects to the BMS module during a GPS EV inspection, he pulls live data sheets — not just a summary reading, but the raw, detailed output from the battery management system.
A full BMS live data report includes:
- Real-time voltage for every cell in the pack
- Real-time temperature for every cell
- Module-level capacity and resistance data
- Historical fault codes and their frequency
- Charge cycle count
- Maximum and minimum temperatures the battery has experienced
- Cell balancing status
- Any active or pending fault conditions
This is an extensive document — the kind of data that a battery engineer would use to assess pack condition. GPS provides this as a full PDF report included with every EV inspection.
- No other mobile inspection service in Australia provides this level of battery data as a standard part of their report.
Per-Cell Voltage Analysis (Up to 200 Cells)
A modern EV battery pack contains anywhere from 96 to 400+ individual cells, arranged in modules. Each cell has a nominal voltage (typically 3.2V–4.2V, depending on chemistry).
Why does per-cell voltage matter?
Healthy cells in a pack should all read within a very narrow voltage range of each other — typically within 20–50 millivolts. When one or more cells start to deviate significantly from the rest, it signals:
- Cell degradation or damage
- Internal short circuit risk
- Reduced pack efficiency
- Potential for sudden capacity loss
The BMS will try to compensate for weak cells through balancing, but it can only do so much. A cell that’s consistently out of range is a problem that will worsen over time.
- At GPS, George checks the voltage of every individual cell in the pack — up to 200 cells per inspection.
This is not a sample check. It’s a complete cell-by-cell assessment. If there are weak cells hiding in the pack, this process will find them — even if the overall SOH reading looks acceptable.
This Is What a Real EV Battery Inspection Looks Like
See beyond basic scans - get a full EV battery inspection with real data
Generic OBD scanners give you a number. A GPS vehicle inspection gives you the data behind it — every cell, every module, every fault code. Up to 200 cells are checked individually, every time.
Per-Cell Temperature Monitoring
Temperature is the other critical variable at the cell level.
Why temperature matters:
- Cells that run hotter than their neighbours indicate higher internal resistance — a sign of degradation
- Thermal hotspots can lead to accelerated ageing in localised areas of the pack
- In extreme cases, thermal anomalies can be a precursor to more serious battery safety issues
- Temperature data also reveals whether the battery's thermal management system (cooling pump, coolant lines, heat exchanger) is functioning correctly
George records the temperature of every cell during the inspection — not just an average pack temperature, but individual readings that reveal the thermal profile of the entire battery.
Combined with the voltage data, this gives a complete picture of battery health that goes far beyond any summary SOH figure.
What a Full BMS Module Data Report Looks Like
The GPS BMS live data report is a detailed, multi-page document that covers:
- Complete cell voltage table (all cells, all modules)
- Complete cell temperature table
- Module-level capacity readings
- Fault code history with timestamps
- Charge cycle data
- Pack-level summary statistics (min/max/average voltage, temperature range)
Here’s a real example from a GPS inspection:
The report below is from a 2020 Hyundai Kona Electric (100kW/150kW, VIN: KMHK281GULU064162) with 35,973 miles on the clock. George ran a full diagnostic scan using the Autel MaxiSys MS909 — one of the most advanced professional-grade EV diagnostic tools available.
GPS Vehicle Inspections – Autel MaxiSys MS909 full system scan on a 2020 Hyundai Kona Electric: topology diagram showing 22 scanned systems including BMS, MCU, OBC, CCM and ADAS modules, all systems green
Page 1 of 2 — Autel MaxiSys MS909 Vehicle Diagnostic Report: 2020 Hyundai Kona Electric (OS EV, 100kW/150kW). 22 systems scanned. Topology map shows all communication networks (C-CAN, P-CAN, B-CAN, K-Line). BMS, MCU, OBC and all major systems return green — 0 fault codes on Page 1.
GPS Vehicle Inspections – Autel MaxiSys MS909 diagnostic report page 2: full system status list including BMS green pass, CCM flagged with 2 DTCs — C182C:00 EVSE Fault and C183F:00 PLC Signal Failure — on a 2020 Hyundai Kona Electric
Page 2 of 2 — Full system status list.
BMS (Battery Management System):
✅ 0 faults. OBC (On Board Charger):
✅ 0 faults. CCM (Charge Control Module):
❌ 2 DTCs found — C182C:00 (EVSE Fault) and C183F:00 (PLC Signal Failure), both logged as History codes. These would not appear in a basic visual inspection or generic OBD scan.
What the scan revealed:
The Autel MaxiSys MS909 scanned 22 separate vehicle systems on this Kona EV — from the Motor Control System (MCU) and Battery Management System (BMS) to the ADAS suite (Forward Collision Avoidance, Lane Keeping, Blind Spot Warning) and the Charge Control Module (CCM). The topology diagram maps every system and its communication network across C-CAN, P-CAN, B-CAN, and K-Line — showing exactly how the vehicle’s electronics are interconnected.
The BMS returned a clean result — 0 fault codes, green status. That’s the good news.
But the scan also found 2 active DTCs in the CCM (Charge Control Module):
Both codes were flagged as History — meaning they had occurred previously but were not active at the time of inspection. A seller or a basic OBD scan might dismiss these as non-issues. But to a trained inspector, these codes tell a story:
- C182C:00 (EVSE Fault): An EVSE (Electric Vehicle Supply Equipment) fault indicates a communication or compatibility issue between the car and a charging station. This could mean the vehicle has had trouble with certain public chargers — a real-world inconvenience for an EV owner.
- C183F:00 (PLC Signal Failure): PLC (Power Line Communication) is the protocol used for smart charging communication between the vehicle and the charger. A PLC signal failure can affect the car's ability to communicate with AC charging infrastructure — potentially limiting charging functionality.
Neither code was active at inspection, but both are documented in the report and explained to the buyer. This is exactly the kind of information that should inform your purchase decision and your price negotiation — and it’s the kind of information that a basic visual check or a generic OBD scan would never surface.
This report is provided as a PDF and forms part of the full GPS inspection report — alongside the written mechanical assessment, photos, and video.
- When you receive a GPS EV inspection report, you're not just getting a pass/fail verdict. You're getting the same data a battery engineer would use to assess the pack — presented clearly, with every finding explained in plain English.
Can You Check EV Battery Health Yourself?
Some DIY options exist — but they have significant limitations.
For a Nissan Leaf, the Leaf Spy app gives a reasonable SOH estimate. For most other models, consumer-grade tools give limited or unreliable data.
- The fundamental problem with DIY battery checks: they give you a number without context. A professional inspector doesn't just read the data — they interpret it. Is 83% SOH good or bad for a 4-year-old Tesla Model 3 with 80,000km? What does a 45mV cell voltage spread mean for this specific battery chemistry? These are questions that require expertise, not just a scanner.
How Long Do EV Batteries Last?
EV battery lifespan depends on several factors:
Factors that extend battery life:
- Keeping charge level between 20–80% for daily use
- Avoiding frequent DC fast charging
- Storing in moderate temperatures (not extreme heat or cold)
- Regular, gentle charging cycles
Factors that accelerate degradation:
- Frequent DC fast charging (particularly older CHAdeMO systems)
- Regularly charging to 100% and discharging to near 0%
- Exposure to extreme heat (particularly relevant in Australian summers)
- High-mileage use in short, frequent cycles
- General lifespan benchmarks: Always check whether the battery warranty is still active and transferable to a new owner.
Also inspecting a hybrid? Battery health is equally important in hybrid vehicles. Read our hybrid car inspection guide for what to check on used hybrid battery systems.
EV Battery Replacement Cost in Australia
If the battery fails or degrades beyond acceptable levels, replacement is the only option. Here’s a realistic cost guide for the Australian market:
Costs vary significantly based on parts availability, labour, and whether a new OEM pack or a reconditioned pack is used. Prices are indicative and subject to change.
These numbers make the case for a professional inspection better than anything else. Spending $389 to avoid a $20,000 surprise is not a difficult calculation.
What's Included in a GPS EV Battery Inspection Report?
GPS Vehicle Inspections provides specialist EV inspections in Sydney and Melbourne. George leads every EV inspection personally, using professional-grade diagnostic equipment that goes beyond what any generic scanner can provide.
Your GPS EV inspection report includes:
- SOH certificate — a formal, documented State of Health reading for the battery
- Full PDF BMS module live data sheet — complete cell-by-cell voltage and temperature data for the entire pack
- Per-cell voltage analysis — every cell checked individually (up to 200 cells)
- Per-cell temperature monitoring — thermal profile of the complete battery pack
- Deep module-level diagnostics — going inside each module for extensive live data
- Charging system assessment — AC and DC charging ports, onboard charger function
- Full mechanical inspection — brakes, suspension, tyres, underbody, fluids
- Body and paint inspection — including paint depth test
- Instant verbal report — George calls you on-site with his findings
- Detailed written report — photos and video of all findings
- PPSR and car history report — included as standard
- GPS comes to the car. Our inspectors travel to the vehicle's location — private home, dealership, or auction yard. You don't need to arrange transport or be present.
Book your EV battery inspection online — same-day or next-day availability in Sydney and Melbourne.
Frequently Asked Questions
Generally, 80% SOH or above is considered acceptable for a used EV. However, SOH alone doesn’t tell the full story — the condition of individual cells matters just as much. A battery with 85% SOH but several weak cells may perform worse than one at 80% SOH with uniform cell health.
A professional EV battery inspection involves connecting to the vehicle’s Battery Management System (BMS) to pull live data — including individual cell voltages, temperatures, fault history, and capacity readings. At GPS, this includes per-cell analysis of up to 200 cells and a full PDF BMS data report.
In some cases, individual modules within a battery pack can be replaced rather than the entire pack — which is significantly cheaper. This is only possible if the fault is isolated to specific modules, which is why cell-level diagnostics are so valuable. A GPS inspection can identify whether a module replacement might be viable.
BMS (Battery Management System) data is the detailed operational data recorded by the battery’s control unit. It includes individual cell voltages, temperatures, charge history, fault codes, and more. This data reveals the true condition of the battery in a way that a simple SOH percentage cannot.
Yes. Every GPS EV inspection includes a formal State of Health certificate that documents the battery’s SOH reading. This can be used in price negotiations with the seller or kept for your records.
EV inspections are priced at $389 (discounted from $449). This includes the SOH certificate, full BMS live data PDF report, written inspection report with photos and video, and PPSR check. See full pricing.
Battery health doesn’t currently affect registration in Australia. However, some insurers are beginning to factor battery condition into EV valuations. An SOH certificate from a professional inspection can be useful documentation if this becomes more common.
Frequent DC fast charging does accelerate battery degradation compared to slower AC charging — particularly on older battery chemistries. Newer EVs (BYD, Hyundai, Tesla post-2021) handle fast charging better. During a GPS inspection, George checks the vehicle’s charging history to assess how heavily DC fast charging was used.
Don't Let a Bad Battery Be Your Problem to Inherit
Book your inspection today - don’t risk a bad battery
GPS Vehicle Inspections gives you the same battery data a battery engineer would use — explained in plain English, delivered the same day. At $389, it’s the easiest money you’ll spend before handing over tens of thousands.
✅ SOH certificate ✅ Full BMS live data PDF ✅ Per-cell voltage & temperature (up to 200 cells) ✅ PPSR & car history included
- ✅ SOH certificate
- ✅ Full BMS data report
- ✅ Per-cell voltage & temperature
- ✅ PPSR & vehicle history included
We operate 7 days a week across Sydney and Melbourne.
