Range Test
Customers ask one question about used and aging EVs: how much battery is left?
By Anthony LimpertASE Master Automobile Technician- SOH tests read the BMS's calculated state, comparing to new pack capacity. Reveals cell-level imbalance or degradation and estimates remaining life.
Answering it professionally means measuring state of health with proper tooling, not guessing from a voltage reading or the range display.
Measuring State of Health
A battery state-of-health (SOH) test is a scan tool job, not a voltmeter job. A simple voltage check tells you the state of charge at that moment, not how much capacity the pack has lost. OE and capable aftermarket scan tools read the BMS's calculated data: individual cell voltages, measured capacity, and degradation percentage compared to the pack's as-new capacity.
That data does three things for you. It quantifies overall degradation against the vehicle's age and mileage, it exposes cell-level imbalance — one weak cell group dragging down an otherwise healthy pack — and it supports an estimate of remaining service life. SOH numbers are also what warranty determinations hinge on, so document the reading, the tool used, and the conditions of the test whenever capacity is in question.
| SOH reading | Interpretation | Action |
|---|---|---|
| 95-100% | Minimal degradation | Document as baseline |
| 85-95% | Normal aging for a mid-life pack | Set expectations; note trend for next visit |
| 75-85% | Elevated loss — real range impact | Check warranty terms; investigate imbalance vs uniform wear |
| Below ~75% | Major finding | Many OEM warranties trigger around 70% — quantify and document carefully |
| Any reading | Only valid from BMS data via scan tool | Voltage checks and dash range estimates are not SOH |
Degradation expectations vary with chemistry, age, mileage, and climate — most OEMs consider 20-30% loss over 8-10 years within normal. Verify the specific vehicle's warranty threshold and use a tool proven for that platform.
- Bring the pack to a moderate, known state of charge and temperature — an extreme-cold or nearly empty pack skews the BMS's data and your conclusions.
- Read SOH, measured capacity, and cell-group voltages with an OE or proven aftermarket scan tool. The BMS's calculated data is the measurement; a voltmeter only reads state of charge.
- Record the test conditions: temperature, SOC, tool and software version. SOH numbers get compared across time and argued over in warranty claims — context makes them defensible.
- Compare the reading against age and mileage expectations, and against the cell-group spread. Uniform loss is aging; one sagging group is a repairable imbalance masquerading as degradation.
- Document everything on the RO — reading, tool, conditions. If the number sits near a warranty threshold, this record is what the claim stands on.
- Answering a capacity question with a voltage reading. Voltage tells you how full the pack is right now; SOH tells you how much the tank has shrunk — confusing them produces confidently wrong answers.
- Testing a cold-soaked pack and reporting the number without context. Temperature suppresses apparent capacity; an SOH reading without recorded conditions invites a dispute later.
- Using the dashboard range estimate as evidence of degradation. Range estimates blend driving history and conditions with capacity — the BMS SOH data is the measurement; the dash number is a projection.
A used-EV buyer asks you to 'check the battery' before purchase. What does a professional answer include?
A scan-tool SOH reading with measured capacity and cell-group voltages, recorded test conditions, and a comparison against age/mileage expectations and the OEM warranty threshold. A voltage check or a full-charge range demonstration is not an answer — SOH from the BMS is the number that predicts the range they will live with and what a warranty claim would hinge on.
An 8-year-old EV shows 78% SOH with all cell groups tracking evenly. Is this a failing pack?
Probably normal aging — most OEMs consider 20-30% loss over 8-10 years within expectations, and the even group spread says no single failure is present. The important moves are checking the warranty threshold (often around 70%) and documenting the reading with conditions, because the trend from here determines whether a claim window opens before coverage expires.
A used-EV buyer asks you to 'check the battery' before purchase. What does a professional answer include?
Missed one? The reasoning above comes straight from the EVH exam bank, so this is the standard you will be held to.