Weak cell group versus a written-off pack on an imported 40 kWh hatchback
An independent workshop receives a 2017 imported 40 kWh hatchback (96 series cell groups, 350 V nominal, 96,000 km). The owner reports range falling from 240 km to about 150 km and has twice seen a power-limiting warning at 32% state of charge. The workshop has a Bluetooth battery dongle, a 1 kV insulation tester and a bidirectional scan tool, but no module-level service information beyond the manufacturer's public procedures. At rest the weakest cell group sits 190 mV below the pack average; under a 55 kW road-load discharge the spread widens to 430 mV and that group reaches 3.05 V while the rest sit near 3.42 V. A controlled 10 A discharge from 100% to the low-voltage cutoff returns 26.1 kWh. An insulation test with the pack isolated and the service disconnect removed reads 2.1 MΩ at 500 V. The owner wants the cheapest repair and asks whether a locally advertised "battery reconditioning" service or a single module swap will restore the original range.
1. What do the voltage and capacity data support - one failed cell group, general ageing, or a measurement artefact? State what you would replace and what you would refuse to do.
Read the worked answer
Repeat the rest reading at a controlled 20 °C with contactors closed and cross-check the group against the manufacturer's cell-voltage data before calling it a fault. A 190 mV rest spread is roughly four to six times what a healthy pack shows, so artefact is unlikely. The decisive evidence is the widening to 430 mV at about 1.4C: a group that is merely low on capacity drifts down slowly, whereas a group with high internal resistance collapses under current.
The 10 A test returning 26.1 kWh looks like 65% of nameplate, but the test stops at the first group to reach cutoff, so it understates the rest of the pack. The pack is ageing; one group is failing. Replace the pack as an assembly, or an OEM-matched stack where the service information explicitly permits it.
Refuse a single-module swap. A new low-resistance group in a string of aged groups takes a disproportionate share of charge and discharge current, and top-balancing equalises voltage at rest, not impedance. Refuse any work involving opening the pack without HV training, insulated tools and the OEM procedure.
2. The insulation reading is 2.1 MΩ. Is the vehicle safe to return to service, and what test sequence and decision criteria would you apply?
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2.1 MΩ at 500 V clears the absolute safety floor most manufacturers use (commonly 100 Ω/V, about 35 kΩ at 350 V), but it is below the 10 MΩ figure many service procedures call for and far below a dry pack. Treat it as contamination until proven otherwise, not as a failed pack.
Sequence: isolate, verify dead at the terminals, allow the specified discharge time, then test each HV component separately - pack, charger, PTC heater, compressor, motor/inverter, DC-DC - rather than the assembled vehicle, because a single low component localises the fault. Never apply a megohmmeter to a live or connected system, and use the voltage the procedure specifies rather than defaulting to 1,000 V. Look for coolant traces, corrosion in the charger connector and water in the underfloor tray. Dry, clean, retest after 24 hours, and compare against the vehicle's own isolation monitoring during a road test.
If one component is below limit, replace that component. Only condemn the pack if the pack itself remains low after drying. Record all readings.
3. Write the explanation you would give the owner about expected range after repair and about the "reconditioning" service being advertised locally.
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Explain the mechanism in plain terms: the pack stores energy, and every cell group also has internal resistance. One group now has high resistance, so its voltage sags first under load; the car reads that sag as an empty battery and limits power - that is the warning at 32%. Shallow cycling cannot add lithium inventory back or reduce that resistance, so a reconditioning service may shift the top balance and hide the symptom for a few weeks at most, while the group continues to age.
Set the range expectation honestly: with a replacement pack the car should return to its age-typical range, not the original brochure figure, because temperature, speed, heater use and tyres still dominate. Quote a band, for example 200–220 km at 20 °C and 140–160 km at 0 °C, and state that the power-limiting warning should not recur. Give the owner a written comparison of pack replacement cost against vehicle value, and record that a partial repair leaves the underlying impedance mismatch in place.