High-Voltage Battery Systems: Diagnosis, Service, and Repair
Architecture and chemistry
Know the hierarchy: cell, module, pack. Modern lithium-ion cells run about 3.6–3.7 V nominal, 4.1–4.2 V fully charged, and roughly 3.0 V at cutoff, wired in series for voltage and parallel strings for capacity. Pack voltage equals series cell count times cell voltage - a 96-cell series pack is about 355 V nominal and about 400 V at full charge. The flat state-of-charge-versus-voltage curve means open-circuit voltage alone is a poor SOC indicator; the BMS uses coulomb counting plus voltage and temperature.
Diagnosis
Essential checks: cell-group voltage deltas (a large spread suggests a weak or failing module), capacity and self-discharge testing, isolation-resistance (megohmmeter) measurement, contactor and precharge-circuit function, interlock loop continuity, and thermal-management performance. Contactors can weld closed, so a pack that measures 0 V at the terminals may read pack voltage inside the housing. Insulation faults often appear only under wet conditions.
Service and repair
Follow the OEM for module replacement; never mix modules of significantly different age or SOC in the same string. Observe busbar torque, ESD precautions, and interlock routing. Support the pack with proper lifting equipment and never weld or heat near it. Quarantine any pack showing swelling, sweet/chemical odor, venting, or impact damage - signs of incipient thermal runaway - away from structures, and never submerge or open a damaged pack.
Trap and rule of thumb
Watch for precharge-resistor failure misdiagnosed as a bad pack, and for BMS self-learn values that need a full charge/discharge cycle after repair. Rule of thumb: if the highest and lowest cell groups differ by more than about 50 mV at rest, investigate before condemning the pack.