Failure modes
Nickel-metal-hydride packs (older Prius, Camry Hybrid) fail from individual module capacity loss — usually one to three modules out of 28 or 34 drift out of spec and drag the pack down. Lithium-ion packs (newer hybrids, plug-ins) fail differently: cell imbalance, BMS calibration errors, and cooling system failures dominate.
Module-level service
A capable independent can now diagnose which modules are weak using a battery analyzer, replace only the failing modules with matched-capacity used or rebuilt cells, and restore pack performance for $800 to $1,500 versus $3,500 to $6,000 for a dealer pack replacement. The safety requirements are non-negotiable: high-voltage PPE, insulated tools, and manufacturer disable procedures every time.
When to send it out
If the pack has thermal damage, coolant intrusion, or a failed BMS that cannot be reset, send it to a specialist. Do not experiment with high-voltage assemblies you do not understand. The learning curve is real and the failure mode is severe.