Damaged cell voltage sense harness

The thin wiring that carries each battery section's voltage back to the pack's sensing electronics has been damaged, corroded, or has developed resistance at a terminal.

These are the only thin wires in the high-voltage system, which is why they are the ones that fail. Everything else in a traction pack is heavy cable and substantial bolted joints, sized for hundreds of amps. The sense lines carry almost no current at all — they exist purely to be measured — and they run on small connectors inside a sealed box that vibrates, heats and cools for the vehicle's whole life. They fret, they corrode where any moisture reaches them, and their terminals lose tension.

Because they measure with essentially no current flowing, any resistance in the path reads as missing voltage. A corroded tap does not produce a noisy signal; it produces a section that appears to have less voltage in it than it does. And that is indistinguishable, to the module, from a section that genuinely is weak — which is how this cause ends up being answered with a battery pack.

The evidence that separates them is behavioural. A battery section has enormous electrical inertia: it changes slowly, moves with load and temperature, and stays inside the range its chemistry allows. A damaged sense line does none of that — it steps instantly, moves when the car goes over a bump, or reports a value no cell could hold. A reading that jumps is wiring, because a battery physically cannot change that fast, and that argument is worth making to anyone who quotes a pack.