Failed battery coolant heater

The heating element that warms the traction battery's coolant has failed, or its control circuit has.

Most people do not know a battery has a heater at all, which is why this failure is so reliably misdiagnosed. A lithium pack works badly cold — the ions move more slowly through the electrolyte, so internal resistance climbs steeply — and below roughly ten degrees the car must limit both how much power it draws out and how much it accepts in. That is why an electric car in winter is slower, regenerates weakly, and rapid-charges at a fraction of its summer rate. So the car heats the pack, before a scheduled departure and while preconditioning for a rapid charge.

The failure is invisible for half the year and then transforms the car. In summer, nothing. In winter: reduced range, soft acceleration, no regenerative braking for the first few miles, and a rapid charger delivering far less than its rating — on a vehicle with no other fault, whose owner concludes the battery is worn out. A dead heater and a genuinely aged pack are nearly indistinguishable in January.

The discriminator is free and it is the season. A car that behaves normally in warm weather does not have a worn-out pack. Ask what it was like in July before accepting a battery quote in January.

Two failure modes worth separating. An element that has gone open-circuit reports as a circuit fault and is straightforward. An element that has failed while its control circuit still tests perfectly reports only as a performance fault — the module commanded heat, the coolant did not warm, and every electrical measurement is normal. That second one is the one dismissed as a sensor problem.