What Happened
Two weeks ago, Tesla service installed a remanufactured HV battery pack in Sylvie (our 2018 Model S 100D). The car had been running on the original pack since new — 86,000 miles — and the degradation had gotten bad enough that it was time. Or so we thought.
Turns out the replacement pack was defective from the start.
The Reman That Wasn’t
The remanufactured battery that Tesla put in didn’t just degrade faster than expected — it was actually bad. Faulty cells, voltage imbalances, the works. The car was throwing alerts, range was all over the place, and after a few days it was clear something was fundamentally wrong.
So back to the service center it went. They confirmed the pack was defective and are now planning another swap.
The $300 12V Surprise
Here’s where it gets frustrating. The 12V battery in Sylvie died — apparently as collateral damage from the faulty HV pack. The HV system wasn’t charging the 12V properly, it ran down, and now it’s dead.
Tesla service says the 12V is a separate wear item and wants ~$300 to replace it.
The Argument
My position: the 12V failed because the remanufactured HV battery they installed was defective. If they’d put in a good pack, the 12V would still be charging fine. The 12V death is a downstream consequence of their bad part. Warranty should cover it.
Tesla’s position: 12V battery is always customer-pay, regardless of cause.
Bottom Line
I’m pushing back. If you install a defective major component and it takes out a minor one in the process, that’s on you. Will update when service responds.
Lessons so far: Don’t assume a remanufactured Tesla HV battery is good just because it came from Tesla. And keep an eye on your 12V health — it’s the canary in the coal mine for HV system issues.